HPWM problem .. pls help!

[Samuel]

Thanks a lot anj!! I did that, i.e OSC 20 and HPWM 1 150 16000, but the motors were turning alternatively.. i.e one wud turn, and then the other and this keeps going in a loop as per my code. But they didnt turn together as they should be. (see code above) I then bumped the frequency down to 5000 and both motors worked well and at a good speed and went thro the subroutines well. i.e forward>halt>rev>halt>main. But now my robot is not going straight!!! it seems like both wheels are not going at the same speed or not starting at the same instant. Is this because HPWM 1 starts and is "followed" by HPWM 2 so there is a delay? but it doesnt seem to be just the start time .. i feel the actual speeds differ ... in the forward direction, one motor goes slower, and in the revers direction the other goes slower .. get it? playing with the frequency seems to affect this .. how do find the right frequency? Trial and error?

Thanks a lot for your help!

[Samuel]

someone help! HPWM duty cycle of 110 and frequency 1400 is giving me good results but my hbridge (L293D, motor driver) is getting HOT! and i'm hearing the high pitch sound!

If i increase the frequency to above 16000, the sound is gone but then both motors dunt turn together, they turn alternatively!

[NavMicroSystems]

The frequency limit for the L293D is 5kHz !!

see Datasheet !

[Samuel]

ohh thanks! i just checked the L293D data sheet and it doesnt talk about the limit of 5KHz .. But i guess it makes sense.

A duty cycle of anything below 100 doesnt push my motors. So the range for my duty cycle would be between 100 and 255? And the PIC with a 20Mhz OSC cannot go below 1221Hz, and my L293D doesnt go above 5KHz. So my frequency range would be between 1221Hz and 5000Hz? Much better .. now that i've narrowed down to something ...

I like ure signature "There are only 10 types of people: Those who understand binary, and those who don't ..."!!

[anj]

i just checked the L293D data sheet and it doesnt talk about the limit of 5KHz

My datasheet ( Unitrode ) doesnt give this limitation either.
16KHz gives a period of 62.5uS. Fig1 in the application info shows rise/fall times and these are less than 0.5us, so the chip shld cope with 16KHz easily

Re getting hot, at low duty cycle ( ie just getting started or creeping ), the motor is not very efficient and can draw a lot higher than normal current. For this, the datasheet is indispensable, as these chips do get hot and they do need thermal heatsinking.
I have done a lot of testing with "small" motors and PWM and found the best low speed performance was at very low period ( I go down to 28Hz for one motor ). I have never tested with larger motors so cant help there.
If you still want to use a 20MHz osc and HPWM but get a much lower output freq, you can run the signal through a deecade counter typ chip, and just pinch the relevant output that suits yr needs.
Or i just found this, never tried it but ut looks interesting
http://www.pbpgroup.com/modules/wfse...hp?articleid=6
Andrew

[NavMicroSystems]

L293D Datasheet

[anj]

Gday
Interesting.
I have an original Unitrode datasheet, and a quick search brings up the current texas one http://focus.ti.com/docs/prod/folders/print/l293d.html
Neither of these specify a 5KHz limit.
They mention doing their rise/fall time measurements at 5KHz, but thats all.
Based on the actual rise/fall times ( your link vs texas link ), they appear to be close, so i am at a loss to understand the difference.
I certainly have run my 293D at higher than 5KHz before.
Dunno