DiddyBorg motor voltage

Submitted by lars_the_bear on Tue, 02/02/2016 - 12:47

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Hi

I note that the DiddyBorg takes 10 x 1.5V cells, but the rated motor voltage is 6V.

How are these cells wired up? Are they just in series to give 15V? If so, what do the extra 15-6=9 volts do? Is there something else in the DIddyBorg setup that needs > 6V? I note that the battery pack has a 9v-style connector, and is described as such in your documentation, but is that just for assembly convenience, or does this battery pack actually produce 9V somehow?

If the battery voltage exceeds the motor voltage, do I have to make provision in my code so ensure that the output of the PicoBorg Reverse does not exceed 6V?

Sorry if I'm missing something.

Best wishes
Kevin

piborg's picture

piborg

Tue, 02/02/2016 - 13:38

The batteries are indeed connected in serial in the battery pack.
Typical rechargeable AAs are only rated for 1.2V, this gives a 12V supply overall from the battery pack.

The BattBorg needs at least 7V to work correctly, otherwise everything else will operate fine at 6V.
The way we ensure the motors do not exceed 6V is the code examples limit the maximum output from the PicoBorg Reverse.
The reason for the 12V supply is that it gives us more total power, allowing DiddyBorg to run for longer between charges.
Roughly speaking the PicoBorg Reverse can run 6V motors for twice as long when using a 12V supply with the same charge capacity, usually measured in mAh.

We work out the maximum power for the PicoBorg Reverse as follows:
Vmotor ÷ Vbattery = Powermax
for DiddyBorg this gives us:
6V ÷ 12V = 0.5 or 50%

As for the 9V style connector it is used to allow for different size battery packs to be used and for easy assembly of the robot.
In theory DiddyBorg will work with a rechargeable 9V style battery pack, but he would not run for very long :)

lars_the_bear

Tue, 02/02/2016 - 13:56

OK, thanks. Since I'll be writing the i2c code myself, I guess some checking with a voltmeter will be required to ensure that I don't set values that blow up the motors.

Or is the relationship between the the PWM value set in the i2c register, and the motor voltage, a very linear and predictable one? That is, if the value can be set between 0-255, does that mean that setting 127 will give half voltage, or as near as makes no difference?

No biggy -- I've got a voltmeter ;)

piborg's picture

piborg

Tue, 02/02/2016 - 16:44

You are correct, the value set for the PWM is roughly linear.
127 should be aproximately 50 % PWM, this in turn would be aproximately 50 % voltage as the motors see it.

mihai.ci@gmail.com

Wed, 03/02/2016 - 14:09

I am also about to finish the setup for my Diddyborg, and I intend to use 10 AA batteries which are normally rated at 1.5v. Is there any chance that the motors can be blown if I messed something up with the Pi installation and the software doesn't impose the correct power limitations? Should I try turning it on first using a simple 9v battery just to make sure things are running ok?

piborg's picture

piborg

Wed, 03/02/2016 - 14:22

If you are using our DiddyBorg scripts they should limit the output to the motors for you.

By default they are coded for a 12V input as rechargeable AAs are generally 1.2V.
If you run them with 1.5V AAs (15V total) it will be fine, but the motors will run a little fast (7.5V instead of 6v).

You can change the scripts easily enough to run at the correct voltage by changing the voltageIn value to the correct number.
For example the default values are:

# Power settings
voltageIn = 12.0       # Total battery voltage to the PicoBorg Reverse
voltageOut = 6.0       # Maximum motor voltage

and with 10x 1.5V AAs it would be:

# Power settings
voltageIn = 1.5 * 10   # Total battery voltage to the PicoBorg Reverse
voltageOut = 6.0       # Maximum motor voltage

We recommend you use the correct voltage values as running the motors with more voltage will shorten their life and if too high or for too long it may cause them to get hot.

mihai.ci@gmail.com

Wed, 03/02/2016 - 14:58

Actually I have 12v motors. My worry was that if the picoborgrev is not correctly installed software wise, is there any chance the motors get powered directly (e.g. bypass the Pi and the motors start when connecting the battery pack directly).
Sorry if this may sound dumb, but I am really a novice with hardware and electronics :).

Thanks again!

piborg's picture

piborg

Wed, 03/02/2016 - 15:53

Generally speaking the PicoBorg Reverse should power up with the motors both set to off.

If you want to be certain that they will not do this you can remove the EPO jumper.
This will prevent any power to the motors until the jumper is fitted or a specific command is sent from the Pi to ignore the jumper state.

If you wish to remove the jumper reguarly or if it is in a hard to reach spot we recommend you attach the two pins to a toggle switch with some wires.

mihai.ci@gmail.com

Wed, 03/02/2016 - 18:41

Thanks again for the quick reply! So, if the EPO jumper is removed, then no power goes to the motors until it is placed back in. That is good to know.

Thanks,
Mihai

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