Sutibility of motor in fighting robot feather weight

[supermotolew]

Hi all its my first delve into this so im looking at what i have around me to use .
the motors being one of the more expensive items im hoping i can use these or get some advice on these motors in a robot drive application.

My reason for choosing these is i have about 20 of them and the have small reduction boxes attached ! ill also be needing to find a sutible pwm controlled ESC thinking 75mm wheels too.

the devision im looking at is the 13.5kg range but if sutible would like to look at other weight ranges
some spec i can see stamped
brushed DC
PN GR53X30 15v special version
15V
3450 rpm
lfm 30A im (presuming peak amp draw) ?
lN 3.5A (normal operating draw ????)
6.25:1 ratio gearbox fitted

any advice would be amazing thanks

 

[kellys_eye]

Unless you're prepared to delve into the dark arts required to 'calculate' the suitability of a particular motor, the best you can do is 'suck it and see'.

You don't state the actual function the motors will be used in - locomotion, leverage etc. Speed? Torque? Which is important to you?

Battery supply? In which case what size? How long for?

Too many variables for us to comment.

 

[supermotolew]

Hi ah yeah I can try to give more info . I’d say torque is fairly important pushing and shoving other bots i think one motor in each corner with a 75mm diameter wheel . I also have many 18ah lead acid battery’s for testing but the final will use a lighter lipo if needed. The bot needs like a maximum 5 min runtime but I’m no where near calculating runtime yet as there are other factors. Essientally what I’m trying to get a feel for at the moment is are they sufficient for drive power in 15kg platform . Should I reduce my wheel size to gain torque or add more reduction what kind of esc would be sufficient I can resume 60a per Chanel if I’m looking at 2 motors each side .
Maybe someone her has experience in building these and has a feel for what’s required under the conditions . Cheers also for all input much appreciated

 

[kellys_eye]

With any given motor you sacrifice speed for torque (if using gearbox for reduction) - calculate your required speed and, from that, you can determine the maximum reduction ratio for the maximum motor rpm.

Your current setup has 3450rpm, a 6.25:1 gearbox (which gives 552rpm at the wheels) and for a wheel diameter of 75mm this equates to a maximum speed of approx 2.1m/s (just under 5mph).

Whether that's fast enough for you is your decision. You can get more torque (pushing/pulling power) by increasing the ratio (to say 10:1) at a loss of some speed if you want to 'push your opponent about'. And thereby lies the key. Speed (avoidance) or torque (bashing power!).

The 15kg weight will only influence the acceleration from a standstill - the heavier, the slower. But drive two wheel or four wheels (much better!) and acceleration becomes a moot point.

The maximum current of 30A (probably the stall current?) and using FOUR motors means a total current of 120A - sounds a lot but an 18Ahr battery would last 9 minutes (theoretically.... probably only around 5 minutes) which is plenty!

See where we're going with this? You can use FOUR motors for more time than the contest allows and have the motors flogging themselves to death all the time and the battery will be ok.

PWM control (via an H-bridge for direction control) using a controller than can handle at least twice the maximum current (so a 50-60A controller) should be a decent starting point.

 

[Bluejets]

Even with the reduction gearbox the output is over 550rpm which I'm guessing would not be anywhere near requirement.
You can work it out with a wheel of say 100mm diameter.
Applying pwm speed control via a uC would be ok BUT your top speed would need to be assessed and applied to give maximum torque down low.
Many many variations and other mechanical properties to consider to get anywhere near any ball park figure. Simply choosing lithium over lead acid would be a major change.
Why are you trying to re-invent the wheel?
Surely there exists similar class bots you can use as a basic design starter.

 

[supermotolew]

hi blue jets , mostly its cost these motors are readily avalible to me .
i think this ESC looks sutible https://www.robotshop.com/uk/sabertooth-dual-motor-driver.html
i have found some decent looking 100 and 125mm wheels so ill recalculate speed on those although the torque worries me. maybe ill have to suck it and see speed looks easy to calculate but performance feel vs torque looks more complicated.