Adjusting PID Settings - DSC2000

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Feb 17 2016

PID Settings

An Electronic PDF copy of this document can be found in the support section of the website, please see SUP0087.

Introduction

PID settings are used in Clisp Studio in conjunction with a DSC2000 to fine tune the performance of a dynamic servo, both single (DSC2000 M) and dual axis (DSC2000 MM and HM). This support document is not designed to give a full accurate account of what PID’s are and how they are calculated, but is designed to help you know how to change the PID settings on your system to improve the performance of the servo based on the control you want to achieve.

The PID controls work in the following way:

P (Proportional)

Minimum Setting – 0.000
Maximum Setting – 30.000
Increments – 0.125

“P” is best thought of as the control of how hard the servo will push; the lower the number the less the system will push; the higher the number the harder the servo will push. This is how hard you “Accelerate” towards the target.

“P” will generally be increased as the control requires a faster reaction from the servo. This could be large amplitudes, high frequencies and square wave forms.

This is has the most dramatic effect on Y Axis.

In figure 1 the feedback (red line) is under shooting the setpoint (grey line). In this case the “P” would be increased so that the feedback would get nearer to the peak and troughs of the waveform required as the servo pushes harder.

I (Integral)

Minimum Setting – 0.003
Maximum Setting – 0.021
Increments – 0.003

“I” is the accumulative error that is allowed between the set points generated by the DSC2000 to produce your required waveform and the feedback from the transducer as the waveform is played back.
This is the correction based on accumulated error from target. The higher the “I” the lower the error allowed.

This is has the most dramatic effect on the X Axis. “I” should be used with caution as it will aslo influence the “P” value.

The graph in figure 2 shows an out of phase load feedback (red line) compared to the set point (grey line); there is an error between the set point and feedback position. To bring the red line closer to the grey line and reduce the error between the set point and feedback (move the red line left on the x axis) the “I” can be increased.

D (Derivative)

Minimum Setting – 0.000
Maximum Setting – 100.00
Increments – 1.000

“D” is best thought of as the control of deceleration of the servo. The lower the number the slower the servo will slow down, the higher the number the quicker the servo will slow down. This can be considered as a deceleration and helps to prevent overshooting.

A value for “D” greater than zero is most useful on waveforms with sudden/large direction changes, i.e. Square/Triangle but has little or no effect on Sinusoidal waveforms as the change in direction is gradual.

This has a smaller effect on the Y Axis than “P”.

The graph in figure 3 shows the red load input (feedback) overshooting the grey set point line. To help stop this “D” can be increased to make the servo decelerate harder.

Suggested starting PID’s

Two Excel spreadsheets can be found in the installation folder of Clisp Studio on your computer’s hard drive. The two spreadsheet gives suggested PID values based on a number of factors including: if you are using a single axis DSC2000, Dual axis DSC2000, the action you require of the servo or servos and the stiffness of the material being tested.

The spread sheets can be found in this folder location:

C:\Program Files (x86)\VJTech\Clisp.Studio\Clisp.Instruments.DSC2000

The spreadsheets are called MultiPIDSettings.csv and SinglePIDSettings.csv.

The SinglePIDSettings spread sheet should be used in conjunction with a single Axis DSC2000 M. The MultiPIDSettings Spreadsheets should be used in conjunction with a dual axis DSC2000 (DSC2000 MM or DSC2000 HM). An explanation of the spreadsheets can be found in figure 6.

These suggested PID’s may require further adjustment based on what is seen during the test.

Changing PID Values Safely

With all PID settings it is suggested that any changes required are done is small increments (the minimum increment) to ensure that the PID’s are not set to values that may cause the servo to go out of control and damage the equipment.

Where to find the PID settings in a dynamic test

The PID is changed for each individual schedule in a test. To change the PID settings first open the menu for the stage of the test in which the schedule you want to change is found.
From the menu select the option Test Plan.

When the test plan opens select the schedule you wish to change the PID settings by clicking on the schedule name on the left side of the test plan, in the example below the stage called cyclic has been selected.

Next select the automation tab, then in the controllers list select the vertical load controller (for single axis DSC2000 M) or either the vertical or horizontal load controller (for dual axis DSC2000 MM or HM).

Lastly select the settings tab to access the PID settings for this stage.

To access the PID settings window press the advanced button, a warning message will appear. If you have any doubts on how to set the PID then contact VJ Tech (service@vjtech.co.uk) as incorrect PID settings could damage your system. Press the cancel button.