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Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
I have to ship my zero-g project tomorrow. All that remains is the segment display for my motors RPM, but I can't seem to get it to work.
Heres the problem: the motor has internal hall effect sensors, from which I plan to read the frequency and thus RPM. The sensors have three wires; power, return, signal. I hook up + to power and - to return.
When I try to look at the signal with a scope, however, I see nothing. Now heres the weird bit. If I attach the scope's ground clip to the + rail, then I can see the signal, but negative, in the scope.
Why can't I see it properly?!?! When I hook the signal wire up to my frequency counter, I get nothing as well.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Check the specifications for the output. Sometimes it can have an output topology that may require additional components to see such as open collector (needs pull-up for example), etc...
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
well thats the kicker; the data sheet simply presents that it has halleffect sensors, and the names of each wire. I think they want me to buy their control solution, as they don't offer any details about using the sensors (i guessed that they need ~5V, seems fine so far).
I guess I need to rig something, but I don't really understand why the orientation of the scope matters.... can anyone explain that to me, or suggest a way to make this work like i want
Registered Member #130
Joined: Fri Feb 10 2006, 08:12PM
Location:
Posts: 15
Have you tried using a pull-up resistor? I've used hall effect sensors in the past and they always needed a pull-up resistor.
I'm wondering if your scope is acting as a pull resistor when you put the ground clip on the + signal and the probe on the hall output signal? I'm sure there will be some resistance between the scopes ground clip and the probe.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
From the optical encoder interfaces i've worked with including motor related, they always were open-collector output and required a pull-up resistor. Remember, no pull-up, and the output will be stuck at floating / ground and never pulled high. Try putting a 10k between Vcc and the Signal.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
well, i got it solved, but it was a lil more complicated.
I tried the pull up, but then my frequency counter was, apparently, low resistance and pulled it back down again. I kept lowering the value of the pull up to bring the voltage up to the point where the PIC could sense it, but I was down to about 200 ohms and still only getting a 2V signal.
I eventually went back to the 10k, plus and opamp in voltage follower mode. everything works great. thanks for the help
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Urgent! Signal problem
