International Conference on Advances in Electrical and Electronics Engineering (ICAEE'2011)
Modulated Laser Signal Sensing in Bending
Guard Control System for Press Brake CNC
Machine
Kyaw Thu Htet, Maung Maung Latt, Zaw Min Naing and Hla Myo Tun
injuries still exists. Because of this exposure, the operators
must be safeguard from point of operation hazards. Best safety
practice is when operators are never required to put their
hands or a part of their body in the point of operation at any
time. Sometimes as the work piece bends up, a hazard may be
created between the part and the front face of the slide. When
this happens, be sure the operators do not place their fingers
on the top or sides of the material, but support the material
from underneath.
The productivity of a Press Brake is determined by the
cycle time to complete a stroke which determines the number
of bends per hour or productivity rate of the machine. Short
cycle times require maximum fast speed operation thereby
minimizing slow speed operation. Cycle time is dependent
upon following factors.
1) Descent speed of the ram
2) Back-up speed of ram
3) Back-up height
4) Mute point setting
5) Controller functionality
6) Safety guarding system employed
This laser bending guard system is designed to provide
minimum impact on productivity while at the same time
providing maximum operator safety protection. [2]
Laser-based press brake guarding system uses a double row
of light bands set 10mm apart vertically to give a 10mm
deceleration zone for the top tool from fast approach to
pressing speed. Below this zone the required bending speed is
maintained, but above this zone a much higher rapid approach
speed can be obtained.
Three laser beams are paralleled with ram and beneath the
upper tool. The first one is parts 10mm from upper beam. The
next one is place 15mm front of first one and the last one lay
at least 5mm under the first one, though it has manually
adjustable according to the thickness of the work piece.
The upper two beams are to detect the obstacles and lower
one is to detect the work piece. If the laser beams encounter
any obstacle the machine stops the downward movement of
the ram and reverses a few inches upward. Once the
obstruction is removed, the operator can continue the bending
process by pressing the foot-switch again.
This process guarantees maximum hand and finger safety
within the area of the three moving laser light beams. During
Abstract—This paper discusses frequency sensing in modulated
laser signal which used in bending guard control system for press
brake CNC machine. Laser beam has suitable propagation angle to
detect any obstacles (flat work pieces and hand or parts of operators)
in bending process. Prevention of auxiliary laser sources and effect
of signal transmission noises, we used the method of modulation
security signal in process. However to far away from missing with
one signal to another of itself, the modulated signals have in separate
frequencies. Each of three laser transmitters feed to the laser receiver
with different data frequencies in the same modulating frequency.
Photo diode detects the laser signal and demodulator circuit purifies
data signal from the modulating frequency. PIC 16F628 is used to
detect the incoming signals which feed from laser receiver and
controlled the bending process according to the received signals.
Keywords—Hand security control, Operator’s safety control,
Laser transmitter, Laser receiver, Bend Guard.
I. INTRODUCTION
P
RESS brakes are one of the most difficult machine tools to
safeguard because of their general purpose-type use in
industry. General purpose means a press brake can be used to
bens from very small parts to very large parts. Production runs
on these machines can be one part to thousands of parts. With
this type of operation, press brakes must be versatile.
There are three basic categories of press brakes:
1) Part-revolution
-Mechanical-Friction Clutch
-Air Clutch (single- or Two- speed)
2) Hydraulic (Down acting and Up acting)
3) Hydra-Mechanical
There are servo-drive electric press brakes that can be
safeguarded. [1]
Most press brake operations do not require the operators to
place their hands or any part of their body into the point-ofoperation hazard; however, exposure to point-of-operation
Kyaw Thu Htet is with the Mandalay Technological University,
Mandalay, Myanmar (phone: 95-09-2300401; fax: 95-02-57360; e-mail:
kyawthuhtet99@ gmail.com).
Dr. Maung Maung Latt was with Moscow Engineering Physics Institute,
Moscow, Russia. He is now with the Pro-rector, Meiktila Technological
University, Meiktila, Myanmar (e-mail: mgmglatt2020@gmail. com).
Dr. Zaw Min Naing is with the Pro-rector of Maubin Technological
University, Myanmar, on leave from the Nanyang Technological University,
Singapore (e-mail: zawminnaing@gmail.com).
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International Conference on Advances in Electrical and Electronics Engineering (ICAEE'2011)
bending of boxes or tubes, in the "nesting mode", the outer
laser beam can be switched off. Therefore, forming
perpendicular flanges is no longer a problem, and operator
safety is still assured by the middle beam.
38 kHz frequency. It has a wide operating voltage range and
even provides visual indication of the output. [3]
Tx = Transmitter Rx = receiver
Fig. 1 Positional diagram of laser transmitter and receiver
Fig.5 Carrier oscillator circuit using 555-timer IC
To obtain three data signals, we used the NAND gate
astable multivibrator circuits. The timing cycle is determined
by the time constant of the resistor-capacitor, RC network.
Then the output frequency can be varied by changing the
value(s) of the resistors and capacitor in the circuit. [4]
L1, L2, L3 =1st, 2nd and 3rd Laser beam
Fig. 2 Positional diagram of laser beams in Bending guard system
II. BENDING GUARD CONTROL SYSTEM
There have three pairs of laser transmitter and receiver
which have specified modulated data signals. For that, we use
the On-off Keying also known binary pulse frequency
modulation method. We specified the 38 kHz of modulation
frequency and three data signals are 1 kHz, 1.5 kHz and
2 kHz. Micro-controller senses the signals which are detected
and demodulated from laser receivers.
Laser
transmitter 1
Laser beam
Laser
transmitter 2
Laser beam
Laser
transmitter 3
Laser beam
Fig. 6 NAND gate astable multivibrator circuit
Laser receiver 1
Data signal 1
Laser receiver 2
Micro-controller
On-off keying method is sensing of carrier frequency.
Carrier frequency present is defined as bit 1 and when absent
of carrier frequency well known as the bit 0. According to this
process, on time and off time duration of clock pulse in data
signal represent the binary bit 1 and 0 respectively. A NAND
gate IC, CD4011, act as a modulator IC in On-off keying
modulation process. According to the on time of both input
signals of the gate IC is represent to output high and which of
one is absent, the output of gate IC means off condition. [5]
The modulated signal feed to the input of laser driver and
the laser driver drive the laser diode according to the
modulated signal.
Control signal
to CNC
Data signal 3
Laser receiver 3
Fig. 3 System block diagram of laser bending guard system
A. Modulated Laser Transmitter
Laser Transmitter blockdiagram
Carrier
oscillator
Data signal
generator
Modulator
Laser
driver
Laser
diode
B. Demodulator (Laser Receiver)
In sensing of laser wave, the wavelength of transmitted
wave is radiated in coherent to the laser receiver. According to
transmitting laser light’s wavelength, about 650 nm, we used
the laser photo diode DTD – 15 (Everlight) [6]. It produces
the response signal according to the laser light. The photo
diode’s output signal is too small to detect from the
demodulator circuit. CA3140E is a high frequency operational
amplifier and useful for amplification of photo diode signal.
Advantages of signal to noise ratio and bandwidth of the
Laser
beam
Fig. 4 Laser transmitter block diagram
There are several ways to generate clock pulses. A square
wave generator is one of those pieces of equipment that has
many uses in the workshop yet few hobbyists actually have.
This simple kit, based on the popular 555 timer IC, generates
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International Conference on Advances in Electrical and Electronics Engineering (ICAEE'2011)
amplifier is help to demodulation and sensing the laser
modulated data signal frequency. [7]
have a problem how long times wait to found the rising edge.
PulsIn command takes 131.1 msec to be timeout period. That
is too long for our condition. So, we use time limitation
depend on frequency of one cycle time. When the signal is
being 0 that time over a cycle of frequency, we defined that is
absent of input signal. And also we defined when the counted
value is not in the range of ± 5% of the definite value, that is
absent of input signal. [10]
Laser receiver blockdiagram
Laser
beam
Photo
diode
Data
Amplifier
Demodulator
signal
Fig. 7 Laser receiver block diagram
0 start
Op-amp’s output signal is passed through the differentiating
RC circuit to reduce the amplification and dc supply noise. It
has demodulated with a high frequency diode and RC time
delay circuit. The output of the demodulator circuit is sampled
with the TTL compatible circuit. The output can feed to PIC
to sense the frequency of the input data signal.
1 start
1 kHz
1ms
1.5 kHz
C. Micro-controller Unit
The system needs three frequency input pins and one
interrupt access input pin. There need 4 input pins and 8
output pins. External interrupt function is used for box
bending selector switch. PIC 16F628A is a flash-based 8-bit
CMOS micro controller and a middle range of microcontrollers. It has enough I/O pin and special function for this
work. [8]
Pin 1, RA2 used for 3rd laser beam indicator LED
Pin 2, RA3 used for blocked and paused system indicator
LED
Pin 6, RB0 used for box-bending function selector switch
Pin 7, RB1 used for input of 1 kHz signal
Pin 8, RB2 used for input of 1.5 kHz signal
Pin 9, RB3 used for input of 2 kHz signal
Pin 10, RB4 used for control signal output to control CNC
machine
Pin 11, RB5 used for box function on/off indicator LED
Pin 13, RB7 used for buzzer alarm control
Pin 15 & pin 16 are external oscillator input and output
Pin 17, RA0 used for 1st laser beam indicator LED
Pin 18, RA1 used for 2nd laser beam indicator LED
Box bending function is initially off condition and when
press the push button first time, the function is on and the next
time of pressing is off the function.
We write the instructions of the program used in PIC Basic
Pro programming. We used the modulated laser signal to
prevent the external sources of laser. Detection of input signal
is right or wrong is handled by control program. We used the
sensing of signal frequency is as follow. [9]
In PIC Basic Pro, PulsIn command is counted in a variable
of the continuous 1 or 0 condition. We can calculate on time
of pulse and off time of pulse in value of the counting
variable. We sensed the continuous 1 and 0 twice, and then
divided by two because we want to be definite. The sampling
is started from 1 is not a problem; we can wait till the falling
edge and sense the next 0, 1, 0, 1 in series, that mean two
cycles of signal frequency. When sensing is started from 0, we
0.67 ms
2 kHz
0.5
ms
Fig. 8 Time duration one cycle for each frequency and 1 or 0
starting condition
Box-bending function selector switch is a push button
switch, pressing that button is being the external interrupt
function. So the program called to the interrupt subprogram
function, indicator LED on/off and sound signal beat.
III. ALGORITHM FOR PROCESSING METHOD
By the program flowchart, the micro-controller has three
functions. It senses the incoming three signals which of each
have in predetermined frequency range. And then, determine
the safety condition of the operators. After that, it produce the
output depending on the condition of safety zone for
monitoring.
At first the system is waiting to get the signal from the
lowest laser beam (L 1 signal shown in Figure 2), after that
upper inner signal (L 2 signal) and then upper outer signal (L 3
signal). That need for ready to use condition, if one of that is
not sensed, the condition is not ready to use and make align
the transmitter and receiver. When the system is ready to use,
the system is sensed the first laser signal which is detected the
object of workpiece. After the workpiece is passed through
the first laser beam, system is checked out the second and
third laser beams for safety. Second laser beam is absent, the
condition is in unsafe and the third laser beam is absent, may
be the condition in box-bending function or unsafe condition.
We used box-bending selector switch is the input from B0
pin because of external interrupt special function. The system
know the sense of the box-bending function switch on/off at
any time of the process, because when the interrupt is come in
from the control pin, the program pause at reach and go to
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International Conference on Advances in Electrical and Electronics Engineering (ICAEE'2011)
work to the interrupt subprogram. The interrupt subprogram is
enough for executing the represented value changes and
produces a sound buzz and monitoring LED depend on the
condition of on or off. The box-bending function is started in
normally closed condition.
Start
Assign var:
A < 100 ?
Start
Delay 10µs
A+1
Yes
Read Pin B =0?
Decleration
0
1
Read Pin B =1?
Call subprog;
Check pin B3
Read Pin B =0?
0
K
1
Call subprog;
Check pin A0
YES
Flag 2 = 1?
Read Pin B =1?
1
Time delay
Cnt + 1
0
Time delay
Cnt + 1
0
Read Pin B =0?
1
Box On ?
NO
Flag 3 = 1?
0
Pause
machine
Cnt / 2
1
Call subprog;
Check pin A1
Flag 3 = 1?
1
Time delay
Cnt + 1
Box On ?
Call subprog;
Check pin A1
1
YES
0
1
NO
0
Time delay
Cnt + 1
0
No
Flag 1 = 1?
1
Goto Start for
next bend
No
Signal = 0
Defined Cnt ?
0
Yes
End
Signal = 1
Call subprog;
Check pin B3
return
Flag 1 = 1
& A=0?
1,1
A=Flag 1
Fig. 10 System flowchart of sensing signal (a)
A 0, Flag 1
Call subprog;
Check pin A0
Flag 2 = 0?
Start
0
Assign var:
Pause
machine
1
K
Read Pin B =1?
Fig. 9 System flowchart of Bending Guard control system
1
Time delay
0
When sensing the signal’s frequency, the PIC Basic Pro
command PulsIn has a function of counting the time period of
on time or off time of the signal. The important thing is the
input signal need to clear from noises, because that may be
getting the error results of measuring frequency. When the
system is started to sense the frequency of the signal, that may
be in two conditions, the first is being the signal of off time (0
interval) and the signal of on time (1 interval). Sensing
process is waited till the next start of condition because of
sure the sampling value and to neglect the lost value of time
interval.
When sensing is started from 0, the system does not want to
wait till the overflow time of the PulsIn command (131.1
msec), so time checking is defined 1 msec by doing looping as
shown in Figure 10 (a). It has a good response time for the
absence of the sensing signal and that has enough time
intervals for sensing for each signal. After that, counted the on
time interval and off time interval in twice to sure the amount
of interval time. When the counting is finished, the amount of
count value is divided by two to get only one time-duration.
Read Pin B =0?
0
Time delay
Cnt + 1
1
Time delay
Cnt + 1
0
Time delay
Cnt + 1
1
Time delay
Cnt + 1
1
Read Pin B =1?
0
Read Pin B =0?
1
Read Pin B =1?
0
Cnt / 2
Signal = 0
No
Defined Cnt ?
Yes
Signal = 1
return
Fig. 10 System flowchart of sensing signal (b)
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International Conference on Advances in Electrical and Electronics Engineering (ICAEE'2011)
In Figure 10 (b), the system is started to sampling the signal
from on time. The signal is already started from 1 and it is just
only need to know the frequency. So, it waited to the end of
on time interval and after that is counting time intervals and
divided by two to get the one time-period of the signal.
The main purpose is to detect the signals which sense the
operator’s safety. These signals are feed to the microcontroller’s input pins respectively. The micro-controller
detected the all signals and it determine in which stage need to
produce the control signal to pause the press brake machine.
IV. RESULTS AND DISCUSSION
V. CONCLUSION
The laser sensing system is started testing with an output of
frequency generator to detect the pulses in frequency. When
the result is sure for the showing of the input function
generator and the allow pass signal, the next step is sensing
from the laser receiver demodulator. The demodulator output
is shown in Figure 11, the oscilloscope’s channel 2 prove is
connected with the output of the receiver and the channel 1 is
measured to the laser photo diode’s op-amplifier output.
In this project, the laser base transmitter circuit is accurate
in specified frequency modulation. The outputs of the laser
transmitters are modulated according to different frequency of
data signal though the carrier frequency is in common. The
laser receiver can sense the laser beam and amplified in clear
condition. The demodulator circuit worked for acceptable
output frequency signal generation. Testing for sensing signal
has in result is remarkable.
Though most of the frequency meters use the method of
counting the rising edges or falling edges of the sensing signal
in the defined time interval, here, use the method of counting
time periods of the on time and off time of the sampling
signal. In the result, this method has the best response time
and gives the exact value of frequency. So, the result of the
bending guard control system has fast response time for
enough to control the CNC machine in defined safety zone.
ACKNOWLEDGMENT
The The author wishes to acknowledge Prof. Dr. Zaw Min
Naing, Pro-Rector, Technological University (Maubin), and
Dr. Maung Maung Latt, Pro-Rector of Technological
University (Meiktila), for their kind permissions, providing
encouragements and giving helpful advices and comments.
Special thanks are also owed to Dr. Hla Myo Tun, Associate
Professor, Head of Department of Electronic Engineering,
Mandalay Technological University, for thoroughly proofreading these paper and giving useful remarks on it. The
authors gratefully acknowledge his lovely friends and many
colleagues at Mandalay Technological University who have
contributed to the development of this paper.
Fig. 11 Output waveforms in laser receiver circuit
Second step is simulating test for three inputs signals and
the interrupt function, as shown in Figure 12. The inputs are
entered from the pulse generator sources.
REFERENCES
[1]
[2]
[3]
[4]
[5]
Machine manual and user’s guide, TRUMPF V130 CNC machine
Bend-Guard for TRUMA BEND (optional part lists)
Guru, “555 Timer and its Applications,” PEDC2DC, Feb. 19, 2006.
http://www.electronics-tutorials/ws/index.html
J. J. Abbott, “Digital Emulation of Pulse Frequency Modulation for
Neuroprosthetic Sensory Feedback”, Member, IEEE, and Sanford G.
Meek, VOL. 15, NO. 1, MARCH 2007
[6] http://www.eetimes.com/design
[7] G. Lochead, “Photodiode amplifiers; application handbook,” Mc GrawHill, second edition, 1996.
[8] http://www.microchip.com , data sheet of PIC 16F628A
[9] Know it all; PIC Micro-controllers, Newnes, 2008.
[10] Programing in PIC Basic Pro, manual book
Fig. 12 Simulation test program for frequency sensing circuit
The final testing is with the PIC based control unit and
three modulated laser input signals.
Fig. 12 Testing for frequency sensing circuit
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