help with noise from motor
jhankle
Posts: 21
I have just bought myself a BS2e, so that I can learn more about electronics. I'm a novice at this: the last time I played with anything resembling electricity would have been, umm, in the late 1960's I guess. And these kinds of little computer things did not exist back in my day, and I am certainly not a EE 
I am trying to control two motors with the BS. They are small motors, around 250-300 mA each. Right now I'm using an H-bridge chip, but plan to switch over to using just a transistor since I don't need direction control, only speed. I want to use PWM to control speed for both. Since the built-in PWM command doesn't execute in the background, I can't use that to control both motors at once, so instead I have worked out my own dual-PWM using 7 bits resolution each, like so, where "duty" is a word containing two separate duty values in each byte:
This works fine, and I can control both motor speeds. I also have a remote controller connected to some stamp pins to control all this - one of the RF units that Hobby Engineering sells. But here's my problem. If I disconnect the motor, the remote control works fine, and controls the BS in the manner I want it to. If I disconnect the remote, I can PWM the motor. But I can't do both at once! I think the problem is that the PWM is causing lots of noise which interferes with the receiver. It just won't listen to the controller if the motor is running, but works with the motor stopped. Everything runs off the same supply: 4 AA NiMH batteries. Common ground for the motor, the BS, and the RF receiver.
I don't have any kind of EE background, only what I have figured out myself in the past few weeks, so at this point I don't really know what to do. If anyone could point me to any advice on the matter, I'd be really thankful.
I have tried putting a capacitor in the motor circuit as described in the BS manual, and this helps slightly, but not much. I have a 100 uF capacitor but that's not going to run the motor even from one pwm pulse to the next, I don't think. I guess I could buy one of those "supercapacitors" which are in a small package. Would that help me any? I can get a pretty small one that's 0.047F or 0.1F. Is that the right way to solve this, or is there some better thing to do? I really don't want to use two separate power sources in my circuit, and don't want to buy a pwmpal if i can help it.
thanks for any kind of help
Post Edited (jhankle) : 1/6/2008 6:54:52 AM GMT
I am trying to control two motors with the BS. They are small motors, around 250-300 mA each. Right now I'm using an H-bridge chip, but plan to switch over to using just a transistor since I don't need direction control, only speed. I want to use PWM to control speed for both. Since the built-in PWM command doesn't execute in the background, I can't use that to control both motors at once, so instead I have worked out my own dual-PWM using 7 bits resolution each, like so, where "duty" is a word containing two separate duty values in each byte:
PWMLoop: pwmcount = (pwmcount & pwmmask) + duty Motor0 = pwmcount.BIT7 Motor1 = pwmcount.BIT15 IF (INS & buttonmask) THEN CheckInputs GOTO PWMLoop
This works fine, and I can control both motor speeds. I also have a remote controller connected to some stamp pins to control all this - one of the RF units that Hobby Engineering sells. But here's my problem. If I disconnect the motor, the remote control works fine, and controls the BS in the manner I want it to. If I disconnect the remote, I can PWM the motor. But I can't do both at once! I think the problem is that the PWM is causing lots of noise which interferes with the receiver. It just won't listen to the controller if the motor is running, but works with the motor stopped. Everything runs off the same supply: 4 AA NiMH batteries. Common ground for the motor, the BS, and the RF receiver.
I don't have any kind of EE background, only what I have figured out myself in the past few weeks, so at this point I don't really know what to do. If anyone could point me to any advice on the matter, I'd be really thankful.
I have tried putting a capacitor in the motor circuit as described in the BS manual, and this helps slightly, but not much. I have a 100 uF capacitor but that's not going to run the motor even from one pwm pulse to the next, I don't think. I guess I could buy one of those "supercapacitors" which are in a small package. Would that help me any? I can get a pretty small one that's 0.047F or 0.1F. Is that the right way to solve this, or is there some better thing to do? I really don't want to use two separate power sources in my circuit, and don't want to buy a pwmpal if i can help it.
thanks for any kind of help
Post Edited (jhankle) : 1/6/2008 6:54:52 AM GMT
Comments
Also... 4 AA NiMH batteries are going to be just under the threshold for the 5V regulator on the BS2e (1.2V x 4 = 4.8) .... Depending on how much "surge" the motors are creating, 4.8V does not leave much voltage overhead for the brownout (about 4.3 volts)
To increase the voltage overhead, add a 5th cell to your 4-pack to provide 6V (1.2V x 5 = 6V)
If you still experience trouble, you can implement a diode capacitor style filter ONLY to the BS2e supply... here your 100uF cap should be fine. This helps the BS2 to become unaware of any short duration voltage surges that might be present on the supply line.· Connection is simple: connect the capacitor across Vin and Vss (remember to observe polarity) ... keep the grounds common, and orient the diode so that the "band" marking goes to Vin.· The remaining diode terminal is now your new Vin to the BS2e.·
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 1/6/2008 7:44:31 AM GMT
"Output Clamp Diodes for Inductive Transient Suppression (L293D)"
As for voltage, I'm using the unregulated input pin to the BS, so I was leery of feeding it 6V. Especially since when first charged, NiMH are around 1.5V for a wee bit until they fall down to their normal 1.2, and 5x1.5 seems like too much for that input, and for the input to my RF receiver? Anyway, I don't think I'm hitting a brownout on the BS2e. I think the problem is more like the general noise in the power lines interfering with the RF receiver's ability to receive the signal from the remote. The BS2e itself seems to keep on trucking. Maybe what I need to do is implement your "cap/diode" suggestion, but on the RF receiver's power inputs? I have the feeling that I need to make that thing less aware of any noise.
thanks again for your expertise!
That's also possible...· One way to check if the BS2e is resetting is to add a bit of code at the beginning of the program to flash an LED or chirp a speaker.
Anyway, the Diode-Cap on the RF receiver would probably be a good idea as well... remember though, that a diode will drop the voltage about 0.6V across he PN junction so now your RF receiver only sees 4.2V ... It depends on the design, but at close range everything may appear to function properly.· Even a 0.6V drop can affect the sensitivity (range) of the unit.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
That might make it tough to power this. With 4xAA, the normal voltage will be 4.8v-0.6v = 4.2 which is too low for the BS and the receiver. But with 5xAA, if the AA's are fresh off the charger that's a 5x1.5V=7.5V supply, or 5x1.2V = 6V after they discharge a little. Even with the diode in there, it might still be 6.9V with fresh batteries, which I guess is way too much to feed something that expects 5V. I could use the regulated input on the BS, but the RF receiver doesn't have a regulated input. This stuff is hard to figure out [noparse]:)[/noparse]
Well tomorrow I will try the capacitor across the power lines instead of on the motor. Maybe, with some luck, that would be enough help even without a diode before the BS & the receiver, and then all will be OK. Thanks for the excellent help.
I think your best bet is to add the extra cell and use the unregulated input for the stamp isolated by a diode and cap. Then add a LM78L05 for the receiver that is also isolated by a diode with a cap and then keep the motors on the same supply with a back EMF diode for each motor. Of course, a separate supply for the motors would be better, but you wanted to use the same supply. Using separate supplies is a much better idea and I think you should consider this apprach.
By the way, you did not let us know what you are using this for?
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D. A. Wreski
I guess I would need two extra cells for that? With just one, I'd be at 1.2x5 = 6V, and it looks like that LM78L05 wants at least 7V on its input in order to maintain a regulated 5V.
>> Of course, a separate supply for the motors would be better
Yeah... I know that you are right, it's just hard for me to do it due to a size constraint. I've spent hours today putting little capacitor / diode filters in front of everything, but no luck [noparse]:([/noparse]. If I replace the motor with a non-noisy entity like my multimeter, or a resistor, then everything works perfectly even with the remote 30+ feet away. With the motor in place, though, I have to hold the remote control within inches of the antenna for it to work if the motor is spinning. If the motor spins at full speed, then it has to be less than 1 inch from the antenna.
I wonder too whether my low frequency PWM is making things more difficult than need be with bigger current surges. I guess that given the BS2e performance, I might be getting under a thousand PWM pulses per second, which is a lot lower than, say, millions
>> By the way, you did not let us know what you are using this for?
It's a tiny (!) little robot. The problem with a separate supply is that the robot can only easily accommodate 4xAAs. There just isn't physically room for more batteries, which is why I was motivated to try to use one power source for everything. But the real "using this for" is, I guess, just for me to learn something new. I've retired recently and I always thought that the best way to try to stay sharp in old age is to keep trying to learn new things. So I am just tinkering around with trying to learn about electronics. My biggest lesson so far is that when I know how I want something to work, it usually doesn't really work that way
thanks very much for your reply!
I've changed over to using a transistor to control the motor instead of an H-bridge (again no improvement). But I have found that if I tie the transistor's C & E directly to the battery (instead of connecting to the same voltage rails as power the BS2e and the receiver), things are *slightly* improved. Still just one battery, but a slightly different connection topology. It's not very much better, but it's noticeable. Now I just have to figure out how to fix it for real.
I guess motors are very unfriendly things to have in one's circuit [noparse]:)[/noparse]
The BS2e has a regulated voltage input. I'm not using it right now, since I'm using the unregulated 5V in. But let's say that I *was* using it. This means the BS2e is going to produce +5V itself. Now, each BS output pin can produce 20 mA, right? Well, the RF receiver needs only 1.5 mA, so what if I use the regulated BS input, and use two BS output pins to power the RF receiver? Would this give it cleaner power?
Is that a crazy idea? Dangerous? Worth trying? I just don't want to accidentally fry my expensive BS!
The simple answer to your question is there is no need to involve the Stamp pins and possibly overloading them. The following comes from the PBASIC Reference Manual:
quote
Pin Name
21 VDD 5-volt DC input/output
If an unregulated voltage is applied to the VIN pin, then this pin will output 5 volts. If no voltage is applied to the VIN pin, then a regulated voltage between 4.5V and 5.5V should be applied to this pin.
end quote
You can "tap" 5 VDC right from Stamp pin 21 (VDD), as it stands right now using the unregulated input (VIN). Why bother with the pin ports when power is so nearby?
Regards,
Bruce Bates
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Excellent! I have high hopes that this will work out. I will try it tomorrow. 6V I can get using 5 batteries, which I might be able to fit. The only slight trouble is that my motors only want 3V, but I can solve that with a resistor I suppose, with a little bit of power loss. (Or maybe it's OK to give them 6V as long as it's at a 50% duty cycle or less?)
Anyway, thanks a lot - I will experiment with this approach.
I am beginning to think that there is just no way to do this without separate batteries. No matter what I try, it seems like having a motor in the circuit makes the RF receiver not work any more [noparse]:([/noparse]
·
Is there any way for you to post a schematic of your setup?· A few moons ago, I·worked in·the robotics research and development department for·a prosthetic facility.· With prosthetics, often times you only get one battery supply (typically 6V) to work with.· ...Although we didn't couple motor operation with RF, we were looking at Myo-Electric signals in the 10's of microvolts to control the motors which can be just as tricky.· Most of our interference was 50Hz/60Hz/100Hz/120Hz electrical interference which could be filtered out fairly easily with a simple notch or High pass filter.· What I'm getting at, is that maybe in your particular circumstance, a·filter might be in order.··
·
A few questions first:
·
What is the base frequency of your PWM signal to the motors?· Is it fixed, or does it vary depending on duty cycle?
How much noise is there on the power line to the RF unit?· Can you scope this to visually identify any frequency components that could be filtered?
How much current do your motors demand?· What is the coil resistance of the motor outside of the circuit, and how many·volts are you providing to the motor?
Is the case of the motor metal?· If so, is it connected to the common ground?· How long are the motor leads?
·
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
> Is there any way for you to post a schematic of your setup?
I will try, but it might take me a few days to make.
> What is the base frequency of your PWM signal to the motors? Is it fixed, or does it vary depending on duty cycle?
I don't know exactly, but you can see the code I use in my first post in this thread. Given the BS2e advertises itself to be ~4000 instructions / sec, I am guessing each loop through my PWM code completes in what, maybe 2-4 mS? I don't know exactly though. Of course given how it works, there could be longer periods of pure on or pure off since several trips through the loop could send the same value. I'm using this loop because I want to control the speed of two motors independently, and I couldn't figure out how to use the PWM command for that. *However*, I think I have eliminated the low PWM frequency a factor. If I control just one motor with the PWM command (higher freq) then I still see the same trouble. That implies to me that my homegrown PWM isn't the trouble.
> How much noise is there on the power line to the RF unit? Can you scope this to visually identify any frequency components that could be filtered?
Unfortunately this is my first real electronics project and I don't own such fancy things [noparse]:)[/noparse] I really wish I had one though, I'm sure it would help a lot.
> How much current do your motors demand? What is the coil resistance of the motor outside of the circuit, and how many volts are you providing to the motor?
On the order of 150-200 mA at full tilt. I am just running one motor right now until I get that much working. I have tried two separate types of DC motors in case it was something odd about one in particular. They are just small inexpensive hobby motors, nothing fancy. The coil resistance (if I measured it right) is 1.5 ohms - does that sound right? Anyway the 'raw' 5V PWM output goes through a diode and a resistor so the motor will never see more than about 3 or 3.5 V. And it's PWMed from there to an effective voltage of 1-3V.
I have noticed that the level of interference varies with the motor speed. If I set the PWM so the motor barely turns, I get a few feet of range. Once the motor is spinning at full tilt, the range is literally less than an inch. (And like I said before, if the motor is not turning the range is farther than I can get away from it in my house - at least 30 ft with no trouble whatsoever).
> Is the case of the motor metal? If so, is it connected to the common ground? How long are the motor leads?
Yes, case is metal and is grounded. I have the motor attached via clip leads to my circuit, and those are maybe one foot in length, plus the motor cable itself, so maybe a two foot total distance. I tried extending that to 4+ feet (to avoid RF effects) but no difference.
Also, I just came back from radio shack with a fistful of capacitors - everything from 0.1uF to 1000uF. I've tried putting them between +5 and GND in various places in the circuit, but no difference that I can tell. I tried putting the 1000uF one across the motor, hoping that it would help to "buffer" the current spikes from the PWM and even things out a little to avoid having a square wave on the voltage supply. But it doesn't make any perceptable difference to the interference - grrr. The voltage source is 4xAA NiMH, high quality Sanyo ones.
I'll try to work on a little diagram. It might be a rough one.
I suspect what you're seeing is the EMI from the "cheap" brush motors. The faster they go the more they arc, the more they arc, the more EMI is produced. This very FIRST thing I would do is to make those motor leads as short as is humanly possible, or replace the wire you have with shielded cable. At least the leads won't act so much like an antenna then.
The suggestion above does NOT negate any filtering you've tried to do to date. Both may be required, or possibly even replacing the motors with those that are more fully encased.
Regards,
Bruce Bates
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I agree with Bruce, need to tackle the source first. A capacitor across the leads of a motor can greatly reduce EMI and also save your contacts from eventually burning up. ...BUT, you should NOT use a large capacitor here as it can overload the transistor driving your PWM signal. Instead, a small .1uF capacitor is more than sufficient for this.
Here is a test:
Forget about the PWM for the moment...keeping the same common ground that you currently have, run the motor directly from your power source or at the 3V tap (after the 2nd AA battery before the 3rd AA battery) on your 4-AA battery pack.
How does the RF receiver fair in this situation?
Note: the Capacitor and Diode should be as close to the motor terminals as possible.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
I will try Beau's experiment tonight and report back
I guess if EMI is a problem that I need a ferrite core around the motor leads?
See if you can follow this bit of troubleshooting technique that I've found helpful in reducing (but not eliminating) EMI or RFI problems. You can spend a lot of time and money filtering things until you're blue in the face, spend all sorts of money on upgraded motors and the like. The "key" to the whole problem is locating the actual source of the problem. If you don't eliminate the SOURCE, you're dead in the water, and all the filters and circumventions may help, but you want to KILL this gremlin if you can.
The following is nothing more than a take-off on an El Cheapo Faraday cage. Let's presume, just for the moment that the EMI --> RFI as seen by the RF transceivers. Wrap a bit of waxed paper, or other insulating material completely around one or both of the motors. This will prevent any short circuits from occurring in the next step. Next take some aluminum foil and wrap it tightly around the waxed paper. Now re-run your tests and see if things are better, worse, or about the same.
If nothing changes, or thing get worse, then I'd consider a ferrite bead or similar method of attack next. If things get better consider how you might encase the motors (leaving room for ventilation) in some sort of metal can. Often older 35 MM film containers made of metal can be used in such an application. You could also use the plastic ones if you line the inside with foil, followed by a layer of insulating material. The can will keep the foil and the "insulation" away from the motor if the motor is properly centered within the can.
Regards,
Bruce Bates
Post Edited (Bruce Bates) : 1/8/2008 3:53:33 PM GMT
*Edit* I've made a little ascii diagram but it doesn't really survive the proportional font on the forum. I'll try to make a graphical one instead.
Post Edited (jhankle) : 1/10/2008 2:41:25 AM GMT
I've replaced my original 2N2222 transistor with an ZTX689B. This makes my receiver work fine *as long as* the motor is not connected as above. I can stub out the motor with a small resistor, and everything is fine. Putting the motor back makes the receiver range go to mere inches.
I've also tried using two separate transistors in a Darlington type arrangement, but that doesn't help. I'm about to try the foil experiment suggested by Bruce.
*Edit* shoot, I screwed it up - the +5V input to the receiver from the BS2e goes to pin 3, not pin 2. But that's not the main point anyway [noparse]:)[/noparse]. Might be some other mistakes too - this is my first time trying to make a diagram of my own. I used gschem to do it.
Post Edited (jhankle) : 1/10/2008 2:13:11 AM GMT
It's all about electromagnetic prophylaxis. Do I sound like a high school health teacher yet?
-Phil
From another post, you indicate that your PWM is about 500Hz .... The disadvantage of having such a low frequency, is that in order to filter it, the filtering components must be larger which unfortunately translates to physically larger as well. Not out of reach, but larger none the less.
What you have been experiencing, I know all to well. The RF receiver simply wants a clean supply, and that is understandable. Although it may not seem apparent, the motor can inject "noise" not necessarily RF noise, but ripple noise on the power supply that can affect the RF unit. What you have suggested somewhat confirms this, but one more experiment would make me absolutely certain that this is the issue.·
Experiment:
Can you supply the RF unit with a separate power supply while the motor is running with another power supply?· (remember to keep the grounds connected)
·
If you want to try a filter, it would need to be at the supply to the RF receiver, and for the frequency at hand it would involve a 1000uF capacitor and a 500uH inductor (<-I can assist you with this). Properly placed, those values
would attenuate a 500Hz ripple on the supply line by -12dB
Reference Post:
http://forums.parallax.com/showthread.php?p=700482
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 1/10/2008 6:01:29 AM GMT
Works fine!
I also tried a similar one: I supplied the C and E of the transistor controlling the motor with a separate power supply (gnds connected together). That also works perfectly! Not quite what I want, of course, but it works
Post Edited (jhankle) : 1/11/2008 2:03:05 AM GMT
Thanks - I guess I'm not a *total* beginner because when I was in high school (back in the 60's) we learned a little bit about electricity, so I was already familiar with resistance and voltage and current and power. What's new to me is all the fanciness with microcontrollers and transistors and PWM and all. But wikipedia has been really helpful - every time someone says something and I don't know what it is, I can go look it up
I did put a capacitor right by the motor (within half an inch) and another on the other end of the motor leads, by the transistor that controls the motor. I don't have a ferrite core yet to try that.
-Phil
Ok.... this tells me that it's not so much an RF issue as it is a noise issue on the power line....· It is not uncommon for an RF receiver to require an exceptionally stable power supply to differentiate an incoming signal.·
·
Phil's suggestion will be fine for increasing the·PWM base frequency, but I honestly think that the brushes on the motor are the culprit, and not so much the PWM signal.· The motor brushes·inject a substantial amount of noise to the power supply, so with a higher PWM base frequency you would end up with similar problems.· Most likely, the motor noise is "washing out" any signal the RF receiver is trying to detect.· Without a scope connected to your power supply so that it can be monitored, it's·difficult to tell.
·
The answer is better quality motors, or a more robust filtering on the supply voltage to the RF receiver.·· What's really hurting you at the moment is the limited overhead of your supply voltage.
A higher supply voltage·would allow·you to place a diode/capacitor filter to the input of your RF receiver eliminating most if not all of your problem.
·
Although the noise affecting the RF receiver is most likely going to be much higher than 500Hz, as the power supply injected frequency is going to be a function of motor RPM and number of armature coils inside the motor.
·
I had mentioned earlier about using an inductor/capacitor filter combination... I am going to change that.· Here the low frequency filtering of 500Hz "might" benefit·from using a simple RC filter.
Using a 5 Ohm resistor and a 1000uF capacitor you·indicated that·you already have,·the roll-off frequency (-3dB)·is about 32 Hz...
other frequencies of interest that could affect the RF receiver:
-5dB at 50 Hz
-7dB at 60 Hz
-24dB at 500 Hz
·
···
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Yes, I have a 1000 uF capacitor, and I know I have a 10 Ohm resistor. I might not have 5 Ohm but I can go buy one.
I don't yet understand the circuit that you drew. I'm not asking for an explanation here though - you've been more than helpful already. I just found the wikipedia page for "RC circuit" and am reading that. It has a lot more math in it than I can remember any more I'm afraid, but I'll keep digging into it and maybe I will be able to make some sense of it eventually. In the meantime I will try to hook it up like that and see what happens.
thanks!
The circuit that I drew, was basically for simulation... it simulates a 5V supply with 100mV of noise... it's RMS, so the scope shows a Pk-Pk voltage of 280mV (RED) which is actually a HUGE amount of noise.
The output voltage after the 5 Ohm/1000uF capacitor attenuates this noise down to about 20mV Pk-Pk (BLUE)
The 10 Ohm is probably ok, but if you have two of them just wire them in parallel to get 5 Ohms.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
(1) Made a faraday cage by enclosing the motor in wax paper and then AL foil. This didn't have any effect that I could tell.
(2) Made a filter such as in Beau's diagram, right before the receiver. It uses a 1000 uF capacitor and two 10 Ohm resistors in parallel. It actually took me a little while to understand what to do. At first I was thinking that I was supposed to use the voltage across the resistor in Beau's diagram as the supply to the receiver (liked red=pos, blue=neg), but that both didn't make sense to me, and didn't actually work [noparse]:)[/noparse]. But then after a little thinking, I figured that the blue wire in the diagram is the filtered positive voltage, and the red is the unfiltered positive voltage, so what I should really do is tie the +5V input on the receiver to the filtered positive voltage here, and the other input to ground.
Doing that made a definite difference. Without the filter, with the motor running, the range is like 1-2 inches. After adding the filter, it seemed at first like my range went up to several feet. Not tens as with a separate supply, but around 5 maybe. But it's strange - it seems a little intermittent, like sometimes it works from 5 feet, and sometimes still has to be within 6 inches. This is still an improvement, since previously it *never* worked from 5 feet.
Does it make any sense to add another such filter as a second stage to this one? I don't have another 1000 uF, but I do have several 100 uFs.
You still have other filtering techniques applied right?· diode/cap across motor terminals, etc.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.