Voltage to each motor

Submitted by borg3757 on Tue, 06/07/2016 - 23:54

Forums:

Hi,

Have been searching around, but confused on a certain part. The motors that come with the diddyborg are 6v. I have noticed that adjusting the power settings significantly increase the speed of the diddyborg.

But what is confusing is if we leave the default settings as power out 6V, wouldn't that get divided by 6 since there is 6 motors? How is 6v output set in the script actually supplying 6v to each motor? Seems like it would be 6v/6 = 1v to each motor?

Thanks for any help.

borg3757

Wed, 06/08/2016 - 06:49

Checked with a multimeter. And makes sense now. Fresh set of dollar store aa's gave me 15.6v from the battery pack, and 15.0v on the picoborg screws. That dropped to around 14.4 v once I started diddyJoy.py.

At 6v out got 6v at every motor. Set voltageOut to 10v and got around 8,4v.

This was flipped on its back. I am sure under load the voltage drop is large.

I am sure some good motors and a good battery setup could make the diddyborg peel out. If I ever get that setup will be sure to make a video :)

Thanks.

piborg's picture

piborg

Wed, 06/08/2016 - 10:46

Glad to hear this makes sense now.

What happens when you add more motors connected this way is that the voltage stays the same, but the current draw (A) increases.

For example if you have three motors taking 6V 1A each then the total power supplied from the board is 6V 3A.
The robot will also has 3x as much torque as having a single motor.

The net result is that the batteries are drained 3x as quickly as with a single motor on each side.
This is why we need so many AAs :)

fgiraldeau

Fri, 06/10/2016 - 22:04

If I understand, the motors on the DiddyBorg are 6V, but the battery supplies 12V. To limit the power, the PWM is set at limited to 50% of duty cycle: 

# Power settings
voltageIn = 12.0                        # Total battery voltage to the PicoBorg Reverse
voltageOut = 6.0                        # Maximum motor voltage
maxPower = voltageOut / float(voltageIn)
PBR.SetMotor2(-driveLeft * maxPower)

However, the max voltage applied to the motor is nonetheless 12V. Can it contribute to reduce the lifespan of the motors?

borg3757

Sat, 06/11/2016 - 00:14

In the above example, it will be limited to 6v. Just out of curiosity, how are you getting 12v? maxpower is set to .5 and that is only under full throttle.

Currently running 12v out and get just over 9.5v on fresh batteries. Under load, the voltage would drop more. Not sure how much as I haven't found a way to test yet. But not concerned much with motor lifespan as replacements are very cheap. But even with that said I don't think what I am doing is any issue. The motors are still ice cold after 30+ minute runs. If they were running 24/7 at a solid 12v while rated for 6v it definitely would burn out faster.

There is about a 30-40% speed increase on the little obstacle course I made. At 6v out it is dead slow, and cannot clear this jump I made due to lack of speed.

They only sell the 83p motors with with wheels here. But pretty sure I may have found a supplier. Want to try the 82p also. But really curious how it would be possible to power them. Rechargeable aa are only 1.2*10=12v total. So how is the pi being powered? Perhaps the battborg has 1st dibs on power and the rest will go to the motors.

piborg's picture

piborg

Mon, 06/13/2016 - 10:48

The BattBorg shares the voltage with the motors, so that is not an issue.
The problem is that the BattBorg needs some current.

If for example we say the Raspberry Pi and its bits are using 1.5 A the BattBorg will need approximately:
1.5 A × (5 V ÷ 12 V) = 0.625 A from the batteries.

Since the BattBorg does some smoothing on its own the 50% of the PWM where the batteries are not being used by the motors is plenty.
Put simply the BattBorg has half of the battery power available to it minimum :)

On a side note this is why we limit the power to 95% on Red Edition, rather than the full 12V.

piborg's picture

piborg

Mon, 06/13/2016 - 10:32

It is true that the PicoBorg Reverse will be putting out 12V for 50% of the time and 0V for 50% of the time.

The reason this works fine is because the motor itself smooths the power supplied to it as long as the switching is fast enough.
In the case of the PicoBorg Reverse the PWM cycle is ~ 7.8 KHz, which is more than fast enough for these motors.

With the PWM like this the lifespan of the motors should be approximately the same as running them with a steady 6V supply.

borg3757

Mon, 06/13/2016 - 18:28

Thanks for examples.

Very interesting. Have to read up on pwm.

Have some 83p, and 82p motors coming. Hopefully. Would like to try 18650 instead of aa as well. Seems just 4 will give enough volts, and they can supply more than enough amps. But 6 makes a nice little semi cube that will fit nicely in the DiddyBorg. And then just use like a toggle or push switch to turn it on and off.

Will report back with some times and stats of what ends up happening. Thanks again.

borg3757

Sat, 07/09/2016 - 12:50

The 83p motors arrived last week. I was expecting more out of the box to be honest. Compared to the overvolted 6v motors it is slightly faster. Around 15% by my calculations. Though, you can feel the extra torque.

Checked with a multimeter and seeing 11.1v to the motor contacts so there is still much more potential. Right now I think I have reached the limitation of what the 10aa can do.

It just needs a little more kick :)

Subscribe to Comments for "Voltage to each motor"