28th Nov 2015
On the previous drivetrain blog post we’d found that cordless drill motors were excessively large and powerful for a piwars robot. We continued the drivetrain search, this time looking for smaller motors that could work in a one-per-wheel (four motors total) configuration. From last time we know that we want a total power output of around 120watts, or 30watts per motor. At this point we were hoping that we could use the small ‘1000rpm’ gearmotors we’ve used on small combat robots before.
They give our beetleweight (~1.5kg) combat robot great performance and it looks like other competitors are looking at them too. From previous testing we also knew they had a stall current of ~6A at 12V (72W input) so they look like they might work here. As before, we went to enter them into the drivetrain calculator , but found the 1000rpm motors weren’t listed in their motor database.So this time we put together our own numerical model (spreadsheet) of the torque, acceleration etc, allowing us to change the parameters and see what happens to the performance: This is something we’ve used successfully many times before, for combat robots to dragsters:
So they’re ok but it looks like they top out very quickly. The top speed is much below the 12m/s we were aiming for. What can we do about that? The usual options are: change the gear ratio (complex for us) or use larger wheels (increasing effective gearing, is more easily acheived but increases the stress on the motors). Another solution is running the motors above their rated voltage (something thats very common in combat robots). This also increases the stress on the motors(overheating). Playing with the drivetrain calc again, we found that increasing the wheel size worked up to a point (about where the motors stopped having enough torque to wheelspin), then the overall time started to increase again:
So we can get a good improvement there but its still not quite as fast as we’d hoped for (1.2s). As expected, changing the voltage also worked but running the motors at 36V is a long way from their rated voltage.
Playing with the inputs more showed a combination of increasing the voltage and the wheel size could get us pretty close to target:
Picking the 95mm, 24V design, we can see from the graph that we have more than enough torque (we’re traction limited) for most of the time and the finish speed is pretty high at ~9m/s, much more than we’ll need for the other challenges. Since the 95mm, 12V performance still looks pretty good, we now wondered if we could run at 12V for most of the challenges, just adding the extra battery pack for the speed challenge. This would reduce weight and stress on the motors when we don’t need the extra speed. As before, we quickly mocked up a drivetrain test platform to confirm the model:
Well that looks promising 🙂 The robot is about as fast as before but it’s now much easier to fit the electronics in. Reliability seems good, apart from the occasional wheel falling off… It also has a tendency to wheelie or roll over, so we’ll have to be careful for component placement to keep the centre of gravity low or limit the peak deceleration a little. Its still a little difficult to get the robot to reliably move slowly though (like we’ll need for the proximity challenge) but now the motors are smaller, we can fit smaller wheels just for that challenge, effectively changing the gear ratio and slowing the robot down:
So that’s the drive train sorted, better get the code started now 🙂