Diagnostics: Impeller Overspeed

I am a diagnostician. It’s in my blood. I was born with a sense of curiosity that drives me to see what others miss. I am fortunate to be able to include diagnostics in my work. Sometimes, I find myself being paid for it, helping others solve problems they can’t solve themselves. Most of the time, though, the analysis falls outside the neat professional envelope. In this case, I’m troubleshooting a nagging problem I discovered in my CatGenie 60. It’s not a functional defect, so it isn’t a huge issue or deal breaker or anything like that. It’s just something I noticed that would make a significant difference in the operation of the appliance. In fact, it’s the only thing I can find about the product that isn’t already brilliantly engineered.

CatGenie is a self-cleaning litter box, essentially a toilet for your cat. I love it because I never have to scoop any thing, and I can leave it for days at a time without so much as thinking about cat shit. Oscar, my tuxedo cat, loves it and has used it without much more than the occasional “bad aim” puddle, which is far better than a hidden spot in a carpeted closet that I don’t discover for weeks. (Don’t ask…)

One of the key functions of the CatGenie is to wash and rinse reusable granules. Part of the cycle is to drain the bowl. This is accomplished with a clever component called a hopper. The hopper is a funnel that feeds the solid and liquid waste into an impeller, where it is ground in to a slurry and pumped into the sewer by way of the drain tube. This excellently designed component even includes the injection of water during the drain cycle, presumably in an attempt to prevent the problem I’m about to explain.

This leads to the title of this post. Impeller Overspeed. Since the hopper must liquify the solid waste in order to pump it out, the impeller must be driven with a strong torque, at least sufficient to break up the bigger chunks into smaller ones that will fit through the drain tube. Once the solids are broken up, though, the strong torque causes the impeller to speed up. At some critical speed, the pressure forcing fluid out of the hopper is sufficient to force all the fluid out before more fluid can fall into the chamber. When this happens, the impeller is running dry. That means there is no fluid to prevent the impeller from accelerating even more. So, the impeller speeds up to a very high speed by the time the pressure in the drain tube has dropped to zero (because the impeller is what generates pressure). When the pressure in the drain tube drops, the fluid stops and flows backward into the impeller chamber again, causing the impeller to slow back down enough to fall below the critical speed.

As you can imagine, this is a dynamic equilibrium. Fluid slings up the drain tube, the impeller speeds up, then the fluid falls back into the impeller to repeat the process. In the process, some fluid crests over the “hump” at the top of its travel and falls down the other side into the sewer. However, because the fluid is spending a lot of time accelerating and decelerating back and forth, not much of it actually leaves the system. This results in slower drain time and unnecessary wear on the impeller blades.

The solution is relatively simple. A speed governor offers a feedback mechanism where the torque applied by the motor is reduced when the shaft speed exceeds a set limit. By adding a shaft speed sensor and a cutoff or throttling circuit, this problem should be eliminated entirely, allowing faster drain times and more predictable behavior without affecting any other features.

Of course, what I would absolutely love to see in the next CatGenie model is wifi-enabled smart appliance features. That way, a support person can be notified directly from the device when it encounters a malfunction, eliminating the need for a customer to make a call to the support team. Presumably, the support staff could even fix the problem remotely or notify the customer that human action is required to resolve the error. Naturally, this mechanism could also be used to automate supply procurement.

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