DC SERVO TRAINER
INSTRUCTION MANUAL
ED-4400
TABLE OF CONTENTS
1. Introduction ......................................................................................... 3
2. General guidelines in using ED-4400 Servo System ............................ 4
3. Experiments using ED-4400 Servo System
Experiment 1 Motor speed and the input characteristics ............ 6
Experiment 2 Motor speed and the load characteristics ............. 10
Experiment 3 Transient response of a motor ............................. 14
Experiment 4 Operational amplifiers .......................................... 18
Experiment 5 Closed loop motor speed control techniques ....... 23
Experiment 6 System gain and motor speed control ....................27
Experiment 7 Bi-directional motor speed control ........................ 33
Experiment 8 Motor speed control efficiency ............................. 37
Experiment 9 Error signals in a position controller ...................... 42
Experiment 10 Closed loop position controller ............................ 45
Experiment 11 Transient response of a position controller ...........48
Experiment 12 Position control with speed feedback ....................52
Experiment 13 Stabilizing an unstable position controller .............55
Experiment 14 Construction of a position controller .....................58
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Auxiliary Equipment:
To maximize the learning efficiency, the following instruments are essential in performing the
experiments listed above. Users are to provide the equipment. These instruments are readily available
from ED Engineering.
- Oscilloscope : An oscilloscope with dual trace capability. The dual trace should be able to set to
generate X vs.Y outputs on the screen. It’s preferred to have a variable persistence adjustments to retain
the captured image on the screen for longer period of time.
- Voltmeter : The voltmeter should have high input impedance. The DC voltage range is required to be at
least 15V.
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1. Introduction
The DC servo trainer ED-4400 from ED Engineering is a closed loop DC servo system designed in modular
construction. The key concept behind the ED-4400 trainer system is to provide practical working
knowledge on closed loop DC servo systems to the users by integrating basic theories and step-by-step
experiments into one subject. To maximize the educational effectiveness, each section ends with a brief
summary to conclude what the user has learned in the experiment.
The modular construction in ED-4400 makes it extremely simple and easy to assemble experimental
systems. All that is required is to make connections between modules as instructed in the wiring
diagram of each section using the patch cords provided. A total of 14 experiments are covered in this
manual. At the end of this manual, the motor characteristics used in ED-4400 and the schematics of
each module are presented as supplemental information.
2. General guidelines in using ED-4400 Servo System
The following guidelines are common to all experiments in this manual. The user should be well aware
of these guidelines before setting up ED-4400 for actual experiments.
• When modules are placed on a panel for an experiment, there is no specific location for each module.
Modules are placed as the user prefers.
• Whenever the servo motor and potentiometer (U-158) are required to be linked together, make sure
it is done correctly. Otherwise, the motor will experience unnecessary loads. Make the connection to
the low speed shaft (1/60 of the motor speed) by hand.
• The high speed shaft of the servo motor can adapt the electronic brake set (U-163). The “0” indication
on the brake means the load from the brake is zero.
• The tacho generator is mechanically coupled to the servo motor. It generates AC voltage and
frequency outputs proportional to motor’s RPM. The Tacho Amp Unit (U-155) converts the frequency to
an equivalent voltage through a F/V (frequency-to-voltage) converter. The converted voltage is used as a
feedback signal.
• When the “OVERLOAD” indicator of the Power Supply Unit (U-156) is on, it means that there is an
excessive current flowing in the circuit. Turn the power off immediately, and check for the cause of the
overload.
• Throughout the experiments in this manual, rotating speed is expressed in volts, which is proportional
to the speed.
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• In order to display the system response in an easy way, the U-162 Function Generator generates a
ramp signal which is in phase with the output. Therefore, the internal time reference of an oscilloscope
is not necessary.
• The ±15V power supply is omitted from wiring diagrams.
• Functional descriptions of each module:
U-151 ............. Dual attenuator (0, 9/10 .... 1/10 attenuation)
U-152 ............. Summing amplifier (gain: 0 dB, EXT, NET)
U-153 ............. Pre-amplifier (gain : 20 dB)
U-154 ............. Motor driver amplifier (10 watts)
U-155 ............. Tacho Amp unit
U-156 ............. DC power supply ( ± 15V 0.2A and Motor Power)
U-157 ............. Potentiometer (Reference) ( 1kΩ or 10 kΩ 5W)
U-158 ............. Potentiometer (Motor Coupling) (1kΩ or 10 kΩ 5W)
U-159 ............. Tachometer (FS 4000 RPM)
U-161 ............. Servo motor (Motor: 12V, 4.5W
Tacho Generator: Approx. 3Vp-p/4000RPM)
U-162 ............. Function Generator ( 0.1~1 Hz, 1Hz~10Hz and Ramp output)
U-163 ............. Magnet Brake (air gap : 4mm, 10 step variable input power: AC 110/220V, 60Hz)
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Module identifications
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Experiment 1 Motor speed and input characteristics
1. Basic theory
In general, a motor is a machine that converts electrical energy into mechanical rotation. The key
elements of a DC motor are a field winding and an armature winding. As electric currents flow through
the windings, torque is developed between these two windings. In ED-4400 trainer system, the field
winding is replaced by permanent magnets. The permanent magnets provide constant lines of magnetic
flux and therefore, the motor speed becomes only a function of the voltage applied to the armature
winding. This relationship is shown in Figure 1-1.
In Figure 1-1, the point “a” occurs because a motor requires a certain minimum voltage to overcome the
mechanical friction from brushes, bearings and other moving parts before it starts to move. Once the
input voltage exceeds the minimum voltage, the speed of the motor begins to increase in linear fashion
as the input voltage is increased. However, this linear characteristics is not maintained beyond the
saturation point. It is because the counter electromotive force in the armature coil is also increased as
the input voltage is increased, and at some point, any further increase in input voltage does not produce
increased electric currents in the coil.
The motor in ED-4400 system is driven by U-154 Motor Driver Amplifier with U-151 Attenuator as a
voltage control. The detection of the motor speed is accomplished by converting the Tachooutput of the
motor (U-161) through the F/V Converter (U-155). The converted output is indicated on the Tacho
meter, U-159. The AC output from the Tacho motor is converted into DC which is proportional to the
motor speed through U-155
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2. Experiment Procedure.
1. Referring to Figure 1-2 and 1-3, place the modules needed in the experiment on a flat surface or on
top of the ED-4400 cover, and connect modules as indicated in the figure.
2. Connect the Tacho-meter U-159 across U-155 meter and GND.
3. Set the angle on U-157 to 180 degrees.
4. Verify that the line voltage is correct (100V or 220V). Plug U-156 line cord to the power outlet, and
turn the power switch ON.
5. Turn U-157 slowly counterclockwise until the motor begins to move. Record the U-157 position and
the input voltage.
6. Increase the input voltage by slowly turning the U-157 clockwise. For every one volt increment of the
input voltage (1V, 2V, 3V .... ), record the U-159 indication.
7. Make a graph on input voltage vs. motor speed using the above measurement data.
Caution: When the motor is saturated, increasing the input voltage will not increase motor speed. Avoid
saturation in this experiment.
8. Make a graph on motor speed vs. motor current using the data obtained in step 5 and 6. Review the
relationships between these two parameters.
9. Repeat the steps 5 -7 several times to reduce the measurement error.
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3. Summary
• The motor speed in a servo system is proportional to the input voltage.
• The motor current is not linearly proportional to the input voltage. At saturation, the motor input
current no longer increases even if the input voltage is increased. The saturation effect is caused by the
counter electromotive force in the armature coil.
• There exists a “deadband” input voltage range in a motor, below which a motor can’t start. Motor
input voltage is required to be greater than the largest value of the deadband to initiate motion. The
deaband is caused by various mechanical frictions in the system.
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Experiment 2 Motor speed and the load characteristics
1. Basic theory
Typical output ratings of permanent magnet based DC motors range from a few watts to several
hundred watts, and this type of motors exhibit an excellent power efficiency.
As was mentioned earlier, permanent magnets in the motor provide constant magnetic flux ( KΦ).
Therefore, the torque (T) generated in the motor becomes a function of only the input current (Ia). Also,
the counter emf (electromotive force) of a motor (Ea) is generated by the action of the armature
conductors cutting lines of force, and is proportional to the speed of the motor (ωm). These
relationships are expressed in the following formulas.
KΦ = constant ..................................... (2-1)
Ea = KΦ ωm ......................................(2-2)
T = KΦ Ia .........................................(2-3)
where KΦ = magnetic flux (line of force) of the permanent magnet
Ea = counter emf in volts
ωm = speed of the motor in rad/sec
T = torque in N.m
Ia = input current in amps
The input voltage and speed of the motor are related to other parameters according to the following
equations:
Vt = Ea + Ra Ia ................................. (2-4)
ωm = Vt / KΦ - Ra T / (KΦ)2 ............. (2-5)
where Vt = input voltage in Volt
Ra = Resistance of armature coil in Ohms
It should be noted that the input current increases as the mechanical load of the motor is increased,
resulting in increased input power. Also, the counter emf keeps the motor speed constant when a motor
is not loaded. The relationship between motor speed and load is illustrated in Figure 2-2.
2. Experiment Procedure
1. Referring to Figure 2-1 and 2-3, arrange the modules and connect them together.
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2. Set U-151 attenuator to “8”, and turn the power switch of U-156 on. Adjust U-157 to obtain maximum
speed on U-159 without saturation.
3. Attach the aluminum disk to the high speed shaft of U-161 as shown in Figure 2-4. Raise the electric
brake setting on U-163 from 0 to 10 by one step each time, and push the button and measure the RPM
on U-159. See also Step 5.
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4. Repeat the measurements in Step 3 by starting from 10, and moving toward 0. See also Step 5.
5. In Step 3 and 4, record the corresponding motor current readings as indicated on U-156 Power Supply
module. This is the current flowing between U-154 (Motor Driver Amp) and U-161 (Motor).
6. Plot the data points obtained in Steps 3 and 4, showing the relationships between brake setting and
motor speed and motor currents.
3. Summary
• When a motor is loaded, the speed of the motor decreases, and the input current increases.
• Overloading a motor causes excessive currents in the motor winding, and could result in damage to
the motor due to the heat generated by the product of the motor voltage and motor current.
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Experiment 3 Transient response of a motor
1. Basic theory
Previous experiments defined steady state motor characteristics. Due to the existence of nonideal
parameters in the real motors, a motor can’t respond instantaneously to a step input. Instead, a motor
responds in an exponential rise in speed. When the input is removed, the motor speed decreases
linearly to zero . This relationship is illustrated in Figure 3-1 and 3-2. It’s obvious that the inertia in a
motor affects the rate of delay in response. When an inertia load, such as a flywheel, is added to the
rotating shaft, the response of a motor is significantly slow as shown in Figure 3-2.
These figures are obtained from an oscilloscope with the motor input and horizontal time signals as
shown in Figure 3-4 are applied to the oscilloscope. The point “a” in Figures 3-1 and 3-2 indicates where
the motor begins to move. The point “b” is where the motor input is removed, and the speed of the
motor begins to fall.
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2. Experiment procedure
1. Referring to Figure 3-3 and 3-5, arrange modules and an oscilloscope and connect them together.
2. Set the oscilloscope for X-Y mode. Apply the Ramp output from U-162 to the X-input of the
oscilloscope.
3. Set the frequency of the Function Generator (U-162) to 0.1Hz.
4. Turn the power of U-156 on.
5. Adjust the gain of X-input (CH2) of the oscilloscope for proper display on the screen.
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6. Adjust U-151 to set the motor speed which is indicated on U-159 below saturation. If necessary, use
U-157 instead of U-151.
7. Adjust the gain of Y-input (CH1) of the oscilloscope for proper display on the screen.
8. Observe the trace on the oscilloscope.
9. Turn the power off (U-156). Attach a flywheel to the high speed shaft of U-161. Turn the power on,
and observe the trace on the oscilloscope.
10. Move the flywheel to the low speed shaft of U-161, and repeat the above experiments.
11. Plot the obtained data.
3. Summary
• Unlike an ideal motor, real motors respond to a step input with an exponential rise in speed.
• The rotational inertia in the motor affects the transient response of a motor. The larger the inertia, the
worse the response.
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Experiment 4 Operational amplifiers
1. Basic theory
A closed loop servo system needs information as to how much the output speed of the motor is
different from the preset input. The detected difference is returned to the system controller as an error
signal. Once the amount of error is defined, the closed loop reacts in a way to reduce the error , and the
loop repeats the process until the error detected becomes zero. The error detection is done by
comparing the input and sampled output voltage using an operational amplifier.
The key elements of an operational(OP) amplifier circuit are the resistors and the gain of the amplifier
itself which is typically in the range of 1000 to 100,000. Several OP amp circuits using U-152 Summing
Amplifier Unit are shown in Figure 4-1. Because the gain of the amplifier “A” is very large, the output of
the amplifier is given by the following equation.
Vo = - (R2/R1)(V1 + V2 + V3) ..................... (4-1)
In equation (4-1), it can be seen that when R1 = R2, the output Vo becomes the sum of the inputs. Also
when a voltage divider network is used as in Figure 4-1 (b), the Vo can be scaled down by a factor of 1/α with the α representing the ratio between the divided resistance to the entire resistance of the
divider network. When a capacitor is placed in the feedback path in parallel with a resistor, as in Figure
4-1 (c), the response of the output to a step input is affected by the time constant of the RC network. In
this case, the output Vo is obtained from;
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Vo = -V .................. (4-2)
where τ 2 = C2 R2
The above expression of Vo assumes that the Vo does not exceed 12V supply voltage. In U-152, setting
the selector switch to “a” will configure the amplifier to Figure 4-1 (a) with R1 = R2. When the selector is
set to “b”, the amplifier will be configured to Figure 4-1 (c) with R1 = R2, and CR2 = 0.1 second. In this
experiment, only Figure 4-1 (b) circuit is utilized.
2. Experiment procedure
1. Referring to Figure 4-2 and 4-3, arrange the necessary modules and connect them together.
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2. Set the selector switch of Summing Amp U-152 to “EXT”.
3. Turn the power of U-156 on.
4. Using a high input impedance (1 Mohm or larger) voltmeter or an oscilloscope, measure the voltage
at U-157 and U-158 terminals (slide side). Adjust the voltage to 0.
5. Set U-151 to 0.
6. Measure the output of the U-152 using a high input impedance voltmeter. Make sure the output is at
near 0 (around 0.01V).
7. Adjust such that the outputs of U-157 and U-158 are +1V respectively.
8. Measure the output voltage of U-152, and observe the relationships to the input.
9. Set U-151 to “5”, and measure the output of U-152.
10. Set U-151 to “0”. Vary the output of U-157 and U-158. Check the summed output appearing at U152.
11. Observe how the U-151 position affects the input and output relationships. When the polarity of the
output changes to “-” ? Examine the summed output value.
12. Notice that when U-151 is set to “0”, R1a = R2 and the gain becomes unity (one). When U- 151 is set
to “10”, the gain is at maximum because R2 is maximized.
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3. Summary
• An operational amplifier is a linear amplifier. The output is proportional to the input, and inversely
proportional to the negative feedback.
• Operational amplifiers are used in error detection circuits where more than two signals are compared
and added together. The high input impedance of an operational amplifier results in negligible signal
loss. The summing output includes the polarity of the input signals.
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Experiment 5 Closed loop motor speed control techniques
1. Basic theory
Quite often, when a motor is used as a source of mechanical force, the motor is required to provide
constant speed regardless of the change in loads. A closed loop speed control system is a self-regulating
system in which the measured speed of the motor is compared to the preset valueto produce an error
output. The detected error voltage is, then, amplified and fed back to the control circuit to compensate
the difference between the actual and preset speed. This self-correcting process continues until the
detected error voltage becomes zero. At this point, the actual speed of the motor is equal to the preset
speed, and the motor maintains a constant speed. Compared to the closed loop system, the systems
built in the previous experiments are identified as an open loop system.
The conceptual difference between an open loop and closed loop systems is graphically illustrated in
Figure 5-1.
In Figure 5-1, it’s clear that a system with feedback is far superior than an open loop system in
maintaining a constant speed against load variations. In a closed loop system, it’s important that the
error signal is amplified to a proper level to eliminate “deadband” effect. For this reason, the error signal
is amplified before it arrives to the input of the Servo Driver (U-154). Also it is critical that the feedback
signal is 180 degrees out of phase to the reference signal to maintain proper control.
2. Experiment procedure
1. Referring to Figure 5-2 and 5-3, arrange the required modules and connect them together.
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2. Set the selector switch of Summing Amp U-152 to “a”.
3. Set ATT-2 of the U-151 to “10” to prevent Tacho output from entering the system. Set ATT-1 to “5”.
4. Turn the power of U-156 on.
5. Adjust U-157 to obtain about one half of the maximum speed. This is same as setting for 2500 RPM on
U-159 meter.
6. Attach an electronic brake U-163 as was done in Figure 2-4. With the brake’s setting increased by one
notch at a time, record the RPM reading at each setting.
7. Measure the error voltage at each brake setting.
Note: There is no feedback signal at this point. Therefore, the error voltage will vary only when the
preset speed is changed to a different value.
8. Set ATT-2 of U-151 to “5”. Adjust U-157 to obtain the same speed as in Step 5 (around 2500RPM).
9. Measure the Tacho output and error voltage at different brake points. Plot the data points on the
chart provided in Figure 5-4.
10. Change ATT-2 setting to “0”. Adjust U-157 to obtain 2500 RPM.
11. Measure the speed and error voltage at each brake setting, and plot the data on the chart.
12. Compare the results between Steps 3-7 and Steps 8-11. Notice that the loop was closed for Steps 8
through 11.
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3. Summary
• In a closed loop system, reduction in motor speed due to a load is compensated, within the limit, by an
error signal which is proportional to the drift of speed and is 180 degrees out of phase to the reference
setting.
• Excessive feedback signals will reduce the reference setting. Therefore, the feedback signals at the
input of the summing amp can’t be larger than the reference signal. The feedback signal should be
adjusted to the right level for given load and amplifier gain.
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Experiment 6 System gain and motor speed control
1. Basic theory
A simplified diagram of a closed loop constant motor speed control system is shown in Figure 6- 1. As
the reference or control voltage is applied to the input of the comparator, and the Tacho generator
produces a signal which is equivalent to the speed of the motor, the two signals are compared at the
input of the summing amplifier through addition of two signals with opposite polarity. The output of the
comparator is, then, an error signal which represents the difference between the preset and actual
speed. Because the error signal is out of phase to the reference signal, this signal compensates the
motor speed in the direction to achieve a constant speed.
In general, the speed of a motor and the error signal have the following relationship.
θ o = K E ................................... (6-1)
where θ o = the motor speed
E = error signal
K = system gain
The error signal is defined as:
E = Vref - Kg θ o ........................... (6-2)
where Vref = reference voltage
Kg θ o = output of the Tacho generator
Replacing E in (6-1) with (6-2) yields;
θ o = K (Vref - Kg θ o) .....................(6-3)
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θ o = ............... (6-4)
In case the K is very large in forward direction, Equation (6-4) is reduced to;
θ o = ............................... (6-5)
From equation (6-5), it’s clear that for a given Tachogenerator constant Kg, the motor speed is linearly
proportional to Vref only, and is not dependent on the deviation of the system gain. This is the most
beneficial advantage of a closed loop motor speed control system.
Similar relationships can be developed for the error signal in a closed loop system. Replacing
θ o in (6-2) with (6-1),
E = Vref - Kg K E ................................ (6-6)
E = ................................ (6-7)
Equation (6-7) indicates that the error voltage E can be reduced when the gain K is increased. In a
practical system, maintaining a high system gain means reduction of the deadband, as well as
desensitizing motor speed to the load changes. Although large system gain is desired in general, the gain
should be limited to an acceptable level. When the gain is beyond the acceptable level, the transient
characteristics of the system will suffer, and it will cause irregular motor rotation. The relationships
between load, error and motor speed are shown in Figure 6-2 at two different system gain levels.
Figure 6-2 Relationships between load, error and motor speed
For an equivalent system diagram of Figure 6-3, the output of the Frequency-to-Voltage converter U-155
should be large enough to provide sufficient feedback signal. Otherwise, the motor will not run at
constant speed. Also, when the gain of the amplifier U-153 is low, the system response will be slow and
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the “deadband” effect will get worse. However, in case the gain is too high, the system will become
unstable.
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2. Experiment procedure
1. Referring to Figure 6-4, arrange all the modules and an oscilloscope and connect them together.
2. Set the selector switch of U-152 to “a”.
3. Set ATT-1 of U-151 to “9” and ATT-2 to “10”. This will minimize the reference setting, and the
feedback will be almost zero.
4. Turn the power of U-156 on. Adjust U-157 to approximately one half of the maximum motor speed
(2500 RPM).
5. Referring to Figure 2-4, attach the disk brake to the high speed shaft of the servo motor, and set the
brake to “0”. Raise the brake setting by one increment, and each time, press the brake button and
measure the motor speed and the associated error signal.
6. Set the U-151 ATT-2 to “5”. Adjust the motor speed to 2500 RPM, and repeat Step 5. Plot the data
obtained in Figure 6-5 (a).
Notes: The same motor speed can be obtained by increasing the reference signal level and decreasing
the amplifier gain. However, this method will reduce the amount of feedback control signal and thus
decrease the over-all ability to control the system.
7. Using U-157, set the motor speed to 2500 RPM. Set U-151 ATT-2 to “5”. Adjust ATT-1 from 0 to 9, and
measure the error voltage at each point.
8. For each point of ATT-1 setting, hold the high speed motor shaft by hand and repeat the experiments
in Step 7. Compute the error deviation ratio as defined by the following equation, and plot the results in
Figure 6-5 (b).
Note: Error Deviation Ratio = error measured with motor stalled error measured with motor running
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3. Summary
• In a closed loop servo system, lower system gain produces larger error voltage, reducing controllability
of constant motor speed.
• Constant motor speed is obtained when the detected motor speed signal is equal to the preset
reference signal. As a motor approaches to constant speed operation, the magnitude of the error signal
becomes very small. Therefore, the gain of the error amplifier requires to be large.
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Experiment 7 Bi-directional motor speed control
1. Basic theory
The closed loop speed control system that has been investigated so far has negative feedbackbased
speed control ability only in one direction. However, in real applications, motor speed control requires
to be available in both directions: forward and reverse. The motor used in ED-4400 changes its direction
as the input polarity changes. The error signal polarity follows the input polarity change at the same
time.
The direction of the rotation is determined by the position of the potentiometer setting referenced to 0.
The speed of the motor which should be constant after proper regulation is linearly variable as a
function of the potentiometer setting.
Figure 7-1 shows the bi-directional response of a motor at two different loads when a squarewave input
signal is applied to the system. The curves labeled as “1” represent the response in forward direction,
and the curves labeled as “2” represent the response in reverse direction.
The Tacho output in ED-4400 is an AC signal which does not discriminate the direction of the motor.
However, when the AC Tacho output is converted into DC in U-155, the input polarity is monitored and
correct polarity is assigned to the converted DC signal.
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2. Experiment procedure
1. Referring to Figure 7-2, arrange necessary modules and connect them together. Set U-152 switch to
“a”.
2. Set ATT-1 of U-151 to “10”, and ATT-2 to “6” or “7”. Adjust U-157 dial to the mid point (180 degrees).
Turn the power of U-156 on.
3. Turn the dial on U-157 to left or right from its 180 degree position and observe the motor direction.
Bring back the dial setting to 180 degrees. Stop the motor by adjusting the Zero Count of U-153. When
the motor stops, fix the Zero Count setting.
4. Turn the dial on U-157 clockwise until the motor speed reaches one half of the maximum speed.
Increase brake setting from 0, and record the motor speed at each brake setting. Insert an ammeter
between U-154 output and U-161 and record the current reading at each brake setting.
5. Turn the dial on U-157 counterclockwise until the motor speed reaches one half of the maximum
speed. Repeat experiments as described in Step 4.
6. Plot the data obtained in Steps 4 and 5.
7. Set the U-157 dial to 180 degree position. Set U-162 frequency to 0.2Hz. Reduce ATT-1 of U- 151 from
10 to 5. Observe the motor changing its direction for every 2.5 seconds.
8. Apply U-155 output to Y- input of an oscilloscope, and U-162 Ramp output to X-input of the
oscilloscope. Adjust X and Y input gains for proper display.
9. Increase the load (brake) and observe the trace on the oscilloscope. Turn U-157 slightly to left as well
as right from 180 degree position, and observe the trace on the oscilloscope. Set U-157 back to 180
degrees.
10. Set U-152 switch to “b” and repeat Steps 7 through 9. Compare the difference in waveforms on the
oscilloscope between setting “a” and setting “b”. Sketch the difference.
Note: When U-152 switch is in position “b”, the system response is delayed and may cause oscillation in
servo motion.
11. Set ATT-1 of U-151 to “7” or “8”, and change ATT-2 from “5” to “9”. Observe and sketch oscillation
pattern in servo motion.
Note: It should be observed that as delays are introduced into the system, oscillation lasts longer as
feedback is increased.
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3. Summary
• The rotational direction of a DC servo motor can be changed depending upon the polarity ofthe
control input signals.
• For a bi-directional motor, the motor speed is not the same between identical forward andreverse
direction settings of U-157 (same magnitude of input signals).
• Any delays in a servo system will slow down the system response and will cause oscillation.The
duration of oscillation depends on the magnitude of the feedback.
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Experiment 8 Motor speed control efficiency
1. Basic theory
The key elements affecting motor speed control have to do with deadband and system responsetime. So
far, previous experiments demonstrated that a higher gain has minimized the deadbandeffect, and
improved over-all system response time. In a practical system, the existence of timeconstants in the
system can add to the delay. Time delays in the error channel means the error signal can’t change fast
enough to catch the change in speed. Such a characteristic has been experimented with U-152 selector
switch set at “b”.
Closed loop motor speed characteristics can be made visual on an oscilloscope. Some of the
characteristic curves are shown in Figure 8-1. When the system gain is large, the system response is very
good as shown in Figure 8-1 (a). The error voltage in this case is significant only when the motor changes
its direction, as shown in Figure 8-1 (b). However, when the gain is not sufficient, the response of the
motor slows down with the final speed reduced than before as in Figure 8-1 (c). Also the error is
significantly increased throughout the operation period as shown in Figure 8-1 (d).
The effect of time delay in the error channel is displayed in Figure 8-2 (a) and (b). It’s clearly
demonstrated that the time delay in the error channel causes oscillation in the system. Oscillation also
occurs in the error signal.
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When a motor is mechanically loaded, by a brake in this experiments, the motor reaches the same final
speed as it would without a load, but at a slower pace as shown in Figure 8-3 (a). The corresponding
error signal is displayed in Figure 8-3 (b). When the load exceeds the rated value, it will overload the
system power supply.
Finally, some of the oscilloscope output due to an injection of electrical delay, by selecting U- 152 switch
to “b”, is shown in Figure 8-4 (a) and (b).
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2. Experiment procedure
1. Referring to Figure 8-5, arrange necessary modules and connect them together. Set U-152 switch to
“a” and do not connect the squarewave output ( ) of U-162 at this time.
2. Set ATT-1 and ATT-2 of U-151 to “0”. Set U-157 to exactly 180 degrees to make the output 0V.
3. Turn the power of U-156 on. In case the motor turns, stop the motor by adjusting U-153 Zero Adjust.
4. To measure the deadband, turn the control on U-157 from its 180 degree position to first clockwise
until the motor begins to move. Record the position, and return bck to 180 degree position. Turn U-157
control to counterclockwise this time, and find the angle where the motor begins to move. Add two
angles together. Set ATT-1 of U-151 to “9” and repeat the above procedure. Compare the difference in
deadband between two ATT-1 settings.
5. Set ATT-1 and ATT-2 to 5 respectively. Set U-157 to stop position (180 degrees). Apply the squarewave
output ( ) of U-162 to the input of U-152, and set the frequency to 0.1Hz. Connect the Ramp output to Xinput of an oscilloscope. Adjust the gains of X- and Y-inputs to see the Tacho output and the error signal
on the oscilloscope. Repeat this experiment with ATT-1 set to “0” first, then to “9”. Sketch the outputs
obtained on the oscilloscope.
6. Set U-152 switch to “b”. Measure the Tacho output and error signal at “0” as well as “9” position of
ATT-1. Sketch the output, and compare the results between the switch setting of “a” and “b”.
7. Reset U-152 switch to “a”. Set ATT-1 to “3”. With ATT-2 set to “0” first, then to “9” respectively, and
U-162 output connected as in Step 5, observe the results on the oscilloscope.
8. Set U-152 to “b”. Repeat the experiments in Step 7. Compare with the results of Step 7.
9. Set U-152 switch to “a”. Attach a flywheel to the high speed shaft of U-161. Set ATT-1 and ATT-2 to
“5” respectively. Observe the Tacho and error signal on the oscilloscope .
10. Repeat Step 9 with ATT-2 set to “9”.
11. Set U-152 switch to “b”, and repeat Steps 9 and 10.
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3. Summary
The impact on the system performance, due to the time delay in the amplifier and the load change, has
been experimented in this section. An oscilloscope with an X-Y display is used to display the
relationships between two variables more effectively. Optimum settings of the system parameters to
maintain stable and constant speed have been experimented also.
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Experiment 9 Error signals in a position controller
1. Basic theory
The basic function of an angular position controller is to provide an output angular position signal which
precisely follows the input angular position signal. The input or output position information is expressed
in terms of the selected angle around a circle.
To achieve the control function, its necessary to rotate a motor until the signal detected for the motor
position is equal to the signal representing the reference or the input position. A potentiometer is used
to convert the angular position to an equivalent electrical signal. Figure 9-1 shows a circuit diagram
which utilizes potentiometers as an angle-to-voltage converter.
The Pi in the figure is the input potentiometer, and Po the output potentiometer. The amplifier (-A) is
configured as an inverting amplifier. Due to the polarity applied to Pi and Po, when the input and output
positions are identical, the output of the amplifier becomes zero.
In general, when the angular position of Pi is θ i, and θ o is the angular position of Po. Also the relative
angular position error between Pi and Po is defined as (θ i - θ o). The converted and amplified output of
the error from the amplifier can be set to Ke (θ i - θ o), where Ke represents a conversion factor. Ke can
be determined for a given system when the actual output voltage of the amplifier is measured.
A closed loop control system can be formed when the error signal is further amplified and applied to a
motor. As the motor reacts to the incoming error signal, and also the motor is coupled to the output
potentiometer Po, the loop is closed. As the loop is closed, error detection and associated motor
reaction processes continue until the error signal is reduced to zero.
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2. Experiment procedure
1. Referring to Figure 9-2, arrange modules and a voltmeter and connect them together.
2. Set U-157 and U-158 dials to 180 degrees.
3. Turn the power of U-156 on. Set U-152 switch to “a”.
4. Measure the voltage at the rotating contacts of U-157 and U-158. In case the voltage is not zero,
adjust each dial for zero reading.
5. Measure the output of U-152. It should be zero.
6. Turn U-157 clockwise 15 degrees ( same as 195 degrees), and measure the U-152 output voltage.
Repeat the process at 5 degree increment for up to 30 degree (120 degree) position.
7. Keep U-157 as in Step 6. Turn U-158 clockwise 5 degrees each time and measure U-152 output
voltage. Make sure the U-152 output is zero when the relative position of U-157 and U-158 is identical.
Measure the contact voltage of U-157 and U-158 (P1 and P2).
Note: The voltage polarity of U-157 and U-158 is opposite each other for the same direction of rotation.
8. Repeat Steps 6 and 7 for counterclockwise rotation.
9. Plot the relationships between the positional difference and corresponding error voltage.
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3. Summary
• The output of the summing amplifier produces zero output when the two inputs are same in
magnitude but opposite in polarity (Vo = V1 + (-V2)).
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Experiment 10 Closed loop position controller
1. Basic theory
In a closed loop position controller system, the positional information from an output potentiometer (Po)
which is mechanically coupled to a motor is fed back to a control amplifier. Then, the reference position
input from the input potentiometer (Pi) is combined with the feedback signal at the input of the
amplifier which drives the motor in proportion to the difference between two signals. When the two
positions are identical, the output of the amplifier becomes zero. A simplified system diagram of a
closed loop position controller which will be used in this experiment is shown in Figure 10-1.
There are three amplifiers in Figure 10-1. The A1 is an error signal generator, A2 is an error signal
amplifier and A3 is the driver for the motor M. As Pi is turned away from Po, the difference between two
potentiometer voltages become an error signal which appears at the input of A1. The error signal is
further amplified through A2 and A3, and drives the motor in the direction to reduce the error voltage
between Pi and Po. Therefore, as Pi is turned clockwise, Po follows the same direction. This feedback
action continues until the output of A1 is reduced to zero. At this point, the voltage measured at Pi and
Po are same but in opposite polarity. For example, if Pi is at +3V, then Po is at -3V, making the sum of
two zero.
The final relative position between Pi and Po depends upon the gain of the amplifiers. For a large gain,
the position of Po can be almost equal to the position of Pi. But when the gain is not sufficient, there can
be an offset in the relative position. This offset is the “deadband” for a position controller.
2. Experiment procedure
1. Referring to Figure 10-2, arrange the modules, including coupling of U-158 to U-161, and connect
them together.
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2. Set U-152 switch to “a” and U-151 to “10”. Turn the power of U-156 on. Set U-157 dial to 180 degrees.
3. Adjust U-153 to make the output of U-154 zero. Once the adjustment is done, do not alter U-153
setting.
4. Set U-151 to “9”. Within +-20 degrees from the original 180 degree setting, turn U-157 either
clockwise or counterclockwise, and see if U-158 follows the movement. U-158 motion should lag U-157.
In case U-158 leads U-157, switch the wires of U-161 motor.
5. Turn U-157 clockwise from 0 degree position by 10 degree increment up to 150 degrees. Measure the
angle of U-158 at each position of U-157. Repeat the measurements with U-157 turned
counterclockwise. Calculate the offset error angle between U-157 and U-158 at each position.
6. Increase the system gain by setting U-151 to 7, 5, 3 and 1. At each U-151 setting, repeat Step
5 experiment. Observe the change in offset error angle as a function of the system gain.
7. Plot the results of Steps 5 and 6. Plot the relationships between system gain and deadband.
3. Summary
• Reducing the system gain worsens the deadband as well as the offset error.
• Increasing the system gain improves the system response and reduces the offset error.
• Angular resolution of Pi and Po affects the position control accuracy. To improve the resolution, a
potentiometer is required to have larger circumference and the winding is prefered to have large
number of turns.
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Experiment 11 Transient response of a position controller
1. Basic theory
When a step input is given to a position controller, the loop takes time to react to the applied input. Also,
depending upon the given system parameters, oscillation can occur at the output during the transient
time period. The major cause of the time delay comes from the added inertia of the moving parts.
Therefore, the higher the inertia, there will be more delay.
Usually, the system gain is preferred to be high to improve system response time. However, when the
gain becomes excessive, it will cause undesired overshoot at the output.
Transient response of a system can be easily observed on an oscilloscope when the system is stimulated
with a squarewave input. Such an arrangement is shown in Figure 11-1.
The function generator in Figure 11-1 provides synchronized squarewave and ramp signals. As it is
shown in the figure, the ramp signal is used to drive the X-input of an oscilloscope. When the output
voltage from Po is fed into the Y-input of the oscilloscope, transient response curves as shown in Figure
11-2 can be obtained. To get the best results, it is recommended that the frequency of the squarewave
be kept below 1 Hz.
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2. Experiment procedure
1. Referring to Figure 11-3, arrange the modules and an oscilloscope, and connect them together.
2. Set U-151 to “10”. Set U-152 to “a”. Turn the power of U-156 on.
3. While monitoring the output of U-153 with a voltmeter, adjust U-153 for zero output.
4. Set U-162 frequency to 0.2Hz. With an oscilloscope in X-Y mode, adjust the horizontal range for best
display on the screen.
5. Set U-151 to “8” and observe U-158 turning to left and right. Adjust the Y-input of the oscilloscope for
best display. Sketch the trace on the oscilloscope on a piece of paper.
6. Set U-151 to 6, 4, 2, 0 in sequence and observe impact on the trace at each time. Sketch each trace on
a piece of paper. At what gain setting oscillation appears in the trace ?
7. Set U-152 switch to “b”, and repeat Step 6. Sketch the resultant response.
8. Attach the flywheel to the high speed shaft of the servo motor. Set U-152 switch to “a”, and repeat
Step 6. Sketch the resultant response.
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3. Summary
• When the system gain is low, the rise time (tr) in Figure 11-2 is long. This means that the response of
the system is slow.
• As the system gain is increased, the response of the system improves. However, if the gain is too high,
it will cause overshoot in the response.
• When the load to a motor is an inertia type, it will slow the transient response.
• Even if there is a delay in the transmission characteristics, oscillation can still take place in the system
response.
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Experiment 12 Position control with speed feedback
1. Basic theory
When the gain is raised in a position control system to minimize the deadband effect, the closed loop
system responded with an overshoot which resulted in undesired system oscillation. One way to
mitigate oscillation is to add a brake which is proportional to the speed to the output shaft. The brake
method may produce a satisfactory result. However, it consumes a significant power and makes
acceleration of the load difficult.
Better way of preventing oscillation is to add a speed control loop to the position control loop. The
speed control loop provides a negative feedback signal from the output of the Tachogenerator which is
proportional to the speed of the motor.
The effect of adding a speed loop is illustrated in Figure 12-1 (a), (b), and (c). An optimum control of the
speed feedback loop produces a system response as shown in (b).
Figure 12-1 Effect of a speed feedback loop to the system response
An actual system with both the speed and position control loops is shown in Figure 12-2. As it can be
seen in the figure, this system is essentially the same system as experimented in the previous two
sections, except that one more loop which is consisted of the Tacho circuit and VR2 is added. To obtain
the waveforms in Figure 12-1, it is needed to replace the input potentiometer with a squarewave input
and connect Po signal to an oscilloscope.
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2. Experiment procedure
1. Referring to Figure 12-3, arrange the modules and an oscilloscope, and connect them together.
2. Set ATT-1 and ATT-2 of U-151 to “10” respectively, and set U-152 switch to “b”.
3. Turn the power of U-156 on. Using U-153 Zero Adjust, set the output of U-153 to “0”. Set an
oscilloscope to X-Y mode. Also set a Function Generator to 0.2Hz. Adjust the oscilloscope X and Y inputs
for best display.
4. Increase the system gain by changing ATT-1 from “10” toward “0” until oscillation is observed. Place
ATT-1 right before where oscillation takes place.
5. Change ATT-2 from “10” to “0”. Observe the pattern on the oscilloscope and sketch the pattern on a
piece of paper.
6. Set ATT-1 to half of the gain setting in Step 4, and repeat Step 5.
7. Set U-153 output switch to “a”, and repeat Steps 4 and 5. Compare the difference in servo time delay.
3. Summary
• A position control system without a speed control loop can generate oscillation when the system gain
is too high. Adding a speed control negative feedback loop can stabilize the system.
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Experiment 13 Stabilizing an unstable position controller
1. Basic theory
For a properly designed system, the transient response effect should gradually decay within a few
seconds, and the system should reach a steady state operation. However, for an improperly designed
system, the transient response can lead into an oscillation which can be sustained over a long period of
time. Such a system is unstable and should be corrected for a stable operation.
The instability of a system is mainly caused by either a long time constant in the system, or an excessive
gain in the system. A closed loop speed controller can mitigate oscillation up to certain extent. However,
in case a highly stable system is desired with a maximum gain, the system needs more advanced
technique than a simple speed control loop. The experiment in this section is limited to a speed
controlled stabilization method. The same experiment system as in the previous section is used for this
experiment.
2. Experiment procedure
1. Referring Figure 13-1, arrange all modules and connect between them. Make sure the coupling of U161 and U-158 shaft is straight.
2. Set ATT-1 and ATT-2 (U-151) to “10” respectively. Set U-152 switch to “b” and turn the power of U156 on. Also set the Function Generator frequency to 0.1Hz.
3. Set the Zero Adjust of U-153 so that the output of U-153 is zero. Set an oscilloscope for X-Y mode
operation.
4. Scan ATT-1 from “10” to “0”, and find a place where oscillation begins to take place in the system.
Leave ATT-1 where oscillation occurs.
5. Adjust ATT-2 to stop oscillation. Explain why oscillation has stopped.
6. Turn U-156 off. Keep U-152 switch at “b”.
7. Set both ATT-1 and ATT-2 to “10”. Remove the squarewave output of U-162 from U-152 input.
Connect U-157 output to U-152 input as indicated by the broken line in the figure. Set U- 157 to 180
degree position. Turn the power of U-156 on.
8. Turn ATT-1 of U-151 from “10” to “0”. Find a place where system begins to oscillate. Leave ATT-1
slightly before where oscillation starts.
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9. Quickly turn U-157 clockwise about 30 degrees, and observe U-158. In case U-158 oscillates, adjust U158 to eliminate oscillation.
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10. Set U-152 switch to “a” and repeat Steps 8 and 9. Compare the results.
11. Maximize the speed feedback by setting ATT-2 to “0”. Set U-152 switch to EXT. Oscillation may occur
due to excessive gain.
12. With U-152 switch left at EXT, connect a 1 MΩ variable resistor to NET terminals. Vary R and observe
the results.
3. Summary
A. Typical problems associated with a position control servo system:
• Increased position error and slow response when the gain of the error amplifier is not sufficient
• Increased position error, slow response and unstable oscillation due to excessive delays in the system
• Oscillation or vibration due to an overshoot during transient time period
• When a servo motor is loaded with an inertia type load, the system response is slow. Also, instability
occurs in the system due to the phase shift of the feedback signal.
B. Requirements for a stable position control operation:
• Optimum system gain
• Optimum speed feedback
• Avoid inertia type load
• Reduce delay parameters in the system
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Experiment 14 Construction of a practical position controller
1. Basic theory
This section is the conclusion of all the preceding experiments. A practical and working position
controller will be built in this section. The key considerations for a stable position controller are
reviewed below:
1. Avoid excessive system gain.
2. Optimum system settings between a moderate transient response and the response time.
3. Increased error due to insufficient system gain
4. Response time vs. delay parameters of the system
5. The impact to a servo motor due to inertia and torque
6. Phase relationship between feedback signal and control input signal. When these two signals are in
phase, then oscillation would occur in the system.
2. Experiment procedure
1. Referring to Figure 14-1, arrange modules and connect them together.
2. Observe the relationship between the speed feedback (ATT-2 adjust) and the response time.
3. Observe the relationship between the speed feedback and transient suppression.
4. Observe the relationship between the system gain (ATT-1 adjust) and response time.
5. Observe the relationship between the system gain and the position control error signal.
6. Turn slightly the position control input potentiometer from “0” position to either left or right. Also try
the following: either increase the amplifier gain significantly, or reduce the speed feedback to improve
the response time. Explain why oscillation tends to occur at “0” position.
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