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Topic: Reading rpm and pulses, what are my best options. (Read 229 times) previous topic - next topic

HugoW

Hi,

I am making a device to check gearing on an RC car powered by a brushless motor. I am not going to use the signal coming over the motor wires to the motor when the ESC is powering the motor, as this is a 'dirty' PWM based signal. I will power the wheels with a cordless drill, turning the brushless motor into a generator. So, I will need to generate two rpm / pulse signals.

The first is the drill. My plan is to mount a magnet to the drill head and a reed contact next to it. But this will need debouncing in the software. Is there an option without debounce?

The second is the sine wave that is generated by the motor when it is turned, as it basically turns into a generator. My idea is to use a larger resistor (1k, 10k, something in that region) and a 4,7V Zener. When the motor is turned, 4,7V pulses will ocour over the Zener (I hope). Are there better options?

BTW, I know there are commercially available brushless motor rpm sensors. I have several. However, I have tested those and they are not overly accurate. You can use them as an rpm indicator (like on a dashboard) fine, but they are not usefull to actually count revelutions.

Any input is appreciated,
Cheers,

Hugo

wvmarle

The first is the drill. My plan is to mount a magnet to the drill head and a reed contact next to it. But this will need debouncing in the software. Is there an option without debounce?
A hall effect switch does not need debouncing. You can also consider adding an RC filter to the reed switch to debounce in hardware. Just make sure the RC time constant is much lower than the expected pulse frequency or you miss pulses.

Also I don't think a single position on the shaft is enough for correctly switching on/off your coils. You'll need at least one position per coil.

Quote
The second is the sine wave that is generated by the motor when it is turned, as it basically turns into a generator. My idea is to use a larger resistor (1k, 10k, something in that region) and a 4,7V Zener. When the motor is turned, 4,7V pulses will ocour over the Zener (I hope). Are there better options?
What is the voltage and frequency you expect to see here?
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

HugoW

Thanks for the reply.

Hall effect sounds like a good option, thanks.

On a brushed motor, I simply get one pulse per revolution with this method. 4,7V over the Zener, the remaining voltage (up to 10V) over the resisitor. With the brushless I don't know. I want to try it and hook it up to a scope, unless other proven options are posted here. If the voltage is not enough to open the 4,7V Zener, I might need a lower voltage one and add an amplifier. When I have a signal, I can calculate how many pulses per revolution the motor gives.

Cheers,

Hugo

wvmarle

Why the zener? Why would you every get more than 5V from the sensor?
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MarkT

Why the zener? Why would you every get more than 5V from the sensor?
The motor used as a generator will generate voltage dependent on speed, its not inherently limited.  This is about the BLDC, not the drill.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

wvmarle

I really thought OP tried to read a signal from a hall effect sensor or reed switch, not the coil voltage...
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MarkT

It is about reading a reed sw or hall sensor for the drill, but the other end is a BLDC inside an RC car - presumably much easier to reconnect the motor wires than retro-fit a temporary magnet/sensor jig,
which would need customization for each kind of RC car...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

HugoW

Well, I am open to all ideas, but indeed my first idea is to glue a magnet to the drill head and use the reed (now Hall effect) sensor to feed the counter in Arduino. The motor I consider to tap two of the three wires and use the zener. However, I just read somewhere the motors we use have a magnet on the rotor, so maybe I can use a hall effect sensor as well here to measure it's rpm. Should be possible with two hands; one holding the drill and one pushing down a sensor on the motor casing, pressing the chassis down on the stand at the same time keeping everything stable during the measurement. I just ordered some Hall effect gizmos from my Chinese friends, so in about two month's time I will receive them and give it a go. Or maybe earlier, if the Dutch customs and mail service cooperate.

Cheers,

Hugo

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