Pi Wars – the Raspberry Pi robotics challenge competition › Discussion Forums › General discussion › Choosing the right motor
12th October 2017 at 10:14 pm #3048
I’m a solo roboteer, and it’s my first time at Pi Wars and also my first time designing a chassis completely from scratch! I am a bit confused about what type of motor to use and which motor to use…
I think I will be using a geared DC motor as it has a good torque and high speeds. Although, I want it to be encoded motors so that I can use them for autonomous robots as well. Unfortunately, a single encoder motor cost around £12 and is very hard to find a suitable one that I can trust as most of them don’t have any reviews at all!
So if anyone could give me some guidance on recommended motors and where I may be able to buy them, it would really help a lot! Are there any good motor models anyone knows of (and suitable drivers)?
Thank you!13th October 2017 at 8:57 am #3050
Encoded motors only ‘help’, they don’t solve all of the problems. Wheels will slip on the variety of surfaces in these very physical competitions and dead reckoning (where you trust where you are based upon how far a wheel has turned) is not reliable. It’s a useful input but it cannot be trusted.
Go for motors which give you the performance/size/cost you wish for and look at the autonomous challenges differently… how could you use a more trustworthy source of information to figure out where you are? Ultrasonic or time-of-flight sensors? Cameras and OpenCV? CMUCam Pixy? A mix of all three?!
If you really want to have rotary encoders you could fit some retrospectively to the motor or output shaft.13th October 2017 at 1:05 pm #3051
I’m a big fan of motors with encoders. We’re sticking with built-in encoders this year too.
For places to buy: try RobotShop and TME.
Choosing a motor is hard. You need to take into account:
– How fast you want the output shaft to spin on your robot
– How much torque you want. This is strongly related to:
– How much power you can supply. In practice this is volts and amps. Think about your motor controllers, your battery, etc.
Find a motor which spins at no-load a bit (maybe 2x) faster than you need for your robot and has a stall torque which is higher than you need (again, maybe 2x).
Then read this: https://www.pololu.com/product/1575/faqs – this tells you how to estimate the curves for your motor and figure out where it will be operating and decide if that’s right for you.
If you want to see how other people choose motors, try these excellent blog posts from previous competitors:13th October 2017 at 1:25 pm #3052
Oh – and buy a spare or two – nothing worse than having your robot break a gear or a motor in the last week before the competition.14th October 2017 at 1:09 pm #3054
Thanks a lot everyone! Choosing motors is so much harder than I expected… Most motors from pololu and their distributors (such as robotshop) are a bit of an overkill and costs a lot! How come, similar motors are almost half the price at ebay and especially Banggood? To my surprise, Banggood actually have a variety of motors but they are 12v and so are most of them I find anywhere else. A good example: Geared motor Banggood Although it does seem to consume a lot of current (not the ideal one, just an example)
This brings me to a few more questions:
1. Are such high torque and high speed motors necessary? As the robot is going to be roughly 1 Kg (+-0.2 Kg)
2. Are there any sources such as banggood where I may be able to find a reltively cheap yet good motor?
3. Why are most of the motors 12v? This requires a bigger battery which has a knock on effect on the weight and size of the robot…
4. What batteries do you recommend? As Li-ion batteries have a low discharge rate whereas Lipo’s don’t provide very good energy densities.
5. Are regular rechargeable AAs actually good for the job? I don’t real have an idea about their ‘C’ rating
6. Are there any easy to use, motor ‘calculator’ online? As the one in the link to the previous competitor’s blog post is broken…
13th November 2017 at 7:04 am #3168
- This reply was modified 5 months, 1 week ago by Vishal.
12V is approximately the voltage of a 3S Lipo battery. I’m going for Lipo, main trouble with them is that you need to be careful to discharge to storage voltage when you finish with them, or they go “puffy”, like mine !13th November 2017 at 7:12 am #3172
Tried to edit post to add this link to energy densities showing lipo very good, edit didn’t show ???2nd December 2017 at 11:39 pm #3445
sorry for the slow reply. Hopefully you’ve found some suitable motors by now. If not, or in case this may help someone else, I thought it was worth answering your questions:
1. are high torque and high speed motors necessary?
No, not really. As long as your robot can do about 30cm/s and climb a 30degree slope, it’ll probably be fine. extra speed is only helpful if you can control it. If you want to be a top level competitor in the straight line speed challenge though, you’ll need more than 2m/s.
2. Places like alibaba, banggood and ebay can deliver real bargains. But at the same time, they often don’t give enough information for you to judge what the performance of the motors is likely to be, so there’s quite a bit of risk involved. I got some cracking gearmotors recently for £3 each (bulk pack though: https://www.ebay.co.uk/itm/Mini-Metal-Gear-Motor-20-1-PACK-OF-10/253082806914?hash=item3aece92a82:g:o3YAAOSwcLxYMsNR but I only bought them after I heard someone else had used them successfully. I’ve previously jumped in after someone before and got different motors from the same listing though, so there’s always a risk.
3. 12V is a common DC voltage for power electronics and devices (like cars) so that’s what most motors are designed for. there are a few more coming out now for 3V or 6V but they tend to be very small. switch mdoe regualtors are very efficient, so you shouldn’t need a larger battery, just a different configuration. for example, if you were initially considering a 5V 1200mah battery, but needed to switch to a 12V one, you could drop the capacity to 500mah and still have the same amount of energy. The annoying thing for me is the knock on affect on motor controller selection. many pi add boards only run at 5V.
4. As steve said above, lipo energy (and power) density is great, you’ll struggle to better it with consumer tech. Just be careful with the charging and discharging to keep within the pack’s specification, as they can be a bit sensitive. Many competitors are quite happy using AA cells in a battery holder, or phone power banks (like from poundland), many of which are actually lipos inside.
5. as above decent Nimh is ok. Unless you have crazy high power motors and are happy with short to medium run times, the C rating won’t be too limiting. For example, a Tiny kit from Coretech robotics is probably about 400mA for the motors and about the same for the pi zero (with bluetooth adapter). If you want it to run for 30minutes, that’s a minimum of a 400mAh battery pack thats needed, capable of 2C. Actually, most NiMH is about 2000mAh nowadays, so it should run for a few hours and the max draw is really 0.4C, when I think they’re mostly rated for 1C. On the other hand, Hitchin Hackspace’s recent entries have used 4 high power motors that draw about 6A at stall plus LEDs and a full size pi, so worst case current draw could be 25A, so clearly they’d have needed many, many AA cells in parallel to meet that, so Lipo was the only practical option.
6. I’ve found an alternative to the motor calculator I had linked to in that blog post: http://www.killerbotics.com/kbtools/TentacleTools/ Alternatively, there’s a simpler one here: https://www.technobotsonline.com/robot-vehicle-speed-calculator.html
That banggood motor you linked to looks very nice, powerful, but much too big for a piwars robot. Two of them back to back with wheels on wouldn’t fit in the A4 footprint. You could get them to work if you really wanted to but you’d end up spending more on batteries and motor controllers than if you’d bought more expensive, smaller motors. Annoyingly, smaller motors often don’t cost any less.
If you’re hoping for a small robot, I would recommend using the 12mm spur gearbox style motors like I linked to from ebay. They’re available from loads of other places, from quick, reliable and expensive to super cheap. If you want ones that are more powerful, the 25mm gearmotors are good. Powerful and expensive from Pololu, or cheap from ebay: https://www.ebay.co.uk/itm/12V-DC-60-120-200-500-1000RPM-Powerful-High-Torque-Gear-Box-Motor/201840681700?hash=item2efea462e4:m:mNvfLpzR56bmCRn9Un-WcEg. The other ones to look at are the motors with yellow plastic gearboxes they’re not very powerful but they are cheap, and several teams have used them successfully: https://www.proto-pic.co.uk/hobby-gearmotor-65-rpm-right-angle-pair.html (other suppliers have them too, in different ratios), or gearmotors from Tamiya, available in a bunch of form factors and all reasonable performance. Technobots has one of the better selections of small robot motors: https://www.technobotsonline.com/motors-en.html
Hope that helps
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