Inverting the direction of a Motor

[willson.thomas]

I know this has probably been answered several times elsewhere in the forums, but I can't for the life of me find where.

I am trying to reverse a motor in software. I have two motors connected to a single axle and want to reverse one of them. I want to use the y axis of joystick no. 2.

I realize that the same thing could be accomplished by reversing the + and - wires on one of the motors but IMHO it is better to do this in the code.

Thanks,
Tom

[GRS]

pwm01 = 255 - pwm01

[willson.thomas]

Thank you very much. Knew it should be pretty simple to do.

[GRS]

No problem =)

[eugenebrooks]

An error here and you could release the magic smoke...

Eugene

[Mr. Lim]

Originally Posted by willson.thomas

I know this has probably been answered several times elsewhere in the forums, but I can't for the life of me find where.

I am trying to reverse a motor in software. I have two motors connected to a single axle and want to reverse one of them. I want to use the y axis of joystick no. 2.

I realize that the same thing could be accomplished by reversing the + and - wires on one of the motors but IMHO it is better to do this in the code.

Thanks,
Tom

These might seem unrelated, but be sure to calibrate your speed controllers when you do this.

The reason is, the Victors don't seem to behave exactly the same backwards and forwards out of the box. We've noticed that setting a PWM to 178 and 78 don't produce close to the same speeds in opposite directions... until you calibrate the Victor.

[Tom Bottiglieri]

PHP Code:

pwm01 = p2_y;
pwm02 = ~p2_y; 


This flips all of the bits, so realistically it does the same as (255-p2_y).

[Alan Anderson]

Originally Posted by GRS

pwm01 = 255 - pwm01

That should be 254, not 255. The valid range of pwm values is 0-254, and the "neutral" value is 127.

Originally Posted by Tom Bottiglieri

PHP Code:

pwm01 = p2_y;
pwm02 = ~p2_y; 


This flips all of the bits, so realistically it does the same as (255-pwm01).

True, but that's not the desired function.

Originally Posted by SlimBoJones

These might seem unrelated, but be sure to calibrate your speed controllers when you do this.

The reason is, the Victors don't seem to behave exactly the same backwards and forwards out of the box. We've noticed that setting a PWM to 178 and 78 don't produce close to the same speeds in opposite directions... until you calibrate the Victor.

Your example numbers aren't quite right. 178 is neutral plus 51, and 78 is neutral minus 49.

But you're right about the problem -- the Victor factory calibration is a little off from the IFI pwm outputs, with its actual neutral point at more like 132. That makes your forward value at plus 46, and your reverse value at minus 54...until you calibrate the Victor.

[Tom Bottiglieri]

Originally Posted by Alan Anderson

True, but that's not the desired function.

How is it not? The problem seemed to be very clear. There are 2 motors running on a shaft. One needs to run in the opposite direction of the other. They wanted to attack the problem in software.

[Jimmy Cao]

I guess it's already been posted, but the following should also work.
(all words in all caps are alliases, as I do not actually know the names of all the values)

PWM_LEFT_MOTOR = STICK_FOUR_X;
PWM_RIGHT_MOTOR = 254 - STICK_FOUR_X;

[T3_1565]

I think switching the wires is the best way to go IMO, just because you don't have to worry so much about calibrating, and any wrong software will do some damage to the motors for sure.... but everything up there seems good lol I would just pwm01 = 254 - pwm01 like was said previous, if you really have a problem switching wires

[JesseK]

Hmm, nevermind, it appears the Victor Calibration procedures are in the IFI manuals.

[Mr. Lim]

Originally Posted by Alan Anderson

Your example numbers aren't quite right. 178 is neutral plus 51, and 78 is neutral minus 49.

It's force of habit. We do all our logic on signed chars by subtracting 128 from the unsigned chars. It makes us off by 1 here and there, but we don't risk over or underflowing on endpoints.

Slightly off-topic, since the new controllers were introduced in 2004, I haven't had an issue with the full 0-255 range on PWM outputs. 255 seems to be a valid value, has anyone had any issues?

I haven't hit 255 on the OI analog inputs though.

[basicxman]

*sigh* that's what i love about H-Bridges, no confusing 0 to 255 just 0's and 1's

[Mark McLeod]

Originally Posted by SlimBoJones

Slightly off-topic, since the new controllers were introduced in 2004, I haven't had an issue with the full 0-255 range on PWM outputs. 255 seems to be a valid value, has anyone had any issues?

I haven't hit 255 on the OI analog inputs though.

Any 255 set in the user code is converted by the Master to a 254 before it goes anywhere. So using 255 doesn't cause any harm, but it really becomes a 254 as soon as you hand it off via Putdata().

You'll never see an OI value of 255 arrive at your user code, no matter how hard you try.