Author Topic: Where Should I Go From Here?  (Read 721 times)

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Offline MastermimeTopic starter

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Where Should I Go From Here?
« on: February 17, 2013, 04:21:08 PM »
Hello everyone,

It's me again...  Sorry about all the questions.  I just ran into an issue that really dropped my spirits. I seem have to inadequate power from my motors.  It could either be  that I have miscalculated my torque (used SOR calculator)  .  It just seems to be very weak.

(2)Motors: 24v 400w each
Motor driver: Sabertooth 2x25
Gear ratio:  2:1
Robot weighs around 120 lbs

Video I recorded describing and showing the problem
VoltX Torque Test 1


If it's my tread system (which I think is very inefficient), what do you suggest I do?  I have invested a lot of money and time into this and I don't just want to consider it a failure and give up

Thanks

EDIT: This topic may be on hold for a little bit.  I first have to figure out if its the motors or my system
« Last Edit: February 17, 2013, 05:55:07 PM by Mastermime »

Offline waltr

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Re: Where Should I Go From Here?
« Reply #1 on: February 17, 2013, 05:57:23 PM »
My first guess is that the motor/gear output does not have enough torque.
But, more info is needed for a full analysis.

If you block up the robot so the treads do not touch the ground how well do the treads move?
How fast do the treads move? Post a Video.

What is the Maximum RPM of the gear motor output and the diameter of the pulley?
This will determine the maximum possible speed.

What were your calculations for the motor/gear requirements (a link to a previous thread is good).
Exactly what are the full motor specs, link the data sheet.

What is the motor's current draw when trying to move and when you are holding the bot from moving?
If way under the motor's stall current spec then either the batter or wiring are inadequacy.
If aclose the the motor's stall current spec then the output torque of the motor/gearing is too low.

Does the threads move freely? if they are too tight then this will rob power for the motors.


Offline MastermimeTopic starter

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Re: Where Should I Go From Here?
« Reply #2 on: February 17, 2013, 07:54:01 PM »
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How fast do the treads move? Post a Video.

Drivetrain Test 1

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What were your calculations for the motor/gear requirements (a link to a previous thread is good).

http://www.societyofrobots.com/robotforum/index.php?topic=15412.0

Quote
Exactly what are the full motor specs, link the data sheet.

Not sure if this will help http://www.monsterscooterparts.com/24vo400wamow.html
Quote
What is the motor's current draw when trying to move and when you are holding the bot from moving?


If  I could test that I would, but my multimeter fuses are rated for 10 amps only

Quote
Does the threads move freely? if they are too tight then this will rob power for the motors.


Not sure what you mean by threads.  If you mean the track, well its really tight, but it has to be in order for the cogs to engage with the drive wheel.  So that probably eats up a lot of power.   I am going to try and find tracks with teeth like a snowmobile track eventually
« Last Edit: February 17, 2013, 08:37:26 PM by Mastermime »

Offline jwatte

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Re: Where Should I Go From Here?
« Reply #3 on: February 17, 2013, 11:50:58 PM »
I would be interested in knowing whether the motor or the driver stalls out when you hold the bot with your hand. Or whether you are seeing voltage drop because of a weak battery.

Thus, the first thing to do: Measure the voltage across the motor when it is stalled! It may be you're seeing less than the nominal battery voltage.

Second: 24V times 25A is just barely above the rated 400W of motor power. The motor may want to draw much more than that temporarily, as the rating may be an average or sustained rating. If the controller chops the current, that will lead to less torque in the motor. Look for motor drivers that can give you more amps, and that have built-in current sensors, so you can get a sense of the current draw without having to have your multimeter in the loop!

Third: 2:1 doesn't sound like a lot of gearing, even with that smallish driving pinion. You may need that low gearing to get the speed you want, depending on the motor RPM/kV rating, but it may still leave too little torque to get the bot moving right.

Fourth: Hook up both treads/tracks, and drive both motors forward at the same time. If it's still simple to stop the robot, then perhaps you're getting too much loss in the tread system. If the first motor has enough oomph to get the bot moving, then the second motor should be able to add significant additional force when both are driving.

Fifth: Try getting some wheels instead, with a diameter similar to your driving pinion. See if that works better! That would indicate losses in the track/tread system.

Offline MastermimeTopic starter

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Re: Where Should I Go From Here?
« Reply #4 on: February 18, 2013, 11:11:46 AM »
Quote
Thus, the first thing to do: Measure the voltage across the motor when it is stalled! It may be you're seeing less than the nominal battery voltage.


When the motors are stalled, I measured 120 volts

Quote
Second: 24V times 25A is just barely above the rated 400W of motor power. The motor may want to draw much more than that temporarily, as the rating may be an average or sustained rating. If the controller chops the current, that will lead to less torque in the motor. Look for motor drivers that can give you more amps, and that have built-in current sensors, so you can get a sense of the current draw without having to have your multimeter in the loop!

The Sabertooth 2x25 has some current sensing in it and according to Dimension Engineering (just emailed them), the red LED indicates overcurrent.  Eventually if I get some funding, I will upgrade to the 2x50 or 2x60

Quote
Fourth: Hook up both treads/tracks, and drive both motors forward at the same time. If it's still simple to stop the robot, then perhaps you're getting too much loss in the tread system. If the first motor has enough oomph to get the bot moving, then the second motor should be able to add significant additional force when both are driving.

I would do this if I could, but I'm having some programming issues with getting both motors to go forward at the same time.   This is the thread that defines the problem http://www.societyofrobots.com/robotforum/index.php?topic=16548.30

Quote
Fifth: Try getting some wheels instead, with a diameter similar to your driving pinion. See if that works better! That would indicate losses in the track/tread system.

I was  thinking about this yesterday.  I just don't know where I could mount the wheels.

Offline jwatte

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Re: Where Should I Go From Here?
« Reply #5 on: February 18, 2013, 01:00:19 PM »
If you measure 12 volts when stalled then either the controller is chopping the current or your battery is too weak. (I'm assuming you're not actually seeing 120 volts?)
If you can't use both motors at the same time, battery is the most likely culprit.

Power is proportional to voltage squared, so 12V instead of 24V means 1/4 the available power. Get controllers and batteries that are up to the task and your robot will probably have lots of life in it!

Offline MastermimeTopic starter

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Re: Where Should I Go From Here?
« Reply #6 on: February 18, 2013, 02:14:24 PM »
Quote
If you measure 12 volts when stalled then either the controller is chopping the current or your battery is too weak. (I'm assuming you're not actually seeing 120 volts?)
If you can't use both motors at the same time, battery is the most likely culprit.

Power is proportional to voltage squared, so 12V instead of 24V means 1/4 the available power. Get controllers and batteries that are up to the task and your robot will probably have lots of life in it

I was actually seeing 120 volts when I stalled it.   The batteries I'm using are huge though!  Two 12v 55 Ah in series!

EDIT: I just received a reply from Dimension Engineering and they said I should start my gearing at 4:1 instead of 2:1

So I think that in combination with treads with "teeth" (proper term?) would really get this going
« Last Edit: February 18, 2013, 03:45:21 PM by Mastermime »

Offline jwatte

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Re: Where Should I Go From Here?
« Reply #7 on: February 18, 2013, 05:15:37 PM »
Quote
I was actually seeing 120 volts when I stalled it.

That sounds like a lot of EMI or kickback or similar. To know what it "really" is, you'll probably need some way of quieting that down (high-voltage capacitors across the motors?) and perhaps use an oscilloscope rather than a multimeter.

55 Ah sounds like a lot, as long as the batteries can actually generate a significant fraction of that at any one time! If it's car batteries, then yeah, they typically can, because they are rated to drive a very hungry starter.

 


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