DC motor to microcontroller electrical problems

Hi, Josh. There are a number of possibilities here, and I'm not really sure what advice to offer to start out with.

As a WAG, I'd guess you've got either a conceptual problem or wiring problem, and have accidentally hooked up the motor so the 12V is getting into the EVB through the motor coil or one of the signal lines. Unless there's optoisolation of the signals going to and from the motor, you must have the GNDs connected together. Potentials only exist from a reference voltage, and if you've got two power supplies, you have to somehow connect them together for voltages from one to achieve potentials relative to the other.

Is it possible you might be able to post the schematic of your circuit? That would give a lot more information.

Cheers Chris

Thanks for the reply Chris. I'll try to get a schematic of the circuit up. It's all just straight wires between the motor and EVB but I'll create one and post it up. As for hooking up the grounds, what you say makes perfect sense. What I was confused about was the separate GND banana jack on the bench power supply. I have a +/- output from the bench power supply. I'd think I just wire the "-" output to the ground on the EVB and ignore the GND banana jack but I don't know. Can you clear up this point while I work on creating the schematic?

Thanks.

OK, Josh. The question of "ground" is one of the perennial topics here, and if you google this newsgroup, you'll see the answer many times over.

Basics for your application is that the thing you call "GND" on your schematic is actually circuit common, the reference voltage from which all other board voltages are measured. This is the - end of the power supply, I'd guess. Actual cold water pipe ground (the green banana jack on the power supply) is connected to the round plug on the line cord, and the chassis of the power supply. You don't need it at this point, and your circuit will certainly work OK without it.

Actually, it might be a bit of a problem, if your motor 12VDC power supply has any leakage current to ground (sometimes a problem with some cheapie power supplies). Best to just set up the negative terminal of your 5VDC power supply with the negative terminal of the 12VDC supply, connect that node to your circuit "GND", and be done with it.

Hope this has been of help. Look forward to more information. While you're at it, you might want to specify the EVB and also the motor setup. Any web links would be helpful.

Good luck Chris

Chris, I appreciate all your replies. Regarding the ground, I have found myself reading many threads on it in the past when I was trying to get to the bottom of "ground loop". Anyways, I think that part is clear to me now. Here's some information on the motor I'm using.

The data sheet is here:

It is an NT DYNAMO BRUSHLESS DMC4336B10003 CL B 36 VDC 2.50 Amp motor. If you decode the model number based on the data sheet linked above, I have the "digital" one which is quite confusing the way it is explained. The motor definitely spins as expected when I feed it a square wave from a function generator, i.e. stopped at 50% and increasing speed either side of 50% in different directions.

The evaluation board I'm using is the MPC5554 Demo from Axiom Manufacturing:

Can you suggest a program to use to make an ascii schematic? I really just plan on making a little block diagram type thing showing how the motor is connected to the EVB. Anyways, hopefully the above information helps. In the meantime, I'm going to try to figure out how to create the schematic.

Thanks,

Josh

Andy's Ascii Circuits

OK, Josh. I'd guess we're getting somewhere (after a fashion). Here are some initial impressions:

• Your Hurst motor has a two pin power connector (36VDC nominal) and a digital connector. The motor docs aren't clear on any connection between the two, but I can tell you for sure that, as with most computer-controlled real-world electronics, you either make sure the computer is powered up first, or do the design in such a way that you won't have these problems. Since you've got "spam in a can" (set board, motor), I'd recommend that you use a relay (driven by the SBC) to turn on and off the 36V for motor power.

• I'd also be using one of the control signals (Run/Stop), but not relying on this for safety or overriding motor control. Keep the safety circuit separate from the uC as much as possible.

• You mention that you've got an enable signal tied directly to the +5V of your SBC. That might not be a good idea. You might want to tie that to an output instead.

• Another thing you might have concern for is the logic level of your signals. I'm not sure from a cursory glance at the schematic about the output voltage level of your signals, but the motor docs show that they want TTL-compatible signals. I'd actually guess this is one of the first places you should look for the source of your original issue. If your outputs are 3.3V, and they're required to source current, they might not be getting up to logic transition level. Not only that, but usually motor control signals have pullups, so it's possible you might be getting current from the Hurst motor logic through those pullups (thus lighting the LEDs). If you can get away with open collector signals, you'll immediately eliminate any possibility of a sneak path for current getting into your 5V power supply from the motor control pins.

• Since there's no indication on the motor docs that the motor control signals are optoisolated from the motor voltage, you'd have to assume they're not. But you also can't just assume the - of the 36V is the same as the "Return" of the digital signal. You might want to arrange to have the 36VDC supply floating (no GND connection or any direct connection to the 36V power supply). Any connect between the 36V and the logic common might be made inside the motor, but it won't have any effect on ground loops that way.

I hope you've at least got a start on places to look. All of this interface stuff is pretty basic once you know it. Sorry I'm not familiar with your SBC -- this is all unfortunately very general advice.

Two other things. If this is a school project and you want to post again, please note whetehr this is a senior project and when it's due. Also, the documentation in the "overview" isn't sufficient in and of itself. You need to get in touch with the apps people at Hurst and ask for more info. Look at what came with the motor in the box, too. There's gotta be more documentation.

Good luck Chris

By the way, I'd start with this for your p.s. hookup:

| | Hurst |Power Supply 1 Digital | Connector | .-------. .--------------. .-----. | | | | | | | | | | | | | | | | | | o-------------o | | | +12 o-----------o o-------------o | | | | | o-------------o | | | | | SBC o-------------o | | | | | o-------------o | | | | | | | | | | | | | | | | | COM o-----o-----o GND | .------o4 | | | | | | | | | | | | | | | | | | | | '-------' | '--------------' | '-----' | | | Power | '---------------------------' Connector | .-------. .-----. | | +o----------------------------------------o1 | | |+36VDC | | | | | (Iso.)| | | | | -o----------------------------------------o2 | | | | '-----' | | | | '-------' | (created by AACircuit v1.28.6 beta 04/19/05

This looks as safe as anything else. Be sure both power supplies are isolated, don't connect the - of the 36V supply to the - of the 12V supply (at least to start with), and leave ground out of this for now.

Cheers Chris

Why does the computer have to be powered up first in most real-world electronics? What electrical issue does this avoid?

I have connected the run/stop signal on the motor to the 3.3 V output of the power regulator on the evaluation board. Someone else at work did this and I didn't think to try a 5 V output instead so I'll try that tomorrow.

This is actually something I'm working on at work. Unfortunately we are a small company and I don't have access to any real hardware experts. Unfortunately I have no more documentation on the motor, but I will call the company tomorrow to see what they say.

Here's a schematic of how I have been connecting my motor and EVB to this point:

| | Hurst |Power Supply 1 Digital | Connector | .-------. .--------------. .-----. | | | Evaluation Board | | | | | | | | | | | | | o-------------o5 | | | +12 o-----------o o-------------o6 | | | | | +3.3Vo-------------o9 | | | | | PWM o-------------o11 | | | | | | | | | | | | | | | | | | | | | | | | COM o-----o-----o GND | .------o4 | | | | | | | | | | | | | | | | | | | | '-------' | '--------------' | '-----' | | | Power | '-----------o---------------' Connector | .-------. | .-----. | | +o-----------------|----------------------o1 | | |+36VDC | | | | | | | | | | | | -o-----------------o----------------------o2 | | | | '-----' | | | | '-------' | (created by AACircuit v1.28.6 beta 04/19/05

Thanks,

Josh

Hi, Josh. Nice to know this is a real problem instead of just a school project.

As always, I rely on the product vendor for interface help. Glad you're going to be talking to the Hurst people. Try to keep them from talking you into the serial interface if possible -- that's just locking in a mystery solution. Since you've gotten this far, you should be able to do it, and they should be able to provide the docs you need. It helps to tell them something about the potential future volume of sales of your toy. If you let them know it's a one-off, they might be a little more reluctant to help. (I hope you're reading between the lines here ;-)

Looking at your diagram, I'd disconnect the jumper between the +36V minus side and the +12V minus side. As long as the 36V supply is floating, you should be OK.

Sneak current paths between SBC I/O pins and interface circuitry can be a problem. On power-up, they can lead to the SBC gate latching up (creating an SCR internally, which fries the chip). But even if it doesn't latch up, this sneak current going into the uC can interfere with the normal startup reset sequence internal to the uC. That can mean the uC doesn't come up in a known state, which can wreak havoc with your program. The uC chip is just not made to do that.

Good luck with your project, sir. Being the "one man gang" that gets it done can be very satisfying. You may get out late, but you frequently walk home with real job satisfaction.

Cheers Chris