PID controllers are everywhere. They regulate temperature, motor speed, power supplies, positioning systems, process equipment, and probably a dozen things within arm’s reach of you right now.
They’re also frequently explained with enough calculus to make them seem more mysterious than they really are. Granted, the I and D in PID stand for calculus terms, but they are easy enough to build into a spreadsheet. Grab a copy and keep it open while you read this post.
The Google Sheet implements a simple simulated PID controller along with a simulated process — the thing we’re trying to control. You can change the controller gains, alter the process, introduce disturbances, and watch what happens without compiling anything or wiring up a heater that might accidentally become a toaster.
The three letters in PID stand for Proportional, Integral, and Derivative. If your calculus is rusty, integral is just how much is building up over time, and the derivative is how much changed just now. Each operates on the error:
error = setpoint - process value
The setpoint is where we’d like the system to be, the process value (PV) is where it actually is, and we would obviously like the error to be zero. Proportional is the most obvious method of control. The more we are off, the more we adjust. The closer we are to the setpoint, the less proportional output we need.
Integral, on the other hand, looks at a running tally of errors. Finally, derivative measures how much things have changed from the last time we looked. The basic cycle time for the spreadsheet is set by dt, which, by default, is 0.1 seconds. Therefore, it takes ten spreadsheet rows to cover an entire second.
Suppose we’re controlling temperature and want it to be 20 degrees. If the temperature is 15, the error is +5. If it’s 22, the error is -2. Our controller’s job is to turn that error into an output. That output affects something — a heater or a motor speed or whatever — that can change the process value. So for a temperature example, the output might drive a heating resistor, and the process value is measured by a thermistor.
The PID tries to drive the heater so that the process value is as close as possible to the setpoint. To the PID algorithm, the actual units of the output and the process values are immaterial. The spreadsheet limits output from 0 to 100 and, presumably, that would be a percentage of voltage or a PWM duty cycle. The setpoint and process value might be in degrees C or F. But the algorithm doesn’t really care.
First, Just P
Make sure the Model drop-down is set to DEFAULT. We’ll begin by setting:
Kp = 4 Ki = 0 Kd = 0
Set the initial process value to 0, the setpoint to 20, the process gain to 1, and the time constant to 2 seconds. With only the proportional term operating, the controller is particularly easy to understand:
output = Kp × error
At the beginning, the error is 20, so the controller asks for an output of 80 (that is, 4 times 20). However, the process doesn’t instantly jump to 80. Our simulated plant is a first-order system implemented essentially as:
PVnew = PVold + dt/tau × (Kprocess × output - PVold)
The actual spreadsheet has extra terms for a bias and disturbance, but you’ll usually leave those at zero. That’s a useful generic model for a surprising number of real things. Turn up a heater, and the temperature approaches a new value gradually. Apply voltage to a motor and its speed doesn’t change instantaneously. Charge a capacitor through a resistor, and you’ve seen exactly this sort of exponential behavior before.
As the process value rises, the error gets smaller. Because the error gets smaller, the proportional controller reduces its output. This works. At least, mostly.
Watch where it eventually settles. With the suggested values, the process value winds up around 16 even though our setpoint is 20. Why? At a process value of 20, the error would be zero. A proportional controller presented with zero error produces zero output. But this particular process needs an output of 20 to remain at 20. Therefore, it can’t ever quite get there.
This is the classic steady-state error of proportional-only control. We could crank Kp upward. Try Kp=8. The process gets much closer to the setpoint. But continually increasing proportional gain isn’t a universal solution. Eventually real systems start overshooting, oscillating, amplifying noise, or otherwise expressing their displeasure. We need another term.
Remember the Error
Set Kp back to 4 and try:
Ki = 0.5
The integral term looks at not just the error right now, but the error accumulated over time. In the spreadsheet there’s an Integral State column. Each row does approximately this:
integral = previous_integral + error × dt
and the I contribution becomes:
I = Ki × integral
Now consider our P controller sitting stubbornly below the desired value. As long as some positive error remains, the integral keeps growing.
That gradually increases the controller output until the remaining error disappears. Instead of settling around 16, the process now creeps all the way toward 20. This demonstrates one of the major reasons integral control exists: it eliminates persistent offset. It also gives us a good excuse to disturb the system.
Select the user process model and set User Model # to 1. This will let you disturb the process value by entering numbers into the User1 column. Leave the first bit of the User1 column at 0. But somewhere farther down the simulation, put a disturbance into that column — perhaps -5. If you are feeling especially salty, try a sequence like: 0.5, 0.75, 1, 1.5, 2, 1.5, 0.75, 0.5, -1, -1, -0.5. That sequence should already be in the template’s User1 column.
You can imagine that as opening a refrigerator door, suddenly putting a mechanical load on a motor, or connecting another load to a regulated power supply.
A proportional-only controller reacts immediately, but once things settle, it again tolerates a permanent error.
The integral controller doesn’t. If the process remains below the setpoint, integral action continues increasing until the disturbance has been compensated. That’s a powerful trick. Unfortunately, integral control has tricks of its own.
Too Much of a Good Thing
Integral action remembers errors, but memories aren’t always helpful. The derivative term responds to how rapidly the error is changing:
D = Kd × (error - previous_error) / dt
If proportional control asks, “How far away are we?”, derivative control asks, “How fast are we approaching?” Or, more precisely in this case, “How fast is the error changing?”
Make the simulated process faster by changing its time constant from 2 to about 0.8 seconds. Then try something deliberately more aggressive:
Kp = 8 Ki = 1 Kd = 0
The response now gets to the setpoint quickly, but it overshoots it. The problem is easy to see in the spreadsheet. While the process is racing upward, it remains below the setpoint, so the integral continues accumulating positive error. By the time we arrive at the destination, the integral term is still pushing. Depending on the process and gains, the result may be a little overshoot, a lot of overshoot, or sustained oscillation.
Since the default plant is only first-order, derivative action doesn’t have much to work with. User Model 2 adds another lag, using the USER2 column as an intermediate process state. That produces more phase lag and makes aggressive PI tuning more prone to overshoot.
Switch to User Model 2 and start with Kd=0. Then note the peak process value. Then try Kd=0.2, 0.5, and perhaps 1.0. Try some negative values. You will find that there is a range where the peak overshoot is reduced slightly, but keep going and the response starts to ring. Push Kd far enough, and the derivative term becomes part of the problem rather than part of the cure. The graphs autoscale, so sometimes what looks like a peak the same size (or even bigger) is really smaller than the previous result. Be sure to read the numbers.
That’s the basic PID balancing act. P reacts to the error that exists now. I reacts to error that has existed for a while. D reacts to where the error appears to be heading. Put all three together, and you have a controller that may respond strongly, eliminate steady-state error, and anticipate rapid changes. However, sometimes you are better off with, for example, just PI or even just pure proportional control. Having the algorithm in a spreadsheet form is a nice way to experiment, especially if you can model the system’s behavior.
It’s Only a Spreadsheet
You can add your own models by modifying USR_PROCESS to call your function or just modify one of the existing ones. DEF_PROCESS is just a simple lag model. USR_PROCESS0 has some random noise, while USR_PROCESS1 lets you inject a disturbance in the USER1 column. USR_PROCESS2 is like USR_PROCESS1 but has a second-order process in the USER2 column as well. Of course, real control systems are messier than these nice models.
The output may have hard limits. In fact, the spreadsheet includes minimum and maximum output clamps. This exposes another classic PID problem: integral windup. If the controller desperately requests an output of 150 but the actuator can only deliver 100, the integral can continue accumulating error even though the actuator cannot respond. When the error finally reverses, all of that stored integral has to unwind. Practical controllers frequently include anti-windup schemes to deal with this. In practical terms, imagine a thermistor gets unplugged, and the system suddenly thinks there is a giant temperature error. It will try to correct it, but it can’t. Then someone plugs the sensor back in. All the accumulated error in the integral term now has to be backed off.
Derivative action causes its own problems. The spreadsheet calculates derivative from the error, which means suddenly changing the setpoint produces a large derivative pulse — the notorious derivative kick. Real controllers often calculate derivative from the process value instead.
Of course, real measurements also contain noise. Differentiation is very good at making high-frequency noise more prominent, so the D term is commonly filtered. We aren’t doing any of those sophisticated things here, and that’s intentional. The point of the sheet is that every number is visible and to make it easy to experiment.
Change a setpoint in the middle of the Setpoint column, and you’ve generated a step input. Change one of the User columns, and you can inject a disturbance. Adjust Kp, Ki, or Kd, and you can immediately see which portions of the controller output changed and why.
To add your own models, modify USR_PROCESS and add a custom function to the SWITCH statement. Then create your custom function. If you need to grab data from the spreadsheet, you’ll see examples of using ROW() and INDIRECT() to get the right numbers. It is fairly straightforward to add motors, thermal systems, second-order plants, dead time, nonlinearities, or whatever other pathological system you’d like to inflict on your controller.
The most important lesson about PID control? There isn’t a magic set of Kp, Ki, and Kd values. A set of gains that works beautifully on one plant may be terrible on another. Change the mass, thermal capacity, load, delay, sample rate, actuator limits, or sensor characteristics and the optimum controller changes with it. Not every control job needs all three terms.
The equations fit comfortably into a few spreadsheet cells. The interesting part is figuring out what numbers to put in them.
Most of our spreadsheet hijinks center around DSP. Except for the ones that simulate computers.
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