Electrical Engineering Technology
SUNY College of Technology, Alfred, NY
Embedded Controller Applications ELET 3144
Spring Semester 2008
Introduction to Servos
The servo motor is a specialized motor that may be commanded to turn to a specific position. The servo motor
consists of a DC motor, a gear reduction unit, a shaft position sensor, and an electronic circuit that controls the
motor’s operation. The next figure shows a typical servo motor used or RC cars or small mobile robots.
Fig. 1 A typical hobby servo motor
1.- Unmodified (closed-loop servos)
Unmodified servos are closed loop in the sense that there is a feedback mechanism that allows the servo to move
to a specific angle. Servo motors use a standard three-wire interface consisting of a power, ground, and control
line. The power supply is typically 5-6 volts. The control line expects a periodic control pulse (pulse train) no
faster than 20 milliseconds. This means that you should not send pulses faster than 20 ms. Pulses slower than 20
ms will work, but you will not get the optimal response out of the servo.
Fig. 2 Typical control signal for a servo motor
Pulses that are far too slow may cause your robot to seem awkward and jerky. Rotation position of the servo
depends on the pulse with (time the pulse is high or near +5V). This is called Pulse Coded Modulation (PCM) or
Servo Pulse Width Modulation (Servo PWM). The following figure shows how the closed-loop servos work.
Fig. 3 Control signals to position the servo in a specific position
Depending on the manufacturer, the range of the pulse width to drive the servo varies. In general, typical values
are in the ranges: 0.9 ms to 1.5 ms to move to the left, 1.5 ms in the center, 1.5 ms to 2.1 ms to move to the
right.
2.- Modified (open-loop servos)
Modified servos are modified so that they can turn freely forever. There is no way of determining how far they
have traveled, or making them only to turn to a certain angle since the feedback mechanism has been removed.
One main advantage of this type of servos is that it can be used to drive a robot wheel continuously while
controlling both speed and direction. Open-loop servos are similar to closed-loop in that they turn one direction
or the other depending on the pulse that are given.
Open-loop servos expect a periodic control pulse of 20 ms just as close-loop servos. The main difference is that
the feedback mechanism stopping the servo from “moving to far” has been removed
The speed of the servo depends on how much more or how much less the pulse with is compared to a “STOP”
pulse. Here the STOP pulse stops the servo from moving rather that moving to the CENTER position. By
varying the pulse widths, you can vary the speed. The following figure shows how the open-loop servos work.
Fig. 3 Control signals to rotate the servo in a specific direction
If a servo is driving the wheel of a mobile robot, you can measure the rotational speed or get the manufacturer
rated rotational speed. Using this rotational speed and the diameter of the wheel, you can calculate the distance
the robot travels
Distance S = π ω D
where
S is the distance traveled in one minute (cm)
D is diameter of the wheel (cm)
ω is the angular velocity of the servo measured in Revolutions Per Minute (RPM)
Using a Modified Servomotor to control a mobile robot using the 68HC11
Most hobby servo motors use a standard three-wire interface consisting of a power, ground , and control line.
The power supply is typically 5-6 volts. The control signal uses a train of pulses, and depending on the length of
the positive-going pulse, will be the speed and direction of the motor. Typical pulse width values to control the
motor in clockwise (CW) and counterclockwise(CCW) directions are given in Fig. 4.
Fig. 4 Typical Servo Motor Pulse Width Position Signals
To generate the waveforms required to control the servo motor using the 68HC11, the Output Compare Register
and the Free Running Counter (TCNT) available in the 68HC11 can be used. The following programs show how
to control a servo motor using the 68HC11.
Program 1. Driving the servo motor in clockwise direction.
; Using the OC2 (PA6) to generate a train of pulses to drive a servomotor clockwise (CW)
; The period of the signal is around 17 msec
; The length of the positive-going control pulse is around 2msec (2000us)
TCTL1
EQU
$1020
TFLG1
EQU
$1023
TOC2
EQU
$1018
ORG
$2000
LDS
$3000
LDX
#TFLG1
AGAIN
LDAA
#%10000000
; Set OM2=1 and 0L2=0 in TCTL1 to clear OC2(PA6) on successful
compare
STAA
TCTL1
LDAA
#%01000000
; Reset OC2F in TFLG1
STAA
0,X
LOOP1
BRCLR
0,X $40 LOOP1
; Wait until OC2F sets and toggle the output
; BRCLR performs a logical AND of the memory location specified
; and the mask supplied with the instruction, and branches if
; the result is zero
; Set the lenght of the zero level control pulse to 15 msec
LDD
TOC2
; Get present TOC2 value
ADDD
#30000
; Add 300000 and store it back (30000 x 0.5 us = 15 msec)
STD
TOC2
LDAA
#%11000000
; Set OM2=1 and 0L2=1 in TCTL1 to set OC2(PA6) on successful
compare
STAA
TCTL1
LDAA
#%01000000
; Reset OC2F in TFLG1
STAA
0,X
LOOP2
BRCLR
0,X $40 LOOP2
; Wait until OC2F sets and toggle the output
; Set the lenght of the positive-going control pulse to 2 msec
LDD
TOC2
; Get present TOC2 value
ADDD
#4000
; Add 4000 and store it back (4000 X 0.5 us = 2 ms)
STD
TOC2
; The total duration of the signal is: Duration in zero level + Positive-going ("1")=15ms+2ms=17ms
BRA
AGAIN
Program 2. Driving the servo motor in counterclockwise direction.
Using the OC2 (PA6)
; The period of the
; The length of the
TCTL1
EQU
TFLG1
EQU
TOC2
EQU
ORG
LDS
LDX
AGAIN
LDAA
compare
STAA
LDAA
STAA
LOOP1
BRCLR
to generate a train of pulses to drive a servomotor counterclockwise (CCW)
signal is around 16 msec
positive-going control pulse is around 1 msec (1000us)
$1020
$1023
$1018
$2000
$3000
#TFLG1
#%10000000
; Set OM2=1 and 0L2=0 in TCTL1 to clear OC2(PA6) on successful
TCTL1
#%01000000
0,X
0,X $40 LOOP1
; Reset OC2F in TFLG1
; Wait until OC2F sets and toggle the output
; BRCLR performs a logical AND of the memory location specified
; and the mask supplied with the instruction, and branches if
; the result is zero
; Set the lenght of the zero level control pulse to 15 msec
LDD
TOC2
; Get present TOC2 value
ADDD
#30000
; Add 300000 and store it back (30000 x 0.5 us = 15 msec)
STD
TOC2
LDAA
#%11000000
; Set OM2=1 and 0L2=1 in TCTL1 to set OC2(PA6) on successful
compare
STAA
TCTL1
LDAA
#%01000000
; Reset OC2F in TFLG1
STAA
0,X
LOOP2
BRCLR
0,X $40 LOOP2
; Wait until OC2F sets and toggle the output
; Set the lenght of the positive-going control pulse to 1 msec
LDD
TOC2
; Get present TOC2 value
ADDD
#2000
; Add 2000 and store it back (2000 X 0.5 us = 1 ms)
STD
TOC2
; The total duration of the signal is: Duration in zero level + Positive-going ("1")=15ms+1ms=16ms
BRA
AGAIN