Setting up a Program with PID parameters and Analog Signals

BHazen · Dec 29, 2011

I am starting to write a program for a machine that needs to control the thickness of synthetic deck boards as they come out of an extruding machine. Using laser distance sensors (2 sets) and analog values, I need to control the speed of the pulling motors on both ends of the line, to keep the thickness of the boards within tolerance. I am looking for info on setting up PID parameters in RS Logix 500 and FactoryTalk ME. I am using AB 1400 Micro w/Analog I/O built in and a Panelview Plus 600. Any info on this would be greatly appreciated.

Lancie1 · Dec 29, 2011

For the RSLogix500 PID instruciton, you need a memory area of 23 words:

Control Block (start address) N7:9
Process Variable (analog input) I:9.0
Control Variable N7:33
Control Block Length (words) 23

You may need to control the PID Control Block Word 0, Bit 1. That is the bit that sets the Auto or Manual Mode for the PID instruction. In manual, the Control Variable is not calculated, so you have to move the Setpoint to the Control Variable manually (if you need to use the manual mode). Manual is often needed for start-up or upset conditions.

Control Word 0, bits 7 (DA - Derivative Rate Action bit) and 4 (RG - Reset and Gain Rate Enhancement bit)apply only to the SLC 5/03, SLC 5/04, and SLC 5/05 processors and to the MicroLogix 1500 controller.
(MicroLogix 1200 and 1500) In Relay ladder logic, access to the high word (Control words 18 and 20 are only allowed through a double word access of the low word.

When using the MicroLogix 1500 controller the following word/bit associations apply to the PD file layout. . Sub-elements 0 - 16, 21, and 22 are addressable by word and/or bit through relay ladder logic. Sub-elements 17 and 19 are addressable by double word and/or bit through relay ladder logic.

Control Block Layout:
_Bit #:_15_14_13_12_11_10__9__8__7__6___5__4__3___2__1__0
Word 0:EN____DN_PV_SP_LL_UL_DB_DA_TF_SC_RG_OL_CM_AM_TM
Word 1 PID Sub Error Code (MSB)
Word 2 Setpoint SP
Word 3 Gain Kc
Word 4 Reset Ti
Word 5 Rate Td
Word 6 Feed Forward Bias
Word 7 Setpoint Maximum (Smax)
Word 8 Setpoint Minimum (Smin)
Word 9 Deadband
Word 10 INTERNAL USE - DO NOT CHANGE!!
Word 11 Output Max
Word 12 Output Min
Word 13 Loop Update
Word 14 Scaled Process Variable
Word 15 Scaled Error SE
Word 16 Output CV% (0-100%)
Word 17 MSW Integral Sum 5/03/Micro 1500 MSW Integral Sum
Word 18 LSW Integral Sum 5/03/Micro 1500 LSW Integral Sum
Word 19 Altered Derivative Term (Low word)
Word 20 Altered Derivative Term (High word)
Word 21 Time of Last Update
Word 22 Setpoint Old Value

jdbeal · Dec 29, 2011

Are you sure that speed is what you want to be controlling? You may get a lot further controlling the torque of the motors instead.

MichaelG · Dec 29, 2011

speed or torque control

If controlling torque and the temperature or product properties change so that it is easier (harder) to stretch what do you do?

With speed same question?

Personally this looks like a very strong contender for a model based controller (Peter may help)

you are measuring the thickness after (read time delay) you have created the correct thickness and there are multiple variables in the system - Temperature, Gap between rollers, product properties (eg density), speed of extrusion etc.

Peter Nachtwey · Dec 29, 2011

MichaelG said:
speed or torque control

If controlling torque and the temperature or product properties change so that it is easier (harder) to stretch what do you do?

With speed same question?

You have a speed or torque limit algorithm but I don't think that will help.

Personally this looks like a very strong contender for a model based controller (Peter may help)

I would have three of PIDs. I would gear the out feed motor to the in feed motor but the gear ratio would be variable. The gear ratio would be controlled by the thickness PID which would increase or decrease the gear ratio so the proper thickness is maintained.

As MichaelG noted, where the thickness is measured will be critical.

This sounds like a big project for a Micro Logix.

Setting up a Program with PID parameters and Analog Signals

BHazen

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Lancie1

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jdbeal

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MichaelG

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Peter Nachtwey

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