Yearly Archives: 2018

A New Chassis For Wall-E2, Part III

Posted 13 January 2018

Back in November of last year I started moving Wall-E2 to his new home in a larger chassis, and subsequently I was able to integrate the new-improved charging system in to this new chassis.

So, today I started some field testing, and discovered a major problem – the motors that came with the new chassis don’t have sufficient torque to handle the operating environment – they stall out much too quickly when encountering any movement issues at all.  This wasn’t a problem for the previous incarnation of Wall-E2, so clearly the motors that came with the new chassis have different speed/torque specs than before.  And, of course, the old motors won’t fit into the new chassis (actually, they do fit, but not with the new battery pack).  What a bummer!

So, I started researching dc motors to determine if I could find motors with the speed/torque specs of the old chassis, but with the physical specs of the motors that came with the new one.

Motors in previous ‘Pirate’ chassis:

The specs for the motors that came with the DF Robots ‘Pirate’ chassis are shown below:

Specifications for the motors using in the ‘Pirate’ robot chassis from DFRobots

With a gear ratio of 160, the motors should produce about 160 RPM at At Wall-E2’s normal operating range of 6-7.5V, and can produce a maximum torque of 0.8 kgf-cm (kilogram force – cm).

Motors in new chassis:

A DRIVE MOTOR GEARBOX REDUCTION RATIO: 1:48
DRIVE MOTOR NO-LOAD SPEED: 220RPM

I couldn’t find the max torque specification, but assuming it’s the same basic motor body, the much lower gear ratio should result in the observed higher no-load speed and lower torque.

So, I’m fairly confident that I now know why the motors in the new chassis aren’t performing as well as the old ones – a much lower gear ratio.  Now all I need to do is find another motor source with the same form factor as the new ones, but with a gear ratio more like the ones from the older chassis:

Pololu:

I found a 120:1 gear ratio right-angle motor at Pololu

Pololu 120:1 right-angle motor dimensions

Pololu 120:1 right-angle motor

These motors have almost the same exact dimensions as the ones in the new chassis, but they have a shorter (5 vs 9mm) and larger diameter shaft (7 vs 5mm).  These are not insurmountable difficulties, but surely I can find a better fit.

Ebay:

So, eventually I ended up back on eBay, the same place where I got the new chassis.  But, this time I was a little more informed about what gear ratio I wanted, and so was able to reasonably quickly locate a set of 4 motors (without wheels – I have too many already!), as shown below

4ea 1:120 right-angle shaft motors with 5mm shaft diameter

So, now all I have to do is wait 2-3 weeks for them to arrive, and I should be back in the low(er)-speed, high(er)-torque business!

Stay tuned!

Frank

 

 

Wall-E2 battery charger module integration

Posted 01 January 2018

What a way to start off the new year!  The battery charger module for my autonomous wall-following robot Wall-E2 has been completed and tested, and now has been integrated into the robot – yay!!

If you have been following this saga, you will recall that I started working on an internal charging module for Wall-E2 well over a year ago, back in November 2016 with this post.  Since then I have gone through several iterations, revisions, and mis-steps (including a semi mind-boggling deep-dive into the details of the Adafruit PowerBoost 1000C specifications in this post).  Last month I finally got a complete system (two PowerBoost 1000C’s integrated onto a single PCB with appropriate control and battery switching circuitry) working, and was able to run extensive charge/discharge cycle testing using a simple test circuit and an Arduino Uno to run it. So, now all I had to do was stuff the whole thing back into the robot.  This task was made possible by my earlier decision to upgrade Wall-E2’s ride to a slightly larger chassis, so instead of trying to cram 2Kg of battery/charger into a 1Kg space, I now had the pleasure of fitting 2Kg into a 3Kg space – nice!   Here are some photos of the integration process.

Battery module shown in the ‘maintenance’ configuration.

Another shot of Battery module in the ‘maintenance’ configuration.

Front cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Front cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Rear cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Rear cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Now that the battery/charger module has been integrated into the robot chassis, I will have to make some minor changes to the robot operating system to accommodate changes I have made along the way, but these should be easy and straightforward.  Then, it will be back to field testing, I hope.

Stay tuned!

Frank