closed-loop control of a bench test drive system for...

8th Brazilian Congress on Metrology, Bento Gonçalves/RS, 2015

1

Closed-loop control of a bench test drive system for automotive shock

absorber.

Marcelo Vandresen 1 1, Cynthia B S Dutra 2

1, Michel F Almeida 3

1, Vinicius G

Sbardelotto 4 1

1 IFSC-Florianópolis;

E-mail: [email protected]

Abstract: On a bench test for automotive shock absorbers the angular velocity of the

excitation mechanism used "scotch yoke" must be constant. It was studied two signal

conditioners and two speed sensors to make the feedback control and PID.

Keywords: control, test bench, optimization.

1. INTRODUCTION

In an automotive shock absorber test is very

important to control the speed at which the shock

absorber is excited and measuring the resistive

force presented the shock absorber. Because this

resistive force depends on the speed at which the

shock compresses and tensioned. The speed

control can be accomplished by several methods

such as, for example, closed-loop control, root

locus method, frequency response method and

PID control. In this work we used the closed and

PID loop control.

The test bench is a prototype that was

developed by this same research group, it uses an

electric motor of 3 HP IV poles and 60 Hz with a

reduction of 1:5 to a pulley that is connected to

Scotch Yoke mechanism that is responsible for

the excitation of the shock absorber. To measure

the magnitudes of the shock absorber the test

bench uses the following sensors: linear position

sensors (potentiometric rule Gefran LTM-100),

force (load cell ITX czcb for 1000 kg), LVT for

speed (linear velocity transducer with a

resolution of 45 mv/inch/s) and an accelerometer

is being implemented (model ADXL345).

However, in order to simplify and reduce

costs a comparison is being made for various

speed sensors in order to just use two sensors in

the test bench, one load cell and one angular

speed sensor that through mathematical modeling

can also measure the position of the mechanism.

For this to work the angular speed must be

controlled, for this two sensors were compared,

the Hall sensor (with phonic wheel 180 teeth) and

optical switch (with phonic wheel 180 teeth).

2. METHODOLOGY

The feedback control uses the measured output

signal and compares it with the desired output

signal and takes this difference as an error signal

that is used by the controller to adjust the actuator

[1].

The PID control is always used in a closed loop

system, in order to obtain a desired response in a

particular period, on both the transitory and

permanent regime [2].

2.2. Actuator

The actuator used is the frequency variator drive

assembly WEG CFW08-Plus and an electric

motor of 3 HP IV poles and 60 Hz. The


2

frequency variator model used is the 220V with

rated current of 7.3 A. Since it is the model

"plus", it has both scalar and vector control, the

scalar control well-adjusted and not used in low

frequency can have a variation of 1 to 2% of the

rated motor speed and the vector control is

optimized in relation to torque and speed

regulation giving a variation of 0.5% of rated

motor speed [3].

2.2. Characteristics of tests

The tests are performed with the frequency

variator in the vector mode. All tests were done

with the same shock absorber "Ranch" Monroe

mark with 9 regulations (heavier regulation was

used) in the same temperature range (60 to 70 °

C). The Hall sensors model "032 906 433 B" and

optical switch model "XPI-A5" were used in a

phonic wheel 180 teeth.

2.3. Controller

For the signal treatment to be used in the

frequency inverter, boards were developed with

the "IC" (integrated circuit) LM331, which is a

frequency to voltage converter and the "IC"

MCP4725 which is a digital to analog converter.

The frequency to voltage converter (LM331)

has slow response time, around 0.1 s [6]. A

graphic was made using this converter for the

feedback loop as shown in figure 1.

Figure 1. Speed vs Time graphic, using LM331 as

feedback.

With the response time delay in the LM331

converser, the reference was lost causing

variations in the test speed, as shown in Figure 3.

For the MCP4725 converter this problem

didn’t happen because its response time is 6

microseconds [4]. Thus it was only used the

MCP4725 converter together with the micro

controller Arduino Mega 2560, which reads the

pulses of the speed sensor and through the

MCP4725 converter sends the value

corresponding feedback voltage to the frequency

converter.

3. RESULTS AND DISCUSSION

3.1. Closed-loop control

The use of feedback system is used in order to

control the angular speed, to achieve it the Hall

sensor and the optical switch was used.

To measure the angular velocity it was used

the Hall sensor model “032 906 433 B” in

conjunction with a phonic wheel of 180 teeth. A

graphic of speed vs. time is shown in figure 2.

Figure 2. Velocity vs Time graphic for the Hall sensor,

without feedback.

As can be seen in the graph of figure 2 the

sensor measured speed with a variation of ± 15

rpm without closed-loop (no feedback) with a

Ranch shock absorber in the heaviest regulation.

With the same shock absorber the test was

repeated with closed loop control, which can be

seen in figure 3.

0

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100

150

0 1 2 3 4

Ve

loci

ty (

rpm

)

Time (s)

0153045607590

0 2 4 6 8

Ve

loci

ty (

rpm

)

Time (s)


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Figure 3. Velocity vs Time graphic for the Hall sensor,

with closed loop control.

According to figure 3, with the feedback

system activated, the angular velocity oscillated

around ± 3.5 rpm. With this there is an

improvement in the system with feedback over

the system without feedback.

For the optical switch model XPI-A5

according to its datasheet the maximum speed

that can be read is 6600 rpm (25 microseconds

per pulse delay change) [5]. Angular speed tests

were made with this sensor without the feedback

activated, which can be seen in figure 4.

Figure 4. Velocity vs Time graphics for the optical

switch XPI-A5, without feedback.

As can be seen in figure 4 it was identified a

variation of ± 6 rpm for optical switch (XPI-A5)

without feedback. Also it used feedback with the

optical switch. Its graphic can be seen in figure 5.

Figure 7. Velocity vs Time graphics for the optical

swich XPI-A5, with feedback.

As can be seen in Figure 7 for the optical

switch, there was a variation of ± 3.5 rpm for the

same shock absorber and the same test conditions

for the hall sensor, but with feedback.

3.2. PID control

The PID control is composed of three variables P

(proportional) responsible for system gain and

depending on system it can cause overshoot, I

(integrator) responsible for the error cancellation

in the steady state, and D (derivative) used to

avoid the presence of oscillations and lead the

system quickly to stability [2].

To determine the parameters by the Ziegler-

Nichols method the step function is analyzed for

a plant (block diagram) of the system or using a

graphic response curve for the system. We opted

for the graphical method for the simplicity of

execution. To determine the parameters

graphically a straight line is drawn at the

inflection point of the step response curve, and

the values of the variables T and L are calculated

as shown in figure 8.

Figure 8. Ziehler-Nichols graphical method. Source:

[2].

Through the graphical method, using the

optical sensor and calculated the T and L, the

values obtained are 0.008612 for L and 0.097488

for T, the PID values calculated are shown in

table 1.

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60

80

0 1 2 3 4

Ve

loci

ty (

rpm

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Time (s)

-20

30

80

130

0 1 2 3

Ve

loci

ty (

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Table 1. T and L relation for the PID control.

PID Control Kp Ti Td

P T/L =

11,32073

∞ 0

PI 0,9*T/L =

0,00775

L/0,3 =

0,324962

0

PID 1,2*T/L =

13,58487

2L =

0,194977

0,5L = 0,

004306

With the values in Table 1 a test speed was

made with feedback and the values of P, I and D

set in the frequency variator, using the same

shock absorber in heavier regulation and using

the optical sensor. The result is shown in figure 9.

Figure 9. Velocity vs Time graphic for the optical

switch XPI-A5, with feedback and PID control.

As can be seen in Figure 10, the angular

velocity varied around ± 2.5 rpm for the optical

switch sensor using feedback and PID control.

Thus, there was an improvement of ± 1 rpm in

angular velocity. There is a suspicion of

mechanical clearance between the shaft pulley

and the axis bearing, for this reason a test without

shock absorber was made, see the figure 10.

Figure 10. Velocity vs Time graphic for the optical

switch XPI-A5, without shock absorber.

According to the result it was identified a

variation of the angular velocity of ± 1.5 rpm, so

there is a great possibility of mechanical

clearance or error during the measurement of

speed.

4. FINAL CONSIDERATIONS

The objective of this project was achieved, the

angular speed of the shock absorber excitation

mechanism was controlled. It was achieved an

angular speed variation of around ± 2.5 rpm

instead of ± 6 rpm, which represents only 41%

variation of the initial condition. Also a problem

of mechanical clearance might have been found

that would cause variations in the measurement

of the angular velocity. The Hall sensor

underperformed the optical switch, one of the

contributing factors can be mechanical clearance

as the phonic wheel approached and moved away

from the sensor at about 0.5 mm. It’s being

studied an implementation of another excitation

mechanism for the shock absorber, another

electric motor will be add together with another

scotch yoke mechanism, this is in order to excite

the shock absorber at both ends and then stay in

accordance to the ABNT NBR 13308 norm.

REFERENCES

[1] Dorf, Richard C., Bishop, Robert H., “Sistemas de Controle

Modernos”, 11th edition, LTC 2011.

[2] Ogata, Katsuhiko, “Engenharia de Controle Moderno”, 5th

edition. São Paulo: Pearson Prentice Hall, 2010.

[3] Weg. “Manual do inversor de frequência”. Accessed May 7, 2015. http://ecatalog.weg.net/files/wegnet/1-577.pdf.

[4] Microship. “12-Bit Digital-to-Analog Converter with

EEPROM”. Accessed May 7, 2015.

https://www.sparkfun.com/datasheets/BreakoutBoards/MCP4725.pdf.

[5] “Photo-Interrupter XPI-A5”. Accessed May 7, 2015.

http://www.digchip.com/datasheets/parts/datasheet/463/XPI-A5-pdf.php.

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