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### Author Topic: Choosing an engine  (Read 922 times)

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#### Cyntrox

• Jr. Member
• Posts: 13
##### Choosing an engine
« on: July 29, 2012, 08:47:02 AM »
Hello, I am trying to follow the tutorial about mechanical dynamics (http://www.societyofrobots.com/mechanics_dynamics.shtml) to choose a set of engines for my next project.

What I'm trying to build is a robot for transporting beer kegs for my local student's pub, with three wheels and two engines. Here are the specifications:

 Est. weight loaded 100 kg Velocity 10 km/h Velocity 277.78 cm/s Acceleration 92.59 cm/s/s

This means I would need an engine RMF of at least 100kg * 83.33cm/s/s * 166.67 cm/s / (2 * pi) = 221044.11, or 110522.06 per motor.

I found an engine (http://www.technobotsonline.com/re-280-1-4mm-metal-gearbox-250-1.html) which seems to fit these specs, with an RPM of 34 and a torque of 6250 giving a RMF of 212500. However, using the wheel size calculation gives a value of 0.02 (cm, I assume?), which is obviously too small. Slower, higher torque engines seem to be a lot more expensive.

Are my calculations correct? Any comments on my design decisions? And should I use external gearing to slow down the engine, allowing me to use larger wheels? Thanks in advance for any advice!
« Last Edit: August 01, 2012, 06:25:04 AM by Cyntrox »

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Choosing an engine
« Reply #1 on: July 29, 2012, 10:35:31 AM »
Hi,

What I'm trying to build is a robot for transporting beer kegs for my local student's pub, with three wheels and two engines. Here are the specifications:

 Est. weight loaded 100 kg Velocity 10 km/s Velocity 166.67 cm/s Acceleration 83.33 cm/s/s
You might wanna rethink and recalculate the project a bit - Rocket motors may be a bit uncontrollable
10km/s = 36.000 km/h might be a bit unreachable (and bad for the beer).
166.67 cm/s = ~6 km/h (brisk walking pace) is a more realistic goal, although it will still take some powerful motors.
You might wanna cut back on the acceleration as well
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### waltr

• Supreme Robot
• Posts: 1,935
##### Re: Choosing an engine
« Reply #2 on: July 29, 2012, 12:23:25 PM »
Quote
two engines
Do you mean internal combustion engines or DC electric motors?

A widely available source of geared DC motors would be from electric wheel chairs. These would have the capability to move the Mass of a keg or two and at a controllable speed.
Get some specs of what is used in wheel chairs and run them through the calculations.

#### rodstar

• Jr. Member
• Posts: 9
##### Re: Choosing an engine
« Reply #3 on: July 29, 2012, 09:27:41 PM »
A 350W DC brush motor will satisfy your needs. As a reference os weight and size, electric go karts use those ones.
« Last Edit: July 29, 2012, 10:19:49 PM by rodstar »

#### Cyntrox

• Jr. Member
• Posts: 13
##### Re: Choosing an engine
« Reply #4 on: August 01, 2012, 06:22:01 AM »
Hi,

What I'm trying to build is a robot for transporting beer kegs for my local student's pub, with three wheels and two engines. Here are the specifications:

 Est. weight loaded 100 kg Velocity 10 km/s Velocity 166.67 cm/s Acceleration 83.33 cm/s/s
You might wanna rethink and recalculate the project a bit - Rocket motors may be a bit uncontrollable
10km/s = 36.000 km/h might be a bit unreachable (and bad for the beer).
166.67 cm/s = ~6 km/h (brisk walking pace) is a more realistic goal, although it will still take some powerful motors.
You might wanna cut back on the acceleration as well
You're right, that would probably shake the beer too much Changed it to 10km/h.

I can see that there's an error in my calculations, though: The two velocities was supposed to be the same, but obviously 10 km/h = 277.78 cm/s, not 166.67 cm/s. I've updated the original post with the new numbers, and, if my calculations are correct this time, I now need 204674 RMF per motor, so the one I linked still seems to fit the bill.

The new acceleration is 1/3 of the desired velocity, so that it should be able to get up to speed in about 3 seconds. That doesn't sound unreasonable to me.

6 km/h would probably be enough for my needs, but I want to make sure that it doesn't fall short, especially as I'm not 100% sure of the weight it will need to carry, so I'm sticking with 10 km/h for now.

Quote
two engines
Do you mean internal combustion engines or DC electric motors?

A widely available source of geared DC motors would be from electric wheel chairs. These would have the capability to move the Mass of a keg or two and at a controllable speed.
Get some specs of what is used in wheel chairs and run them through the calculations.
As it is to be used indoors, combustion engines are pretty much out of the question.

I'll check out the specs on electric wheel chairs, thanks for the idea!

A 350W DC brush motor will satisfy your needs. As a reference os weight and size, electric go karts use those ones.
That sounds like overkill. At 12v, it would draw almost 30A!

#### rodstar

• Jr. Member
• Posts: 9
##### Re: Choosing an engine
« Reply #5 on: August 01, 2012, 08:47:01 AM »
Quote
That sounds like overkill. At 12v, it would draw almost 30A!

Those motors work at 24vdc so amps would be the half... if you develop/get the appropiate controller/driver it won't be a problem...

If amps still thrill you, there are also 80/100W (also 24vdc) about the size of a beer can... you could add an encoder, use a servodrive like a geckodrive's G320X, add an AHRS and perform a self balancing robot (inverted pendulum physics) with only two wheels... blah but that depends on you your resources for the project (time, research, knowledge, budget, and mainly: interest on it...)

pd: electric ones are called motors, not engines

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Choosing an engine
« Reply #6 on: August 03, 2012, 07:42:24 AM »
Hi,

6 km/h would probably be enough for my needs, but I want to make sure that it doesn't fall short, especially as I'm not 100% sure of the weight it will need to carry, so I'm sticking with 10 km/h for now.

Here's a little info for reference...
The average cyclist outputs around 100W, at up to ~30km/h.

Pedal assist (where the motor kicks in to help when you pedal) is, by law (in DK), limited to 28km/h (by limiter circuit or mechanical restraint) and a motor of max. 250W.

Quote from: http://en.wikipedia.org/wiki/Bicycle_performance
Amateur bicycle racers can typically produce 3 watts/kg for more than an hour (e.g., around 210 watts for a 70 kg rider)

Power vs. velocity (speed) is a square law and assuming just 25km/h on a single gear "granny bike" with 100W of input as the reference, a speed of 8km/h works out to:
100W / (25/8)^2 = 10.24W
You'd likely want a faster acceleration than what this bicycle produces, so double up to 20W.
You want two motors, so halve this to 10W each motor.

Assuming a 70kg bicyle driver compared to a 100kg keg, you need close to twice the power (square law as well: (100/70)^2=2) and it works out to 20W each motor on reasonable flat ground.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?