In the past I’ve made some smaller RC controlled robotics out of random spare parts like old cordless drill motors and a spare battery but now I’m trying to build something a little more substantial and eventually autonomous. I was given eight 8AH SLA batteries I plan to form into a battery pack, but this will weigh 40+lbs by itself, not including the frame, drive system or any option components like arms or attachments I work with. The size is fine as I actually want a medium to large finished product.
My main issue is picking a couple of 12v motors (differential track driven) that move roughly 150 - 200lbs at a decent speed and doesn’t drain my 64AH of battery capacity in less than an hour or two. I’m kinda bad when it comes to torque / wattage and math but want to do things a little more professional this time so I need a little bit of information.
To get a requirement of 1053 oz-in torque per motor for a 200lb application. Would a couple of these motors work?
I guess my problem is I’ve not really dealt with non-scrap parts before so I don’t know how big a motor specifically made for robots should look so those seem kinda small. I’m used to grabbing old wheelchair or treadmill motors and tinkering with them. I’d go up to $100-$150 per motor for the right set of motors but I apparently have no idea what I’m doing.
Also I’m open to a lighter weight battery system but haven’t found any good high density li-po packs or anything that aren’t more than my entire budget for the build ($500)
This motor won’t work since the stall torque is 958.2 oz-in and you need a torque of approximately 1000 oz-in for your project.
The stall torque is when the motor’s shaft is locked and doesn’t turn while a voltage is applied to it. It is pretty much the worst case scenario and you want to avoid it because it will make the motor overheat and it will break pretty quickly.
When choosing your motor, you need to find one where the torque at peak efficiency (or rated torque) matches your requirement. It is normally around one third or a quarter of the stall torque (sometimes more sometimes less but it’s a good rule of thumb to keep in mind).
The closest match I could find is this motor:
It is much slower but way stronger.
If this option doesn’t work for you, I suggest either using four motors or making a custom geared motor.
For example, using this motor:
With this GearBox:
Will give you a motor with the following specifications:
Rated Speed: 266 RPM
Rated Torque: 1344 oz-in
The output shaft is very different though (0.5" diameter and it’s a keyway shaft), it also requires greasing and putting it together yourself but it’s not that hard.
I have no problem with the motor+gearbox combination and it seems like a good option since it’s so fast and isn’t that expensive but I guess that changes my battery situation. I have salvaged 12 volt SLA’s so what power system should I look at for 18 volts? Especially since I need a lot of power density for such a heavy application.
It may be another newbie question but I only know the 12v - 24v / 6v - 12v - 18v stuff and don’t want to overvolt those motors with 24 since I currently only have 12.
Since you have 8 sets of batteries, you can consider to series them to become 4 x 24V battery pack. Heavy duty robots work better with higher voltage motors, because the current will be lower.
My issue is that the suggested motor+gearbox is 18v and I have 12v batteries. I can make them 24v and use a step down converter to 18v for the motors but I can’t really find one that can handle more than a few amps. My motor controller is a sabretooth 2x32A to handle the amperage in a 200+lb application so I’d assume I can’t just use a 3 or 5 Amp step down converter.
So I think i’d need a 24v motor or 18v power system?
The rated voltage (18V in this case) can normally be used with higher or lower voltage without a problem as long as you stay relatively close to the nominal voltage (30% higher and 50% lower is a good rule of thumb).
Using a 12V or 24V battery pack with an 18V motor shouldn’t be a problem.
With a lower voltage it will be a bit slower and a bit weaker and with a higher voltage it will be a bit faster and provide more torque.
Using a converter is not suggested, as you said, they are normally limited in the amount of current they can output.
Since you have 8 sets of batteries, you can consider to arrangement them to end up 4 x 24V battery pack. Rock solid robots work better with higher voltage engines, in light of the fact that the present will be lower.
