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bcs99
Joined: 22 Nov 2003
Posts: 32
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| single supply motor control/drive circuit |
 Posted: Sat Apr 24, 2004 12:22 pm |
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I have two 24VDC 6A motors that I would like to control with a 18F452 and drive using MOSFET h-bridge circuits. I would also like to use only one power supply for everything. There are posts here saying that it can be done, but I am having trouble finding any information on implementation.
Any help would be greatly appreciated.
Bob |
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Neutone
Joined: 08 Sep 2003
Posts: 839
Location: Houston
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| Posted: Sat Apr 24, 2004 6:11 pm |
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| Microchip has application notes on this topic. Or at least very close. |
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valemike
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| Posted: Sat Apr 24, 2004 8:41 pm |
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Do you want to drive these two motors independently or at the same time, with identical PWM signals?
You might want to look into the motor control PICs that have up to 6 individual PWMs (PIC18F??331 (something like that)), where you can program in the deadtime intervals if necessary.
The 452 only has 2 (?) PWM outputs, and it might not be enough to drive two h-bridges, let alone one, if you are going to be firing each highside/lowside segment independently. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Apr 25, 2004 12:13 am |
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| Quote: | I have two 24VDC 6A motors that I would like to control with a
18F452 and drive using MOSFET h-bridge circuits. |
We did this years ago, by using two LT1158 chips and four MOSFETs
to control one 12v, 6A motor.
We used the circuit shown in Figure 17 of the data sheet:
"10A Full Bridge Motor Control". We ran our system from a 12v, 24A
power supply. The schematic shows that it can work from 10 to 30v.
We used a 16C77 running at 5v. We used the Sign/Magnitude Drive
logic shown in the schematic. It worked very well after we got
the bugs out. http://www.linear.com/pdf/lt1158.pdf
You will have to heatsink the MOSFETs. We used an aluminum
extrusion, "C" cross section, 6" (15 cm) long, 1/8" (3 mm) thick.
I recall that this was overkill.
Problems that occurred:
1. If too much load is placed on the motor, it will stall. The current
will go way up. Hopefully, this will be detected by the fault
circuit provided in the schematic and shut down the bridge before
the over-current circuit in the 24v power supply trips, and shuts
down the entire piece of equipment. You may have to adust
the sensing resistor for the LT1158's and maybe adjust the over-
current trip level in the power supply.
2. Do not abruptly change direction on the motor. Pause for a few
ms before reversing direction, so that you don't get some "back
emf", I suppose, which will be detected by the power supply as
an over-volt situation, thus causing the power supply to shut down.
3. Do not go with an excessively cheap power supply. Though, if
management comes up with this idea, it is their fault and you
are basically the teflon man in this case, though you will still
have to fix all the problems it causes, resulting in a lot of wasted
time.
4. Use the recommended components in the schematic. I think we
tried to substitute a different schottky diode than the BAT85, simply
because it was easier to get. But it had higher reverse leakage
current and didn't work as well. It worked fine with the
recommended diode. The "BAT" series are good diodes.
| Quote: | | I would also like to use only one power supply for everything. |
I'm not quite sure what you mean by this. You'll need to buy a
15 Amp, 24 volt power supply. Then you'll likely need some sort
of switcher to get it down to 5v for the PIC, and whatever other logic
you're using. If you use a linear regulator, even 100 ma will require
dissipating 1.9 watts. National makes the "Simple Switcher" chips
and has WebBench to help you design it. Digikey stocks the cheap
Talema inductors for it. |
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bcs99
Joined: 22 Nov 2003
Posts: 32
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| Posted: Tue Apr 27, 2004 10:18 pm |
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Sorry I have not got back sooner. Your replies are great!
As far as the single supply question goes. I have used separate power supplies (12V/5V) in the past. I wanted to know how to protect the logic supply from motor noise and power sags etc. Is there not some way to have them both on the same power and yet isolated from interference?
Bob |
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valemike
Guest
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| Posted: Wed Apr 28, 2004 11:34 am |
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I'm not a motor guru, but this is what i've seen...
When you have high voltage motors, you will want to separate them with optocouplers.
For low voltage motors, it's okay to mix your 5V ground and motor ground w/o optocouplers. You would filter out motor noise with the generic answer: capacitors.
I've seen 100pF caps in parallel with the motor, then ferrites in series to your driver IC. I don't have the specifics though.
And i think i heard before that if you truly want isolation from noise, then you would want to use optocouplers, and then use a 1:1 transformer so that your motor is truly galvanically isolated from your PIC.
Don't quote me on that though. I'm a programmer (or at least think i am), not a motor guy. |
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Ttelmah
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| Posted: Wed Apr 28, 2004 3:27 pm |
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| bcs99 wrote: | Sorry I have not got back sooner. Your replies are great!
As far as the single supply question goes. I have used separate power supplies (12V/5V) in the past. I wanted to know how to protect the logic supply from motor noise and power sags etc. Is there not some way to have them both on the same power and yet isolated from interference?
Bob |
I'd suggest you look at the little 'Traco' power modules, like the TMR1211. These comprise a tiny module (the example given is about 1" * 0.5" * 0.5"), containing a complete switching inverter, and transformer, with the better examples, having opto-coupled feedback regulation. The example given accepts 9 to 18v DC 'in', and gives 5v regulated out, at up to 400mA. The efficiency is good (typically better than 80%), the price is usually less than would be involved in making a simple supply, and the output is isolated from the input (allowing you to connect the 'common' rail to whichever side of the motor driver is most convenient. This sort of module is common inside all sorts of equipment, and allows a lot of simplification in supply design...
Best Wishes |
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