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Date:	Wed, 21 Dec 2011 21:13:42 -0500
From:	Ethan Swint <eswint DOT ramu AT verizon DOT net>
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Subject:	Re: [geda-user] Design suggestions...
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On 12/21/2011 12:11 PM, Rob Butts wrote:
> My application has two 12 v dc motors that I want to operate in both 
> directions with speed control. I have two potentiometers to determine 
> speed control for each motor. I have one push button and three to five 
> switches for state control.
>
> I initially didn't plan on speed control for each motor and I had a 
> PICKit 2 so I opted for the Low Pin Count development board with the 
> setup for the pic16F690 microprocessor.  The documentation for that 
> microprocessor says the pmw can be set up for one, two or four 
> channels but it has just one pwm module.
>
> The processor that microchip suggests has more pins and I'd have to 
> purchase more development hardware.  I have the code completed and 
> working for the five input state controller and the two adcs 
> monitoring the motor speed potentiometers.  Does anyone have 
> suggestions for speed and direction control of the two motors where I 
> don't have to purchase new development hardware?  I thought about two 
> micros in a master slave configuration but that would make development 
> and prototyping on my desk difficult.
>
> Thanks,
> Rob 
The four pins will be sufficient - you'll need a gate driver.  Put each 
motor in an H-bridge configuration, i.e. totem-pole two mosfets on each 
side.  The low-side transistors will be your direction selectors - turn 
on the left for CW, turn on right for CCW, low-frequency, run a GPIO pin 
from the MCU.  The hi-side transistors will take the PWM signals (2 for 
each motor, 4 total) - the gate drivers will have a boot-strap power 
supply to drive the hi-side gate.  I'd suggest a toggle switch for 
direction & running and the pot just to set the speed - otherwise 
finding 'zero' speed on the pot will be hard.  You can just take the max 
raw ADC value and scale that to your max PWM counter value, a simple 
multiply-by-constant.  PWM frequency of 15kHz won't annoy most people 
over 30yrs, but 20kHz would be better.

On your layout, keep the loop tight from +12V across the 2-FET totem to 
ground, bypass with a few hundred nF of ceramic caps so that the 
parasitic inductance on the PCB won't over-voltage the FETs and toast 
them - typically 10x capacitance of a FET's capacitance, given on the 
spec sheet.  Several caps in parallel will get their ESR lower so that 
they handle the transient more effectively, too.

The flyback current from the motor will go through the body diode of the 
FETs, but make sure you have sufficient time of zero PWM on the top when 
reversing, e.g. >50uSec.  If you accidentally turn on the top FET while 
the bottom is turned on (or the body diode reverse recovery), you can 
over-current the FETs and toast them in one PWM cycle.

No capacitors across the motor - you effectively have two mismatched 
voltage sources in parallel and the current spike will kill the FETs.  
No resistor in series with the motor is needed, either - that just 
wastes energy.

-Ethan

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