Motors and Batteries

[SeagullOne]

Okay, in my continuing quest to make my robot lighter and smaller--but still not too small--I'm once again searching for lighter-weight motors and, of course, batteries.

First there were wheelchair motors. Too heavy. Next I tired bicycle motors, which were much better, but required 24V, which meant more battery power and I wasn't sure about the current draw.

I'm looking now at CIM motor gearboxes from Bane Bots. They have wonderful torque, are a little lighter than the bicycle motors I was planning to use, and look like they will definitely do the job.

As for the robot's battery, I have two 12V 10AH battery packs. I was planning on wiring them in parallel to make a 20AH pack, but that means adding more weight to the robots. I'm wondering if I can use just one of them and use the second pack as a spare. But, I don't know if both those powerful motors will draw less than 10 Amps, even with the gear reduction. The specs for the battery say the maximum recommended discharge should be no more than 13 Amps for a short time.

I've looked carefully at the specs for the motors and the batteries I have.

http://banebots.com/pc/P80K-nnnn-0005/P80K-433-0005
CIM Motor Gearboxes

http://www.all-battery.com/rectangularnimh12v10000mahbatterypackforsolarpanelemergencelightrobot11613.aspx

Battery pack (again, I have two of these).

My I'm aiming my robot to weigh 50 lbs or less. Any higher than fifty pounds will be unacceptable for me. So far, I've calculated that if I use one battery pack, my robot will have 25 lbs worth of components (motors, batteries, servos, laptop, etc...). That gives me roughly another twenty five pounds worth of hardware to hold it all together. The more lightweight the better, of course.

Okay, here's my big question. Is there a calculation or formula I can follow to estimate the current draw of a given motor (taking factors like torque, voltage, efficiency and such)? I want to make sure one 12V 10AH battery pack can/will support those motors with the gear reduction and the total load of the robot. I don't want to use two battery packs if at all possible, but if the current draw will be too much, I'm either planning to scale my robot down even further or just forget my limit and add about five pounds to the robot with a second battery pack.

Of course, I intend to mount a fuse should the current draw ever exceed the recommended load. I just want to be as sure as possible that this combination of motors and battery will suffice using any arithmetic out there.

[rbtying]

Do you have a link to the motor you will use with the gearbox?  The CIM motor they carry (http://banebots.com/pc/MOTOR-BRUSH/M4-R0062-12) has a 133A stall current, with 2.7A free run, so I'd guess that 10A may not be enough for these... or even 20A. 

[Soeren]

Hi,

Okay, here's my big question. Is there a calculation or formula I can follow to estimate the current draw of a given motor (taking factors like torque, voltage, efficiency and such)? I want to make sure one 12V 10AH battery pack can/will support those motors with the gear reduction and the total load of the robot.

Well, you linked to the battery and the gearbox, but not the motor and some of the crucial info is in the motors datasheet.

The battery page says "Standard discharging: 10 Amps (Warranted) and Highest discharging rate for short-time 13 Amps"
It also says "Weight: 3.65 Lbs Oz." and I guess the last "Oz." is just a mistake, so you won't add 5 Lbs by using two. If needs be, you can offset this by choosing light weight materials for the chassis.

[SeagullOne]

Hi guys! Thanks for the reply.

Here's the link for the motor. It's identical to the CIM motor used for FIRST robotics. I forgot to mention that the specs for the motor I listed in my first post was just for the motor and not the gearbox. http://banebots.com/p/M4-R0062-12

Thanks again, for clarifying that for me, Soeren, about the weight of the batteries. I guess they always felt a little heavier to me than 3.65 lbs...

Also, I found Admin's glorious robot energy and battery calculators. I guess I forgot they were there. I tried using them and reading the tutorials, but the math didn't look quite right to me...with a mass at 50 lbs and a desired acceleration of 1 ft/s, 12V motors, 65% mechanical efficiency, among other entries I listed for my robot, it said the battery pack I needed would also need around 2000 mAH. I guess I have to re-read the tutorial, maybe try doing the math myself and see what value I'm misplacing or omitting...

Anyway, I tried calculating how the motor would perform with the gearbox--but I don't know if I'm doing it right. The CIM motor has a stall torque of 343 oz-in. With a 36:1 ratio gearbox, that goes up to 12357 oz/in, or 64 ft/lbs. That's a lot. Now, the specs say that the motor also runs about 45 oz-in per amp. With a 36:1 gear reduction, that goes up to 1620 oz in per amp. With that data, at 5 only amps, the motor will be pushing the robot at about 46 foot pounds. With two motors, that should be around 10 amps, and even then I think that will be more torque than will be required to move the robot.

However, the above calculations still look fishy to me. I'm suspicious that I'm leaving a lot of factors out of the picture.

[SeagullOne]

Okay, I see what I was doing now. It was kind of dumb, actually. 

I was looking at the wrong value. Based on my studies, I don't think the CIM motor is, overall, power efficient enough for my robot. It's too big, too slow, got good torque but...it just eats up amps.

Instead, I think I'll wire the two battery packs in parallel and by 2 of this motor:

http://www.robotmarketplace.com/products/0-PDX256.html

It's much smaller, much less efficient in energy conversion than the CIM, but it looks to be much more powerful for its size and overall better suited. It's lightweight too. Assuming my robot weight 35 pounds, two of these motors, as indicated by the robot motor factor calculator, could push the robot very efficiently at around 20 amps--if the robot is going up at 5 degree incline. On level floor--which is mostly what the robot will be roving around on anyway, it would easily be done. And, they only weight a little more than a pound each. Heck, my robot could weigh even a little more than 35 lbs and it would still suffice, based on my guesses.

What do you guys think?