Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: Bajanick on June 17, 2008, 07:44:59 PM
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I am considering using two of these and theyre RC control modules for my R/C Rover using wheelchair motors.
http://www.diverseelectronicservices.com/html/mc7_motor_controller.html
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What's the stall current of your motors and at what voltage do you plan on driving them?
- Ben
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Dont know the stall current exactly but I would think it must be pretty high because they are wheelchair motors.
I am going to run them at 12V. They are used at 24V but I rewired them and tested them at 12V and they still have plenty or power.
Not sure of the best method of testing the stall current.
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Haven't used it but this may be a cost reduction assuming your stall current isn't over 25amps per motor channel.
http://www.dimensionengineering.com/Sabertooth2X25.htm
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You can measure the resistance of the motors using a multimeter, then use I = 12V / R to figure out the stall current.
- Ben
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The controllers from diverse electronics have a card that you can use to hookup an RC receiver and control the motors with a standard RC controller.
I had this silly idea of making some kind of fuse strip with several different ratings of fuses in oder, hookup a battery and lock the motor shaft, whichever fuse blew would give a close amp rating.
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So you don't own a multimeter or have access to one?
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LOL, I do have one. :)
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So then the resistance of your unpowered motor would be...?
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So then the resistance of your unpowered motor would be...?
I just checked, Each motor is .6 ohms. 12V divided by .6 is 20. So the stall current is 20 amps? Is that correct?
???
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Well, that gives you a best-case estimate for the stall current. You know that it at least more than 20 A. Unfortunately, you can't say much more than that because the resistance you measured is so small, which means it might be dominated by the resistance of your multimeter or its wires themselves. It's possible your motors have a resistance of 0.1 and the other 0.5 is just coming from the multimeter and its wires.
To get a more accurate measurement you need to put a known current through the motor and measure the voltage drop across it while it's not spinning. I'm guessing you don't have a power supply capable of functioning as a fixed current source?
- Ben
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No, I dont have such a device.
I would have thought that wheelchair motors would have much more than 20 amp stall current, I was thinking somewhere near 80 or more.
The author of a book I am reading states that he has used these controllers with wheelchair motors with no problems.
The wheelchair electronics has a built in breaker that I kept installed and I also plan on installing fuses.
Is there another way of getting a more accurate stall current reading?
Thanks :)
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My wheelchair motors may not be the same as yours. My took 11 amps testing at 12 volts. stall while being feed thru a MOSFET
No, I dont have such a device.
I would have thought that wheelchair motors would have much more than 20 amp stall current, I was thinking somewhere near 80 or more.
The author of a book I am reading states that he has used these controllers with wheelchair motors with no problems.
The wheelchair electronics has a built in breaker that I kept installed and I also plan on installing fuses.
Is there another way of getting a more accurate stall current reading?
Thanks :)
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My wheelchair motors may not be the same as yours. My took 11 amps testing at 12 volts. stall while being feed thru a MOSFET.
How did you measure this and what was your power source? What was the on resistance of your MOSFET? The reason I ask is that this sounds lower than I would expect for a wheelchair motor.
- Ben
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Look into the motor drivers used on battlebots . . . they are often rated 30A-80A . . . but can be a bit pricey . . .
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Well I went ahead with a fuse test starting with a 3 amp fuse, then 5, 10, 15, 20 and 30.
The 20 lasted about 3 seconds but the 30 did not blow at all even after 10 to 15 seconds of keeping the motor locked.
There is a built in breaker on the wheelchair chassis that I left in place but it did not blow even when I had the motor stalled for a long time, I hope the motor did not get damaged, It seems to still run ok. The breaker must be somewhere near 50 amps because it was wired for 24V operation and I rewired the motors and batteries for 12Volts.
The battlebot controllers are great for an ROV because of the direct RC hookups but yea, they are pricey.
I have vantec 22 but it is only rated to 20 amps and I dont want to kill it. Looks like I will have to rig up something with 30 amp auto relays and forget about speed control :'( or postpone this project and start something else. ???
These are very nice but are only rated at 25 amps. http://www.robotmarketplace.com/products/0-SYREN25.html
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If your motors are less than 30 A stall (which it sounds like they are), you could consider using a TReX dual motor controller (http://www.pololu.com/catalog/product/777). It works up to 16 V and can control two independent, bidirectional DC brushed motors and one unidirectional DC brushed motor, delivering up to 30 A peak and 13 A continuous per bidirectional channel (and 15 A continuous for the unidirectional channel). The bidirectional outputs can be combined to deliver a continuous 25 A to a single bidirectional motor. Addition of heat sinks can increase the continuous current.
Out of the box it will function as a 5-channel ESC with optional channel mixing. Just plug your RC receiver channels into the channel inputs and it will drive the motors as commanded by your RC transmitter. It also has the unique feature of being able to switch between RC and serial control on the fly, allowing you to seemlessly mix autonomous and human control of the motors. When the master interface is RC or analog, input channel 5 determines which interface (serial or RC/analog) is in control of the motors. When serial is in control, the serial control source can request the values on the input channels and use them to make decisions on what to do with the motors.
Basically, the TReX should be able to do for you what you would need two MC7 motor controllers and an RC interface unit to do, all for under $100. This, of course, depends on how much continuous current your application will require and how cool you can keep the motor driver ICs, but it sounds like you'd be able to make it work with a TReX.
To give you one idea of the scale of things this motor controller has been used to drive, you can take a look at Probotics America (http://www.proboticsamerica.com/), which uses the TReX to drive its RC educational robots. We will be coming out with an even higher power, higher voltage version at some point, but it's not on the immediate horizon.
- Ben
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Thanks for the info.
That looks like a good controller but with only 13 amps continuous, wouldnt it (or a fuse) blow out if the rover got stuck or hit an object?
I have some other motors that I think these controllers would pair up nicely with for another project.
I have used a pololu controller before and it worked good.
Thanks.
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The VNH2SP30 motor drivers used by the TReX have thermal shutoffs that protect them from overheating, so it would probably be ok if the rover got stuck in the sense that it would cut power to the motors once the drivers got too hot. The drivers would be ok again once they cooled down.
Your application would definitely be on the edge of what the TReX is designed for, which is why you would probably need to modify it by adding heat sinks or some other way to cool the drivers if it's something you want to try.
- Ben