Amazing to find this group! Just curious if anyone has used one of these as an rpm sensor successfully.
Welcome to the community.
Just to mention also that this kind of sensor cannot work with SimpleFOC. It’s a conceptual problem.
This sensor measures the RPM by measuring the commutation waveforms. So it assumes the motor is following the commutation, which will work ok for drones and RC cars most of the time. But it isn’t actually measuring the motor position, or speed. If the motor is stalled, this sensor will happily report the RPM number that matches the commutation patterns being sent to the motor, it doesn’t know that it isn’t actually moving.
When doing FOC (closed loop) control, SimpleFOC needs to know the rotor position to control exactly those commutation waveforms that the sensor is measuring. So it needs a sensor that measures the actual, real position of the rotor. It can’t work with a sensor based on the commutation, because that’s the quantity being controlled, the output of the algorithm.
Thank you very much for the reply. I was curious because these sensors are used for governor mode on rc helis and the circuit adjusts to compensate when a motor is getting bogged down and gives more juice to keep a constant rotor rpm. The magnet sensor is a great option but the motor i am using to drive my turntable is shaftless.
You mean outrunner? From the picture, the shaft extends through the bottom. You can attach a diametrically polarized (magnetized) magnet to the bottom of the shaft with epoxy, and an SPI absolute angle sensor such as AS5047P underneath, and run a perfect closed loop. At that current you won’t need current sensing.
I was thinking about that but the shaft only extends by a mm or so. I was reading the manual for that very sensor on Digikey, but they say to not attach the bipole magnet to a ferrous item. Would the epoxy be enough of a buffer? It would be a great solution.
Without having a picture of the bottom of the motor is hard to say. Generally, attaching a diametrically polarized magnet to the motor shaft directly won’t create a problem, I’ve done it. The worst case you need to break it off and probably use a plastic housing to avoid direct contact, but that’s overkill. Just get a really strong and thick magnet to overwhelm the shaft field distortion, such as this one:
Do NOT, ever, heat the rare earth magnets up, for example using heated glue or such, they will permanently demagnetize. Use only metal rated epoxy.
N52 is the strongest commercially available at reasonable prices. It will demagnetize at about 80oC. Keep it cool.
Many, many thanks. Just ordered one (AS5047P).
The project so far is great fun. With an esc and a servo tester it worked as a proof of concept but need much more accurate speed control.
That’s pretty amazing!
Make sure you get a fast MCU and a nice driver board to drive the motor for SimpleFOC.