this post was submitted on 10 Jul 2023
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Showerthoughts
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I'm actually a retired electrical engineer, used to do aircraft avionics. Anyway it's a simple thing to measure amperage loading on the motor to set a threshold for sleep and wake. But in engineering there's always more than one way to accomplish a task. An optical sensor and timer would be another way to do it.
If you were using a vfd in order to control the speed of the escalator, a lot of vfds have intelligence built in, so you could just wire it up and have the vfd take care of everything. On the other hand, I can see a bunch of reasons why a current might work really well for a short term demo and start to fail immediately after. Start to get things gummed up or also trying to deal with very small people riding the escalator, the trigger point might be difficult to keep straight.
You might be surprised how well you can gauge amperage demand with mechanical demand. I mean it's a linear relationship with respect to voltage so it's pretty easy to scale it. Though you're right, manufacturing tolerances and system state could make it unreliable.
It would be more reliable to use an optical sensor, but the cost of a current gauge is a few lines of code and a sense resistor along with the controller which would already be there for other functions. That's compared to the greater cost of an optical sensor with emitter and receiver.
It's almost always about the cheapest way to do something rather than best. There was a time engineers would do something because it's best even if it cost a little more, but those days are long gone, except maybe for NASA. With design often contracted to cheap labor markets, cost considerations rise well above all else. The new adage is make stuff for half the cost and make the customer buy twice at twice the price.