Converting a Mule into an articulated 4 wheel vehicle. A 'Mule' is/was a hand trolley with a motorised elevatable platform.
What for? Just for fun, want to be able to control it using the R2D2 droid inventor app via LittleBits boards, why not? I might be able to use it as a design testing platform or autonomous lawnmower , we'll see.
It’s a long story, I came across some old Mule's which were no longer needed. A 'Mule' is/was a hand trolley with a motorised elevatable platform. One shown is lifting a keg. The lifting screw or acme thread was used to raise and lower the platform, well this thread uses a thrust washer that wasn’t up to the task of lift the specified weight and tended to destroy itself, hence the ‘no longer needed’
So, I was able to get a few of them for free so I disassembled them for parts and recycling, but couldn’t bring myself to throw away the wheels, base structure and thread section (another project for another time, might be plasma cutter ball screws, maybe). I started mucking around with the parts and put them in an articulated configuration, then sat a solar panel on top and instantly thought lawn mower. Plus it looks like a NASA Mars rover, sort of.
What for? Just for fun to start with, but ultimately to create an autonomous lawn mower, hence the solar panel in the pic above. But firstly, I wanted to make sure it was controllable, so I decided to use my Star Wars Droid Inventor Kit as the user interface. The Droid Inverter Kit made by LittleBits™, used Bluetooth connection to communicate between the app and droid mainboard. This then outputs three 0 to 5VDC signals, I’m using two, one for forward, reverse, and the other for turn left and turn right, all interfacing using a joystick, centre position 2.5vdc.
Firstly, the parts, all I had was wheels and base frames, I needed motors, steering actuator, centre pivoting steering mechanism, batteries and motor controllers. But interesting enough each wheel had a tooth belt pulley plastic moulded directly to the side of the rim. I worked out this pullet pitch to be 8mm and rounded teeth type. I found some pullies that matched with 20 teeth and a 12mm hole and some matching belts.
I had two spare Baldor 90vdc motors from a different recycling adventure, these has 60:1 ratio gear box with ½ inch output shafts. These were perfect for adding the tooth belt pulley too, all I had to do was machine the pully centre to ½ inch and drill a grub screw in the flange. But these motors were where the problems started…
The 90vdc motor, well I’m going to be running them from eight seal lead acid 12 volt batteries, so 96volt is difficult to control. It is really hard to find a PWM motor controller that can run at this voltage without paying HUGE dollars. So, I had to make my own. After a bunch of experimenting, including blowing up a heap of transistors, I found IRLI640GPBF, which is an N-Channel 200V 9.9A MOSFET in TO220FP package. The key to these ones, apart from its VDS, was the gate can be driven from 5 volt TTL, it has a VGS of 5.0V, this is important because you have to had the FET fully switched on from only 5volts or else the on resistence it not at its minimum and it’ll get hot very quickly and fail.
But this was not the only problem, next was the problem with high side drivers. I had to allow for reversing motors, so I needed a full H bridge configuration, but high side drive is tricky with charge pumps and stuff. So, I dumbed it down. I’m using relays for the high side and PWM N channel FETs for low side to vary the speed. I also needed a shoot though protection circuit to make sure there was never a situation where the high switch and low side matching pairs we’re on at the same time. I used some discrete logic gates to help achieve this.
With the motor control solved (well sort of) I moved onto the interfacing board between the Little Bits and the vehicle. This was to be accomplished by a microprocessor, a PIC Microchip 16F88. This micro would read the two analog inputs from the Little Bits board for direction and steering and convert these into PWM for drive motors and left right control for linear actuator.
I then bought some pillow block bearings and found some 6mm thick steel flat bar and built the centre pivoting steering mechanism, also had to weld some 50x20x3 RHS for bolting too. I then mounted the actuator making sure the end to end stroke allowed for full travel of the actuator without damaging or jamming the machine. I mounted a resistor POT to the pivoting centre for feedback to the micro so it would know its position, another analog input.
After mounting the motors, building and installing some battery holders, I was nearly ready for a test run. But I needed a 96 volt to 24vdc switch mode power supply to run the linear actuator and then another step down to 5vdc to run the micro. I used a MEAN WELL 60W 24V / 0~2.5A Railway DC to DC Converter Part# RSD-60H-24, and then a MURATA POWER SOLUTIONS OKI-78SR-5/1.5-W36-C, this is a small foot print LM7805 equivalent regulator but switch mode, very neat and quite efficient.
Now for the real test run. It works! Except I need to better adjust the motor speed when turning.
View from above:
View from underneath:
The next step is swap the LittleBits board for either a Particle Photon WiFi micro processor or a Raspberry Pi and add a camera or two and maybe some ultrasonic sensor or LiDAR and a neural net and a bunch of stuff I know nothing about, like OpenCV. With the end goal of an autonomous lawn mower without an underground perimeter wire. Going to be a long and difficult project, probably wont be completed any time soon or at all, but I can dream...
Stay tuned.
