Tuning a PID controller consists of choosing the best values for the proportional, integral, and derivative gains in order to satisfy some performance criteria of the closed-loop system. If we’re able to characterize or model the plant that we want to control, then it’s possible do find the best gains either through direct calculation or through simulation.
If we go back to the day when computers weren’t exactly around and there was no such thing as a digital PID, folks had to come up with methods that could be applied directly to the plant or process. Developed respectively in the 1940s and 50s, the well known Ziegler-Nichols and Cohen-Coon methods were the way to go about PID tuning back then. One problem with those two methods is that they result in a fairly aggressive set of gains that can potentially lead to an unstable (or a reduced stability margin) closed-loop system. That undesired characteristic becomes even more critical when a digital PID is employed.
Because we will be dealing mostly with DC motors, I’ll go over a method where the motor speed response is approximated by a first order system. The identified open-loop system is then used to determine the PID gains for the corresponding closed-loop system. In a future post we will go over characterizing the motor individual plant parameters, thus obtaining a better model.
So, let’s try to keep the math under control and start with characterization of the first-order system.
If we look back at the digital filter post, we can extend the low-pass filter equation to a more general form of the first-order system, given by:
where the steady-state gainis the ratio between system output and input. As seen in that same post,is the response time of the system. By determining those two parameters, the first-order system can be fully characterized.
The block diagram below represents the closed-loop system with a PID controller and our first-order system, the latter now expressed as a differential equation. While technically correct, no one uses a time-based representation of the system as shown in the diagram. Mostly because it is really difficult to work with differential equations whose inputs and outputs are all interconnected in some way.
Thankfully, a pretty smart guy named Pierre-Simon Laplace (1749 – 1827) came up with the Laplace transform. It allows for mere mortals to treat complicated convolution problems involving differential equations as simple algebraic ones. We can then redraw the diagram using the corresponding Laplace transforms, as shown in the more familiar form below.
Because we will be doing some algebraic manipulation let’s use the following transfer function notation:
Doing the usual “with a few steps, it can be shown that” the closed-loop system transfer function in our case is:
It has the characteristic equation of a second-order system, with damping ratio(zeta) and natural frequency
After all this mumbo jumbo, we’re finally getting to a point where, for a given first-order plant (with knownand), we can determine the PID gains,andthat would give us the desired closed-loop response, characterized byand. In other words, by selecting a damping ratio and a natural frequency, we can determine a set of PID gains that make our closed-loop system behave as desired.
PID Gains Selection
If we take a look at the last two equations, there are three unknowns (the PID gains) for two known parameters (and ). A good starting point is to make . That way, we can solve the two following equations, for and.
- in rad/s is the natural frequency of the closed-loop system. The larger its value the faster the system’s response time. In practical terms, you cannot make this number arbitrarily high since you may run into saturation of the controller output. While, for most applications, short saturation periods are fine, it’s better to keep them at a minimum.
- is the damping ratio of the closed-loop system. Values close to 1 (critical damping ratio) will keep the oscillations and overshoot to a step input under control. In some closed-loop positioning applications, overshoot isn’t an option. Hence, a damping ratio greater than 1 is required.
Finally, note that the PID gains should always be positive. Therefore,.
DC Motor PID Tuning
It’s time to use the hardware setup and code from the Motor Speed Control post to do some exploration of this PID tuning method. First, a step input is applied to the open-loop motor in order to determine the first order response parametersand.
For our little DC motor, with the Raspberry Pi setup, the steady state gain is 20.9/0.5 = 41.8 rad/s/u. Remember that, in our case, u is the normalized PWM output between -1 and 1. The response time of the open-loop system is 0.184 s.
To improve the parameter extraction, a zero-phase filter was used. The code that automatically determines the parameters from the system response can be found on my GitHub page.
Next step consists of choosing values for the closed-loop system’s natural frequencyand damping ratio. The graphs on the left show the system response to a step input for a natural frequency of 2 Hz () and. Using the last two formulas for the integral and proportional gains
with all the values we gathered so far, would result inand. We can observe that the motor now takes 0.11 s to reach the target speed (instead of 0.184 to reach 63% of it). That comes of course at expense of using the full capacity of the PWM with some brief saturation.
The reasoning behind my choice ofandis that I wanted a faster response with limited saturation. 2 Hz was the right amount for that. I also wanted a damping ratio slightly greater than one to avoid any oscillations. At this point you might be asking yourself: why is the closed-loop response showing an oscillatory behavior then! The answer is a combination of:
- The motor is in fact a second order system, not a first order one as our approximation suggests. So, higher order modes may start showing up in the closed-loop response
- The PID is discrete, not continuous (as that was the basis for all the deductions above). And we’ve seen that the digitalization of the PID tends to promote a more oscillatory behavior
- The actual closed-loop system includes the transfer function of a low-pass filter for the motor speed. So, the overall system is not composed only by a PID and a plant, as we initially assumed.
Acknowledging that the choice of gains was fairly straightforward, the resulting closed-loop response is quite good! And we haven’t incorporated the derivative gain yet. While it’s usage is not always necessary, it tends to be quite helpful in the case of closed-loop DC motor control. A good starting point foris a couple of orders of magnitude smaller than the proportional gain. So, after starting with 0.001, I ended up with a value of 0.005. The addition of the derivative term really decreased the overshoot and virtually removed the oscillations.
Finally, the next set of graphs shows the results when choosing different natural frequencies and damping ratios. Notice the effect on the response time, the overshoot and the saturation of the controller output.
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