Electronics > Electronics
RF Interference?
pterrus:
I'm using this encoder and decoder to build a remote control for my robot. It works perfectly except for one crippling glitch: I lose comm every time the drive motor is running!
If I put the decoder in "latch" mode, then the motor will run forever as soon as I hit the "drive" button (I checked, all the decoder outputs are frozen while the motor is running). If I put it in "momentary" mode then the motor keeps hiccuping off and on as it gains and loses the the signal!
If I unplug the motor and put a multimeter on the header, it works perfectly, it seems like the problem only occurs while the motor is actually running.
At first I thought it was an EMI issue, but then I realized that if I'm in latch mode, I can get the motor to stop by turning the remote control (encoder side) off and on again (just off won't do it), which doesn't make sense at all. Maybe it can receive the first packet just fine but not any subsequent packet?
I am stumped on this one, help!
pterrus:
If it helps, I opened up the plastic motor housing and found that there was already a capacitor across the terminals. Not sure if I should replace it or what. (The robot's base is a toy truck)
Here's what it looks like: http://imgur.com/5ahdp
Soeren:
Hi,
--- Quote from: pterrus on June 30, 2012, 02:44:43 PM ---If I put the decoder in "latch" mode, then the motor will run forever as soon as I hit the "drive" button (I checked, all the decoder outputs are frozen while the motor is running). If I put it in "momentary" mode then the motor keeps hiccuping off and on as it gains and loses the the signal!
If I unplug the motor and put a multimeter on the header, it works perfectly, it seems like the problem only occurs while the motor is actually running.
--- End quote ---
An easy one then, as it's clear that it is caused by the motor and what remains is to find what "feature" of the motor is the culprit.
Possible candidates:
Noise injected into the power lines (cure: Filter the power lines).
Noise radiated into the logic (cure: Shielding).
Too soft (weak) power supply (cure: Add buffer caps or upgrade the supply's current capability).
That's assuming that your circuit is designed (and build) in a sensible way - Always add the schematic when you ask questions about a circuit - we can't catch possible errors without a complete schematic, that is a correct representation of how you build it (not how you wanted or planned to build it, but how everything is connected up as is).
--- Quote from: pterrus on June 30, 2012, 02:44:43 PM ---At first I thought it was an EMI issue, but then I realized that if I'm in latch mode, I can get the motor to stop by turning the remote control (encoder side) off and on again (just off won't do it), which doesn't make sense at all. Maybe it can receive the first packet just fine but not any subsequent packet?
--- End quote ---
If it works fine without the motor, stop guessing about its ability to receive packets.
From what you wrote so far, there's nothing excluding EMI (which can cover several aspects).
I could shoot of my best guesses as to what's happening, but life's too short for trouble-shooting in the dark, so post the schematic and a couple of sharply focused pics of the setup. Add info on the motor (voltage and current) and your supply (does it happen with freshly charged batteries or only when it's semi-discharged?)
Test with a dummy load (power resistor or light bulb) equal in (dynamic) resistance to the motor and tell what happens (add an LED with a 470 Ohm series resistor for easy indication of when it's running).
If the problem persists (with the dummy load), you need to beef up your supply with caps and if it goes away, you need to filter the power lines to the motor.
Twist the motor wires in any case (2..3 twists/inch should do) and make sure the motor encasing is grounded through a heavy gauge, well soldered, wire.
If you have access to an oscilloscope, try it on the supply lines to the logic for starters.
pterrus:
--- Quote from: Soeren on July 01, 2012, 09:31:08 PM ---Hi,
--- Quote from: pterrus on June 30, 2012, 02:44:43 PM ---If I put the decoder in "latch" mode, then the motor will run forever as soon as I hit the "drive" button (I checked, all the decoder outputs are frozen while the motor is running). If I put it in "momentary" mode then the motor keeps hiccuping off and on as it gains and loses the the signal!
If I unplug the motor and put a multimeter on the header, it works perfectly, it seems like the problem only occurs while the motor is actually running.
--- End quote ---
An easy one then, as it's clear that it is caused by the motor and what remains is to find what "feature" of the motor is the culprit.
Possible candidates:
Noise injected into the power lines (cure: Filter the power lines).
Noise radiated into the logic (cure: Shielding).
Too soft (weak) power supply (cure: Add buffer caps or upgrade the supply's current capability).
That's assuming that your circuit is designed (and build) in a sensible way - Always add the schematic when you ask questions about a circuit - we can't catch possible errors without a complete schematic, that is a correct representation of how you build it (not how you wanted or planned to build it, but how everything is connected up as is).
--- End quote ---
I'm at work right now, but I'll draw up a detailed schematic later if the first few ideas don't work. For now, the encoder and decoder circuits are wired up exactly as shown in the pdf I linked to above, both powered with regulated 5V. Here's a quick block diagram of the downstream stuff I'm driving the motor with:
--- Quote from: Soeren on July 01, 2012, 09:31:08 PM ---
--- Quote from: pterrus on June 30, 2012, 02:44:43 PM ---At first I thought it was an EMI issue, but then I realized that if I'm in latch mode, I can get the motor to stop by turning the remote control (encoder side) off and on again (just off won't do it), which doesn't make sense at all. Maybe it can receive the first packet just fine but not any subsequent packet?
--- End quote ---
If it works fine without the motor, stop guessing about its ability to receive packets.
From what you wrote so far, there's nothing excluding EMI (which can cover several aspects).
I could shoot of my best guesses as to what's happening, but life's too short for trouble-shooting in the dark, so post the schematic and a couple of sharply focused pics of the setup. Add info on the motor (voltage and current) and your supply (does it happen with freshly charged batteries or only when it's semi-discharged?)
--- End quote ---
I'll post a pic of the setup later and try to get a reading on how much current the motor is drawing. Vcc going out to the motor is about 8.5 V.
--- Quote from: Soeren on July 01, 2012, 09:31:08 PM ---Test with a dummy load (power resistor or light bulb) equal in (dynamic) resistance to the motor and tell what happens (add an LED with a 470 Ohm series resistor for easy indication of when it's running).
If the problem persists (with the dummy load), you need to beef up your supply with caps and if it goes away, you need to filter the power lines to the motor.
--- End quote ---
This is a good plan and I'll try it. I can find the motor resistance with just a multimeter across the leads, right?
--- Quote from: Soeren on July 01, 2012, 09:31:08 PM ---Twist the motor wires in any case (2..3 twists/inch should do) and make sure the motor encasing is grounded through a heavy gauge, well soldered, wire.
If you have access to an oscilloscope, try it on the supply lines to the logic for starters.
--- End quote ---
The motor wires look plenty twisted, but I don't think the motor casing is grounded (see picture in my second post). I'll definitely try that first. I don't have an oscilloscope unfortunately.
I don't know if this helps narrow it down or not, but the base I'm using (including the motor) was originally an RC truck, and there was obviously no interference issue in its former life.
pterrus:
By "beef up your supply with caps", do you mean put a cap between my unregulated bus and ground?
That's easy to do, so is there a down side to doing this or should I just do it?
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