Quanser Academic makes a line of products, which you can see demo'd at
their YouTube channel, that are sold to university laboratories as teaching aids for control theory. Today I noticed a funny undocumented quirk about one of those products. I have notified Quanser of this, and there is some indication that they will update their documentation to reflect this quirk. However, because their documentation is not on-line, it's hard to see how that update will ever reach people who have already bought these products.
I TA a
graduate control lab, and today's lab had the students design and implement an LQR controller that made use of a full-state observer to control Quanser's
flexible joint. The motor angle is measured by a shaft encoder (which is the output we use to drive our observer), and the arm angle is measured by another encoder. We don't need the measured arm angle because we're using a full-state observer, but we still capture it for comparison to our estimated arm angle.
The students build a controller that stabilizes the plant to a moving equilibrium that tracks a square wave. At every transition, the observer trajectories had some peaky transients, as is expected, but it appeared like they were always in the wrong direction for the arm angle and its speed. I dismissed this as peaking, but one group wanted to investigate further, and so I had them track a sine wave instead. What did we find? The arm angle and speed estimates were always EXACTLY 180 degrees out of phase with the measurements.
I was shocked, and so I looked into it. Visually, I could tell that the FLEXIBLE JOINT's ANGLE ENCODER is mounted UPSIDE DOWN with respect to the motor's angle encoder. That means that rotating the motor to the "right" produces a positive angle even though rotating the JOINT to the right produces a NEGATIVE angle! The observer had it right; it was the MEASUREMENTS that were incorrect! (I built a minimal example to test my hypothesis, and what I say matched what I observed; the opposing rotations lead to equal signs)
Nowhere is this documented. Because this angle is not directly driven (it's coupled to the assembly via springs) and the arm+spring system is naturally stable and fast, this POSITIVE FEEDBACK doesn't lead to destabilize the system unless you have extremely high gain. So I'm guessing no one ever noticed. Multiplying the measurement by -1 restored intuition. I drilled down into some Quanser demo code, and I found that they do the same thing in their controllers. I have no idea why they would
not document this in their rotary joint docs. Do they expect everyone to go through all of their MATLAB and Simulink code?
I could understand this error if the flexible joint was a generic component made by a different company, but the joint is
solely for the purpose of connecting to the motor, and so it would make sense that its encoder orientation would
match the motor encoder orientation! At the moment, we need different gains for both.
