A Special Control Function for Dynamic Load Control
Written for the General Public,
even though it is about high tech industrial robotics.
A company hired me to write a special computer program that would
control a robotic machine, because I knew how to program up this particular type of control function. They also had me write a technical manual to tell the company's technicians how to operate the test machine.
Below is an excerpt from that technical manual -- kinda re-written for the public. It's missing some proprietary information, and a whole bunch of techno-babble. You're welcome.
The machine is like a giant 'trash compactor', similar to the one in the first Star Wars movie. It squeezes trucks, or it pulls on chains
attached to the seats and seat belts. It's for safety and reliability testing because federal law requires that trucks protect the people inside during collisions. It has to withstand 11,000 lbs for 90 seconds, and only squish 2 inches!
The name of the control function refers to a specific math formula in calculus.
It's the exact same function that makes the Cruise Control work in cars. Here's how.
The P.I.D. Dynamic Control Function
"Proportional - Integral - Derivative" Active System Control
PID is an industry-standard mathematical
function, which is optimized in this Control Program to provide facilities for
responsively controlling the load applied to the hydraulic cylinders of the
vehicle integrity test stand.
PID control helps smooth out the sudden changes in loading that
are caused by broken fasteners or bent frames in the vehicle under test.
The PID controller compares how much the engineers want the machine to squish the truck with how much the truck is actually being squished. If there's a difference, it reacts slowly and smoothly to adjust the hydraulic cylinders that do the squishing. By comparison, most machine controllers just turn things all the way on or all the way off for periods of time. Microwave ovens work that way.
How It Works
PID can be compared to the cruise control in an
automobile. A cruise control knows when
to give a little extra acceleration when going up the hills, and to let off the
acceleration going down the other side, in order to maintain a preset speed.
On the Structural Integrity Test System, if a bolt on the object under test breaks suddenly, the PID function reduces the force applied until the slack is taken up. This way the system does not overcompensate for the sudden amount of slack. Overcompensation can cause the system to apply more tension or compression than is desired, and can have catastrophic results.
PID is a mathematical function that compares the amount of tension or compression (as measured by a sort of 'fish scale' called a "strain gauge") to the target load value at that specific point in the target load profile. The result of this comparison is called ‘Error’. The rate of error is then minimized by the PID function over time, in terms of hundreds of milliseconds. The net result is that sudden changes in load or force do not overwhelm the system, while still maintaining the current target load value.
The data acquisition hardware monitors the signal from the strain gauge at a rate of 50 samples per second, or 20mS per sample. No gains in performance will be achieved by increasing the sample rate.
Serious problems may occur if the sample rate
is changed. The PID values are optimized for this sample
rate. (This sample rate can only be changed by qualified computer programmers with qualified test engineer support. Any change to the sample rate will require
complete reassessment of all PID tuning values.)
WARNING: REFER PID TUNING TO AN EXPERIENCED PID TUNER!
Extreme hazard to equipment and personnel can result from
inexperience with PID tuning!
Blog Comment: At first, an un-tuned robot flails around uncontrollably. As the roboticist tunes the PID control function, the machine begins to act right. Some robots can be tuned well enough to perform surgery.
For Further Information
Omega Temperature Control Handbook –Proportional control
How I learned about the P.I.D. control function was that one day at work there was nothing to do, so I read and article in the Omega Handbook in this link just to look busy. A few years later a company hired me to write this here program. 😀
Thing is, the math works to smoothly control lots of things, like heaters & freezers, cars & trucks, and nuclear reactors too!
( Okay, fine. Yeah, I'm a geek. I love this stuff! )
