piclist 2001\04\23\153642a >
Thread: Newbie - RC receiver to 2xPWM outputs with a 16F84
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face BY : David Cary email (remove spam text)



Dear people designing H bridge drivers:

You probably have already figured out that "servo" PWM is very different from
"motor drive" PWM.

"servo" PWM: typically repeats at 20 to 70 Hz; 1 ms lo for reverse; 1.5 ms lo
for neutral; 2 ms lo for full forward, hi for rest of period.
"motor drive" PWM: typically repeats at 20 KHz to 100 KHz; solid lo for off; 1/2
period lo, 1/2 period hi for half-speed; solid hi for full on.

Brian Reed <TakeThisOuTpiclistspamRemoveMEREEDONLINE.COM> on 2001-04-21 08:35:13 PM wrote:
> Subject:     Re: [PIC]: Newbie - RC receiver to 2xPWM outputs with a 16F84
...
{Quote hidden}

I agree that we only need 1 PWM and 1 DIRection bit per H bridge, but I don't
understand the rest of your description.

Bob Blick <EraseMEbblickRemoveMEspamTakeThisOuTSABER.NET> on 2001-04-21 08:55:27 PM wrote:
> if you design your
> H-bridge correctly(it only takes 2 small transistors extra) you can prevent
> shorts and get a braking channel for "free".



Do you have a pointer to a "correctly-designed" H bridge ?


The H bridge circuit I'm about to install on my robot has the DIRection bit go
directly to the "left" half of the H bridge (actually a small transistor,
flyback diode, and a SPDT relay) which drives one motor lead. The "right" half
of the H bridge is controlled by the PWM, and drives the other motor lead. (PIC
PWM output goes through 2 inverters to give it a little more power and convert
TTL to +12V, then that 0 to +12V PWM signal connects to a power PFET on the top
right and a power NFET on the lower right).

(Pardon the crude ASCII diagram)

          +12V   +12V
           |      |
           o Bhi-o[PFET
          \       |
           \--(M)-+
                  |
           o Blo-|[NFET
           |      |
           GND   GND

Bhi and Blo are both connected to the 0 to +12 V PWM signal.
The relay is controlled by the direction bit.
Not shown: 4 flyback Schottky diodes, connecting every motor lead to every
battery lead.

I haven't actually applied power yet, but I'm planning on
 Full forward: DIR = Forward, PWM = 0xff
 slow forward: DIR = Forward, PWM = 0x20
 stop: DIR = Forward, PWM = 0x00
 stop: DIR = Reverse, PWM = 0xFF
 slow reverse: DIR = Reverse, PWM = 0xDF
 full reverse: DIR = Reverse, PWM = 0x00.

Does this make any sense ?

(The relay in the left half H bridge is break-before-make. Right now, the 2
transistors in the right half H bridge are make-before-break, but the
transistion time is so small that I'm hoping it doesn't matter -- I *think* I
can tweak that circuit to get independent Bhi and Blo break-before-make if I
really need to, without any changes in software).

I was looking at High-side switching with 2 NFETs, but right now I think it's
simpler to use this PFET for motors that pull less than 10 A (when stalled):
$1.05 IRFR5305-ND (DPAK) -55 Vdss, -25 A Id P-CH   (Price from Digikey on 2000)

Here's some H bridge links I've found useful:

 http://www.4qd.co.uk/
 http://www.4qdtec.com/pwm-01.html
 Power H Bridge Driver  http://www.PicNPoke.com/projects/mdrive.pdf
 IR Design Tip DT-98.2 Bootstrap Component Selection For Control IC's
http://www.irf.com/technical-info/designtp/dt98-2.htm  (for H bridges and half
bridges that use N type transistors on the hi side).
 High-side switching
http://www.irf.com/_FKK4aEd/technical-info/appnotes/an-978.pdf  MOS-gate drivers
(MGDs), the bootstrap circuit, brushless motor driver

-- David Cary

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