pid pick&place robot

8/7/2019 PID Pick&Place robot

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The Implementation of PID Controller in the Pick and Place

Robot

H. Ferdinando, H. Wicaksono and R. WibowoDept. of Electrical Engineering, Petra Christian University, Indonesia

Abstract A control system for pick and place robot

is implemented. The control algorithm uses the PID

(Proportional-Integral-Derivative). The system 3-DOF

robot with specific task, i.e. to pick certain a letter

blocks and places it to the desired location. The robot

is controlled with the AT89S51 microcontroller. The

command for the specific letter blocks comes from a

computer. The computer and the AT89S51 are

connected via RS-232. The time sampling of the system

is 5ms. After tuning the PIDs constants, the ratio of

the constant for P, I and D is 20:0.1:34. The

experiments show that the rise time of the system is1.41s; the settling time is 4.03s and the maximum

overshoot is 1.62%. It is recommended that the

constant for Proportional controller is between 2 and

70, the greater the constant the longer the rise time.

Big value of the constant also contributes to the

number of oscillation. For the Integral controller, it is

10 and 50, the greater the constant the more the

oscillation occurred. This makes the motor run

abruptly.

Keywords PID, pick and place robot, AT89S51

I.II. INTRODUCTION

The PID (Proportional-Integral-Derivative) controller

is the most popular control algorithm in industries. The

power of this controller lies on the simplicity of the

control algorithm. Although there are many new control

algorithms developed nowadays, this controller still exists.

There are many applications of the PID controller in

industries, e.g. robotic arm, conveyor, heating process,

etc. The goal is one, i.e. to reach the setting point fast with

small overshoot.

This paper discusses the role of the PID controller in

controlling pick and place robotic arm. The robot will pick

certain block of letter and then place it in the desired

place. The control algorithm is applied to the movementof the main body. The controller is the AT89S51

microcontroller. It receives command from computer via

RS-232. The final goal is to get system with fast rise time,

fast settling time and minimum overshoot.

This paper is organized as follow; the first section

gives introduction to the project. The PID controller is

discussed briefly on the next section. For those who do not

familiar with pick and place robot can read the following

section, then it comes the design of the system for

mechanics, hardware and software part. It continues with

experiment for the system and conclusion closes this

paper.

III. THE PID CONTROLLER BRIEF INTRODUCTIONThe PID controller combines three control algorithms,

i.e. Proportional, Integral and Derivative controllers. Each

controller has its own constant to adjust the output of the

controller based on the error signal. Each controller hasdifferent contribution to the response of the system.

To get a good PID controller system, one must tune

the constants. Although it seems independent, the

constants do not. This gives complexity in tuning them.

The tune process, however, can be done via several

algorithms such as Ziegler-Nichols, Cohen-Coen, etc [1].

Each algorithm has its own advantages and disadvantages.

The power of the PID controller lies on its simplicity.

One only set three constants in order to get good response.

It is simple for the number of parameter is only three.

Besides, it can be implemented without digital controller.

One can make the PID controller with Operational

Amplifier and several passive components. Off course this

kind of PID controller has limited capabilities, but one canuse it to control simple system.

The more complex PID controller uses digital

controller such as microcontroller, PC (personal

computer) or PLC (programmable logic controller). These

equipments give better PID controller but with higher

cost. The chosen controller is based on the complexity of

the plant.

IV. PICK AND PLACE ROBOTA pick and place robot is a material handling robot that

can work 24 hours a day without worries or fatigue [2].

This robot is a common robot in industries. The task is topick some object and place it in the desired place.

When it is controlled, then the position control is the

main issue. It must be controlled such that the robot can

pick the object at certain place accurately and place it at

the desired place accurately as well. It is the role of the

controller to achieve this goal. One can use many

algorithms to achieve it but the goal is one, i.e. to pick and

place object accurately, fast and with small overshoot.


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The application of the pick and place robot is in

assembling production process, for example. Here, the

robot must pick certain part and place it to the specific

place. The assembling process of the cassette uses this

robot.

Another application is insert machine for the electronic

printed circuit board (pcb). Here, the robot picks the

component, inserts it to the hole, cuts the lead and bendsit. The insert machine can finish the whole component in a

pcb fast, compare to the human.

V. MECHANICS DESIGNThe mechanic design is based on figure 1. The primary

and secondary arms can rotate 300o

and the gripper can

move up and down (see figure 2).

Fig. 1. Mechanic diagram of the robot

Fig. 2. How the robot move

The robot is supported by a main body. The main body

has base plate. On the base plate the letter blocks are

placed. Figure 3 shows the whole base plate from top. The

letter block is picked one by one and placed at the desired

place

Fig. 3. Base place (top view)

VI. HARDWARE DESIGNFigure 4 shows the block diagram of the system. The

system uses several additional circuits. They are discussed

in this section.

Fig. 4. Block diagram of the system

The position sensor for the system is a potentiometer.

It is a linear ten turn potentiometer. The PID controller is

implemented in a microcontroller MCS-51 family, i.e.

AT89S51 [3]. The number of position sensors is two, one

for the primary arm and the other for the secondary arm.

The gripper uses two limit switches. Figure 1 shows the

position of the potentiometer (close to the motor 1 and 2)

To read the current position, the AT89S51 needs A/D

converter. The ADC0809 is used. The signals, however,cannot be read directly for the signal range does not match

to the range of the ADC0809 [4]. For this reason, an

additional circuit is used. It is called zero and span circuit.

The arms must be able to turn CW and CCW. For this

purpose, an H-bridge is used to drive the motors. The H-

bride uses LM298N [5]. This chip can provide current up

to 3A.

AT89S51PC

ADC0809 Potentiometer1

Potentiometer2

Limit switches

DAC0808

Comparator

Triangle wave

generator

H-bridge

RS-232

2

1

3

letter

place/slot

arm

gear box

main body

motor 1

primary

armmotor 2

secondary

arm

motor 3


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The movement of the arms must be able to be

controlled. It means when the current position is far from

the desired position, then the arm must move fast, and

vice versa. For the motor is DC motor, it needs to control

the voltage drop at the motor terminal. This leads to the

PWM (Pulse Width Modulation) system.

The PWM circuit using a simple triangle wave

generator, an input reference signal and a comparator. Thecomparator will give output either HIGH or LOW

according to the status of both inputs. The input reference

signal is produced by the AT89S51 via DAC0808. For the

output of the DAC0808 is current [6], then the current to

voltage converter is used. The DAC0808 gets data from

the AT89S51. Figure 5 shows the simplified PWM circuit.

The frequency of the PWM signal is 1kHz. This frequency

depends on the frequency of the triangle wave. This value

is chosen for the motor will turn abruptly in low frequency

of PWM.

Fig. 5. Simplified PWM circuit

VII. SOFTWARE DESIGNThe software in the AT89S51 is written in C [7] for

the computation uses floating point number. To use

assembly language can lengthen the development time.

The AT89S51 receives command from a computer. User

inputs the command using the HyperTerminal. The

command is simply one letter. It means the robot must

pick that letter and place it at the pre-set place.

The robot moves its arm one by one instead of

simultaneously. This makes the implementation simple.

A. Time SamplingThe control algorithm uses PID. For this purpose, there

are three constants for the controller. It also needs a time

sampling in the computation. The time sampling is 20ms.

It is assumed that the motor has slow response for the load

is big.

The AT89S51 must be set such that it will repeat a

bunch of task every 20ms. The interrupt timer is used for

this purpose. Those tasks are reading the current position

of the primary arm, compare it with the desired position

from PC (via RS-232), calculate the PID control action

and drive the robot. When there is time remaining, then

the system is idle, waiting for the next interrupt timer.

Figure 6 shows the part of interrupt timer routine.

void InitCounter(void){

Counter=0;

EA=0; // disable all interrupts

TH0=256-18;// TL0 = 238

TMOD=0x22; // chose mode 2 setiap timer

PT0=1; // bit prioritas timer0ET0=1; // enable interrupt of timer0

TR0=1; // start timer

EA=1; // enable all interrupts

}

void Counter_Bit() interrupt 1 using 1 {

Counter++;

if (Counter==0){

SamplingPID=1;

}

}

Fig. 6. Part of interrupt timer routine

The flag SamplingPID guards the main program such

that when the time is come, then the PID process will run.As soon as the process starts, then this flag will be cleared.

This makes the main program waits until the next time

sampling.

B. PID ControllerThe constants of PID parameter could have floating

point value. For this reason, the implementation uses C

language.

To make the system simple and easy for modification, the

constants are floating point. Figure 7 shows the PID

controller implementation in the AT89S51.

void CalCulatePID(void){YN1=Y;

ErrN2=ErrN1;

ErrN1=Err;

Err=SetPoint-CurPoint;

T=0.005;

Y= YN1 + KP*(Err-ErrN1) + KI*Err*T +

((KD/T)*(Err+ErrN2-2*ErrN1));

if (Y>255) Y=255;

if (Y


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(a)

(b)

(c)

(d)

Fig. 8. PWM signal for duty cycle (a) 25% (b) 50% (c) 75% (d) 100%

The experiments show that the duty cycles do not fit to

the design. For 25%, 50%, 75% and 100%, the

implementation results 25.34%, 50.23%, 75.57% and

99.53% respectively.

The problem of those deviations is that there is small

error in the frequency of the triangle wave. If the triangle

wave is 1 kHz exactly then the duty cycle of both design

and implementation will be the same. This deviation is not

major problem since the controller will compensate it. The

problem is how to find good combination of PIDs

constants which can compensate that deviation. Goodcompensation is shown when the system can reach the

goal of the design related to rise time, settling time and

maximum overshoot.

B. PID ControllerThe initial constant for PID controller in 20ms time

sampling is 2.3, 3.5 and 0.026 for P, I and D respectively.

Figure 9 shows the result.

Fig. 9. Response of the system

Rise time, settling time and maximum overshoot are

3.36s, 32.62s and 30% respectively. This result is not

good. The rise time must be small and so must the settling

time. The maximum overshoot is too big for control

position.

To change the constant to 2.3, 70 and 0.001 for P, I

and D respectively does not give better results. This

combination makes the oscillation continue forever.Figure 10 shows it.

Fig. 10. Bad combination of PID parameters

This combination should not be used for the settling

time is at infinity, although the rise time is good. The

system becomes unstable for the Derivative constant is too

small.

C. Sampling TimeThese results indicate that the PID constants must be

tuned in order to get good performance. It is also

interesting to see how the sampling time influences the

system. It uses the best combination of the PID controller.

Time Sampling 20ms: The PID constants are 9, 8.5 and0.026 for P, I and D respectively. Figure 11 shows the

result. The orise time is 2.98s with settling time and

maximum overshoot are 6.28s and 0.9%. The result is

better than that of figure 9. But the rise time, however, is

still not satisfying. Also the settling time is considered

slow.

Kp=9 Ki=8.5 Kd=0.026

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16

Fig. 11. One of response system with sampling time 20ms

From many experiments later, it is difficult to find

good combination of PIDs parameter in order to haveTime (s)

AD

Cvalue

ADCvalue

Time (s)

Time (s)

ADCvalue


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good performance. The hypothesis is that the sampling

time is too slow. With sampling time 20ms, the system

gets control action every 20ms. This makes the system

cannot response as fast as possible. Therefore, to choose

20ms as sampling time is not good. It is necessary to use

smaller sampling time.

Time Sampling 5ms: from the previous sub-section, thenew sampling time is 5ms. 5ms is chosen for the motor is

loaded with its arm. The arm is little bit heavy. So 5ms is

enough to get better performance.

The PID constants are 15, 10 and 0.1. Figure 12 shows

the result.

Kp=15 Ki=10 Kd=0.1

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8

Fig. 12. Response of the system with sampling time 5ms; Kp=15, Ki=10and Kd=0.1

The rise time is 2.43s, the settling time is 5.82s and the

maximum overshoot is 2.43%. The result is better

compare to the previous sub-section. But it is still

important to improve its performance.

Figure 13 shows another experiment with different

combination of the PIDs parameter. The chosen

parameters are 20, 34, 0.1. The rise time of the system is

1.41s with settling time 4.03s and maximum overshoot

1.62%. The performance of the system is improved again.

Although the PIDs parameters seem independent,they do not. This makes the tuning process more difficult.

Kp=20 Ki=34 Kd=0.1

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

Fig. 13. Response of the system with sampling time 5ms; Kp=20, Ki=34

and Kd=0.1

D. PIDs Parameters ExplorationIt is necessary to explore the PIDs parameters on this

robot. The purpose of this experiment is to get some

insight about the PIDs parameters in the pick and place

robot.

One parameter will be varied while the other two

parameters are constant. The sampling time is 2ms.

Proportional Parameter: the starting point of this

experiment is the result from figure 13. The I and D

parameters are 34 and 0.1 respectively with variation in Pparameter.

Figure 14 shows the result of this variation. The

experiment shows that the smaller the value of P

parameter, the better the rise time. It is shown that the rise

time is better than Kp=20 with sampling time 5ms, it is

around 0.4s. The settling time is also improved, i.e.

around 0.5s.

0

20

40

60

80

100

120

140

0 500 1000 1500 2000 2500 3000

Fig. 14. Result of the P parameter variation (black: Kp=2, white: Kp=35,

grey: Kp=70)

Integral Parameter: the starting point of this experiment

is the result from figure 13. The P and D parameters are

20 and 0.1 respectively with variation in I parameter.

Figure 15 shows the result of this variation. From rise

time point of view, the system with big I parameter has

fast response. Small value needs longer time to reach the

setting point.

0

20

40

60

80

100

120

140

0 500 1000 1500 2000 2500 3000

Fig. 15. Result of the I parameter variation (dark grey: Ki=10, white:

Ki=20, light grey: Ki=50)

Derivative Parameter: the starting point of this

experiment is the result from figure 13. The P and Iparameters are 20 and 34 respectively with variation in D

parameter.

Figure 16 shows the result of this variation. The plot of

the response for several experiments is difficult to see.

The responses are almost the same, except that the D

parameter cannot be greater than 1. With D parameter is

equal to 1; the response has large steady-state error.

Time (s)

ADCvalu

e

Time (s)

ADCvalue

Time (ms)

ADCva

lue

Time (ms)

ADCvalue


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0

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500

Fig. 16. Result of the D parameter variation

E. Overall ResponseAfter all explorations of the PID parameter, it is

necessary to see the overall response of the system. For

the robot is positioned to pick and place a letter block, the

experiments is about the performance of the robot to pick

the letter block for each letter only.

The initial position is between 5th and 6th slots in the

base plate (see figure 3). The slot is a place where the

letter block is placed. The primary arm will move to the

target letter and the performance is measured. The

performance of the system is represented by rise time (0-

90%), settling time (5%) and maximum overshoot.

Time Sampling 20ms: Table 1 shows the summary of

this experiment with time sampling 20ms. The PID

parameters are from sub-section 7.3.1, i.e. Kp=9, Ki=8.5

and Kd=0.024.

From table 1, it shown that the further the letter, the

longer the rise time. But this is not always the case. A and

Z as the furthest letter do not have the largest rise time.

The largest rise time for the left hand side is for letter

B. If letter B is omitted, then the rise time is increasing

from letter M to A. This is the desired situation. The

anomaly of rise time for letter B could be due to the

mechanic, since the mechanic is not balance.

The maximum overshoot of the system for the left

hand side is also varied. Letter M has the largest value.

This is due to its position. To set the P parameter smaller

will solve this problem but the rest of the letter will have

bad performance.

For the settling time, there is no specific information,

except that the settling time for letters between J and D

has small value.

The rise time for the right hand side letters is wore

than the left hand side is. Here, the rise time changes

abruptly from letter to letter. The stability of the mechanicgives its contribution for this problem.

The maximum overshoot is interesting. The value is

large for the letter close to the initial point. To reduce the

P parameter value will solve this problem but the overall

performance will be bad.

The settling time for the right hand side seems random.

There is no specific pattern.

TABLE 1PERFORMANCE OF THE SYSTEM FOR EACH LETTER WITH TIME

SAMPLING 20ms

Letter

position

Rise Time

(s)

Max. Over Shoot

(%)

Settling Time

(s)

A 2.92 0.00 3.90

B 3.14 0.00 10.96

C 3.02 2.83 38.10

D 2.86 3.01 11.62

E 2.84 1.06 7.46

F 2.68 0.00 3.92

G 2.48 0.00 3.12

H 2.50 0.00 2.90

I 2.46 0.00 2.94

J 2.31 14.54 21.36

K 2.24 6.38 12.14

L 2.22 0.27 6.36

M 2.04 21.21 30.72

N 3.3 21.92 39.40

O 2.14 27.61 20.66

P 2.24 7.40 9.32Q 3.66 10.00 17.6

R 2.54 19.69 16.84

S 2.64 10.81 7.1

T 2.66 3.33 6.50

U 4.20 8.51 6.80

V 2.44 4.25 10.34

W 4.26 14.00 27.42

X 3.22 4.67 6.80

Y 3.24 7.01 15.22

Z 3.56 4.13 26.86

Time Sampling 5ms: Table 2 shows the summary of this

experiment with time sampling 5ms. The PID parameters

are from sub-section 7.3.2, figure 13, i.e. Kp=20, Ki=34and Kd=0.1

The overall performance is better than the previous

experiment. From the experiments, it was known that

sampling time 5ms has better performance than that of

with sampling time 20ms.

Here the rise time for both sides seems random, i.e.

there is no specific pattern. But the largest rise time for the

left hand side is still letter M. The rise time for the right

hand side has almost the same pattern as in the sub-section

7.5.1.

That phenomenon is also happened for the maximum

overshoot and the settling time. This makes the analysis

about mechanic instability must be considered.

IX. CONCLUSIONS The time sampling of the system determines the

performance of the system. Choosing suitable

time sampling makes the tuning process easier.

The comparison between 20ms and 5ms shows

that time sampling 5ms is better than the other.

Time (ms)

ADCvalu

e


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TABLE 2PERFORMANCE OF THE SYSTEM FOR EACH LETTER WITH TIME

SAMPLING 5ms

Letter

position

Rise

Time

(s)

Max. Over

Shoot

(%)

Settling Time

(s)

A 1.62 0.83 8.05

B 1.76 0.00 2.43

C 1.70 4.71 4.90

D 1.64 0.00 3.99

E 1.67 0.00 8.00

F 1.62 0.00 2.22

G 1.86 9.21 5.31

H 1.58 1.42 8.37

I 1.44 1.58 5.41

J 1.79 14.5 1.49

K 1.24 0.00 1.37

L 1.59 17.5 2.96

M 1.89 30.3 9.59

N 1.74 0.00 2.45O 1.46 14.8 6.46

P 2.05 3.70 3.36

Q 1.43 10.0 2.80

R 1.40 0.00 5.61

S 1.40 10.8 78.20

T 1.91 12.5 44.20

U 1.20 0.00 11.55

V 1.43 1.06 37.65

W 1.58 30.0 24.51

X 1.42 6.48 11.05

Y 1.33 8.77 10.69

Z 1.41 5.78 16.16

Different time sampling has different PIDparameter combination. So, to change the time

sampling means to tune the combination of the

PID parameters.

The mechanic gives contribution due to itsstability. Small instability can be compensated by

the controller but it is difficult to compensate the

large instability.

Large P parameter value makes the system haslarge rise time. This is interesting for many

literatures state that large P parameter value will

give small rise time [1]

The I parameter value must be large in order toget smaller settling time. I parameter contributesto the elimination of the steady-state error.

The D parameter value does not have biginfluence, except that the value must be smaller

than 1.

The combination of PID parameters for timesampling 5ms is Kp=20, Ki=34 and Kd=0.1.

REFERENCES

[1] Katsuhiko Ogata. Modern Control Engineering 5th ed. Prentice-Hall Inc.

[2] Pick and Place Robot Work Cell [http://www.robots.com/pick-and-place-robot.htm]

[3] Atmel Corporation. AT89S51 Datasheet. January 3, 2005[http://www.atmel.com/dyn/resources/prod_documents/doc2487.p

df][4] NationalCorporation.ADC0809 Datasheet. October 18, 1999

[http://www.national.com/ds.cgi/ad/adc0808.pdf]

[5] L298 Datasheet. Italy: StMicroelectronics. October 6, 2000[6] National Corporation. DAC0808 Datasheet. January 3, 2005

[http://www.national.com/ds.cgi/da/dac0808.pdf]

[7] Hartanto, Budi. Pembuatan Program C, Yogyakarta: ANDI, 2003

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