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Unread 02-13-2018, 07:31 PM
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AMT Encodedrs: Faulty Output Voltage

We're using the AMT102 encoders mounted on the mini Toughboxes on our drivetrain. One of the encoders is outputting the correct voltage through the B channel, however the A voltage is giving off ~100 mV. I would assume the channel is busted, but when the students hooked it up to an oscilloscope, the correct waveform could be seen. Is there any reason for which one of the channels could be working but at a much lower voltage than VDD - 0.8 (according to the datasheet)?
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Unread 02-13-2018, 07:39 PM
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Re: AMT Encodedrs: Faulty Output Voltage

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Originally Posted by lucas.alvarez96 View Post
We're using the AMT102 encoders mounted on the mini Toughboxes on our drivetrain. One of the encoders is outputting the correct voltage through the B channel, however the A voltage is giving off ~100 mV. I would assume the channel is busted, but when the students hooked it up to an oscilloscope, the correct waveform could be seen. Is there any reason for which one of the channels could be working but at a much lower voltage than VDD - 0.8 (according to the datasheet)?
The outputs are only rated for 2ma, so if the pin or wire is shorted to ground, that would drag the voltage down a ton. I'd check for a short in your cable or swarf in the pins. Other option could be a bad input on the Rio.
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Unread 02-14-2018, 04:53 AM
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Re: AMT Encodedrs: Faulty Output Voltage

Add to the list the possibility of the DIO pin being configured as an output, set low in software.
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Unread 02-15-2018, 05:15 AM
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Re: AMT Encodedrs: Faulty Output Voltage

I tested all the pins on the encoder and there aren't shorts. Both channels on both encoders measure ~724 ohms to ground (if I remember correctly). I'll tell the students to try using another DIO port, but I don't think it'll change much, seeing as the problem is with the encoder's output voltage. In the meanwhile, we're working on a voltage amp with a MOSFET to jump the voltage up the 5V.

Any other ideas on what could be the issue?
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Unread 02-15-2018, 06:35 AM
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Re: AMT Encodedrs: Faulty Output Voltage

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Originally Posted by lucas.alvarez96 View Post
In the meanwhile, we're working on a voltage amp with a MOSFET to jump the voltage up the 5V.

Any other ideas on what could be the issue?
We've used a bunch of these encoders and no amplification has ever been necessary.
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Unread 02-15-2018, 07:53 AM
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Re: AMT Encodedrs: Faulty Output Voltage

Quote:
Originally Posted by lucas.alvarez96 View Post
We're using the AMT102 encoders mounted on the mini Toughboxes on our drivetrain. One of the encoders is outputting the correct voltage through the B channel, however the A voltage is giving off ~100 mV. I would assume the channel is busted, but when the students hooked it up to an oscilloscope, the correct waveform could be seen. Is there any reason for which one of the channels could be working but at a much lower voltage than VDD - 0.8 (according to the datasheet)?
If it spits out the correct voltage when it is measured open circuit (which is what I assume you were doing with the oscilloscope), but it is low when connected in circuit, then the problem is almost certainly what the output is connected to.

It could be a short circuit in the wiring. It could be a software error, not putting the DIO port as an output, like someone suggested.

Back in ancient days, (i.e. when I was young) I was a teaching assistant for a digital electronics class. (EE 249. I wonder if the class is still taught) Students were constantly coming to me with "broken chips", and I would always tell them that perhaps I could take a look at it. The problem, every single time, was always that two outputs were wired together.

I would say if the device works correctly when measured on the scope, then the device works. I don't know of any failure mode of such a device that would allow it to work there but not in circuit. I would look first to the software, and then make sure that the only connection of the A channel is to the signal pin of the DIO port.
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Unread 02-15-2018, 08:07 AM
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Re: AMT Encodedrs: Faulty Output Voltage

Is it the AMT 102-V? We're looking for an encoder and I don't know much about them, Thanks!
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Unread 02-15-2018, 08:58 AM
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Re: AMT Encodedrs: Faulty Output Voltage

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Is it the AMT 102-V? We're looking for an encoder and I don't know much about them, Thanks!
That's the type we have used in the past, and are using this year. I've been testing with them, and they are easy to use and have worked well for us so far.

I don't have enough expertise with other styles to do a meaningful comparison with other encoders. I just know that when I tell the robot to go six feet and stop, it goes six feet, and stops.
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Unread 02-15-2018, 09:34 AM
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Re: AMT Encodedrs: Faulty Output Voltage

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Originally Posted by David Lame View Post
If it spits out the correct voltage when it is measured open circuit (which is what I assume you were doing with the oscilloscope), but it is low when connected in circuit, then the problem is almost certainly what the output is connected to.

It could be a short circuit in the wiring. It could be a software error, not putting the DIO port as an output, like someone suggested.

Back in ancient days, (i.e. when I was young) I was a teaching assistant for a digital electronics class. (EE 249. I wonder if the class is still taught) Students were constantly coming to me with "broken chips", and I would always tell them that perhaps I could take a look at it. The problem, every single time, was always that two outputs were wired together.

I would say if the device works correctly when measured on the scope, then the device works. I don't know of any failure mode of such a device that would allow it to work there but not in circuit. I would look first to the software, and then make sure that the only connection of the A channel is to the signal pin of the DIO port.
I'm sorry, I wasn't too clear. I measured the 500 mV while connected to the scope, so the problem isn't the connection. I would consider it busted, but the waveform is ok (but 10x lower).
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Unread 02-15-2018, 10:21 AM
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Re: AMT Encodedrs: Faulty Output Voltage

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Originally Posted by lucas.alvarez96 View Post
I'm sorry, I wasn't too clear. I measured the 500 mV while connected to the scope, so the problem isn't the connection. I would consider it busted, but the waveform is ok (but 10x lower).
Ah. Ok.

If you were measuring it open-circuit, and it was outputting a low voltage, that's definitely odd, and could suggest something wrong with the encoder itself. If it was connected to anything else at the time, it could still have a waveform of the right shape, but wrong voltage.

I would suspect a bad input voltage, except that channel B apparently works.
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