Software Control Systems

An Introduction

by Dayle Kotturi (dayle@slac.stanford.edu)

Contols Department

SLAC National Accelerator Laboratory

June 25, 2009

Software Control SystemsOur scope today:What are control systems? Control systems you may knowCreating a new control systemHow could it fail?How could it run forever?Making friends with usersWhat else is there?

What are control systems?

A control system manages devices so that they respond to the commands of the operators within the limits of safetyA control system should be able to run foreverA control system should be smart enough to detect problems and to recover from them

Control systems you may know

Very small: pacemaker, hearing aidSmall: coffee maker, microwave oven, vending machine, ATM, carMedium: building’s heating ventilation air conditioning (HVAC) systemLarge: factory assembly line Very large: telescope arrays

Creating a new control system

The situation:It is your first day on the job for LCLS projectYou are excited because the LCLS photon (X-ray) beam is a billion times brighter than any other terrestrial light source and you are now a part of it

 

Description of the problem

LCLS uses screens:to image, attenuate and measure the energy of the X-Ray beammade of boron carbide and other compounds to handle high peak power levelsthat get damaged by burns from the intense radiation levelsthat contain holes typically the size of a pin hole that need to be moved so the beam interacts with an undamaged portionthat are imaged by cameras to determine the location of the beam and defects For simplicity in this example, the screen translates in one axis only

Your task

Write a driver that accepts screen position requests and moves screen in responseYour driver should run forever. I.e. wait for input, move screen to that position, repeat Use function calls from the application programmer interface (API) that came with the hardwareYou read the manual and found a function called setPosition(x) You see that the units are in microns

Your first control program

Where to start?

Drawing a blank, you decide to

Start with the world’s most common computer program, helloWorld.c. In C programming language, it looks like this:

#include <stdio.h>

int main() {

printf(“hello world\n”);

}

And make it run forever

#include <stdio.h>#define TRUE (1)#define FALSE (0)int main() { while (TRUE) { printf(“hello world\n”); }}

Running it hangs computer

#include <stdio.h>#define TRUE (1)#define FALSE (0)int main() { while (TRUE) { printf(“hello world\n”); }}Why?

100% CPU usage is too much

That tiny snippet will take 100% of the CPU! Fix it this way:#include <stdio.h>#define TRUE (1)#define FALSE (0)int main() { while (TRUE) { printf(“hello world\n”); sleep(1); /* let CPU do other tasks */ }}

Better now

Compile it:gcc –o helloworld helloworld.c Run it:./helloworldOutput is (1 line per second)hello worldhello world …Stop it:Ctrl-C

Ready for something useful

xDrive.c:#include <stdio.h>#include <3rdPartyCommands.h> /* for setPosition() */#define TRUE (1)#define FALSE (0)int main() { float x; while (TRUE) { printf(“Enter desired x position in microns:”); /* read desired position */ scanf(“%f”,&x); printf(“Driving to %f um\n” x); setPosition(x); sleep(1); /* let CPU do other tasks */ }}

Are we there yet?

This interface will work

Assumption:

operators are willing to run the control at a command prompt and type in the desired position when they need to change it (not so user-friendly, but okay for now)

Where could things go wrong?

Readbacks are important

xDrive is open loopIt has no knowledge of the actual position of the screenYou can set the position, but you don’t know if it ever got thereTo add this, you make use of another function in the screen’s API: getPosition()

Readbacks are important#include <stdio.h>#include <math.h> /* for abs() */#include <3rdPartyCommands.h> /* for set,get Position() */#define TRUE (1)#define FALSE (0)#define xTolerance (0.001) /* xActual must be this close to the xDesired */#define xMinimum (0.)#define xMaximum (20000.)int main() { float xDesired; float xActual; while (TRUE) { /* check if correct position */ xActual = getPosition(); if (abs(xActual – xDesired) < xTolerance) { /* motor got where it was asked to go. Ok to get next request */ printf(“Enter desired x position in microns:”); /* read desired position */ scanf(“%f”,&xDesired); printf(“Driving to %f um\n” xDesired); setPosition(xDesired); } else { /* something could be wrong. Investigate further */ if (xActual >= xMaximum) { printf(“upper limit\n”); else { if (xActual <= xMinimum) { printf(“lower limit\n”); } } } sleep(1); /* let CPU do other tasks */ }}

Now could it run forever?

Yes, if you address these issues:1. Interference from other signals in the

area could affect the readbacks to make faulty values

2. Ambient temperatures could increase and overheat the screen’s motor

3. Power could disappear and come back on by itself

4. The motor could get stuck at a limit5. System resources (eg.memory, file

descriptors) could run out over time

Robustness is possible

Solutions to each are: 1. Read back a series of values and take average

them into a single value or throw away outliers, dismissing them as noise

2. Add temperature sensors and chill/warm the equipment to neutralize the effect of diurnal and seasonal variations

3. Save positions on another machine and restore them on start up if power goes away

4. Define an operating range inside the motor’s absolute range of travel so that it can’t get stuck at the limit

5. Allocate resources at start and reuse them as needed. This way, you can’t run out

Adding operator interface

Create a user interface to control the screen

Operator needs to enter the desired position, start/stop/reset and readback the actual position

Example User Interfaces

open loop mover

open loop mover with readback

closed loop mover

closed loop driver

What else is there?

Given this simple example, here are some ways complexity may arise:

1. There is no API of function calls2. The API functions do not work3. The system must be interrupt driven4. The system needs to be multi-threaded to do several tasks

simultaneously5. Semaphores are needed to protect resources shared by

threads6. State machines are needed to take care of doing all the steps

to run in a given mode of operation and the transitions between modes

7. Other machines need to interact with your system8. Other machines may need to control your system some of the

time, but at other times your system must control itself

The take away Software control systems are a large

and diverse field You can work on controls systems all

over the world Even though systems can look

complex, at the lowest level, they are always very simple and either work or don’t work

It is very rewarding to get something to “just work” and to see it run forever.