UNITRAC

The UNITRAC (UNIversal TRACtion system) is a simple 3D printable conversion for the Black Gladiator-Tracked chassis, It’s designed to transform the large wheelbase platform into a self-contained propulsion system that has all the torque you need (dual motors ya see!) and a small enough profile to fit through a gutter or drainpipe.

Basic assembly instructions can be found on the attached instructable and the rest is up to you! Put it wherever a track would normally go or string several together like sausages for an exploration bot. And considering its nearest competition costs thousands it’s the perfect choice.

 

I began this project under the name “building better robots”.

I have since learnt to name things after they’re finished, not before.

If you pick up any popular science or engineering magazine you will notice that robotics is a growing field, with new designs and ideas emerging constantly to fill gaps and meet demands we didn’t even know we had. So many robots, so many uses, so many choices. The trick was to get back to basics.

The goal of this project became building  a simple, adaptable and expandable platform that would make life just a little easier, and a little simpler for users: a universally compatible propulsion system. A unit that can be built easily and is easy to build onto or into existing designs. It would need to be small enough to drive through a drain pipe but with enough grip and torque to move considerably larger equipment on rough terrain, along with the mounting space to attach it. 

The solution I have devised, built and documented is a UNIversal TRACtion system, or UNITRAC, for short.

Overview

The project’s aim was to design and build a universally adaptable, easily accessible, robotic propulsion platform for under $100,  then make the 3D files and instructions available for free (uploaded to instructables) so that anyone else could build (and improve upon) their own copy.

This platform could be applied to a whole range of tasks that require moving instrumentation for specific tasks. Examples being: a pair of UNITRACs moving a robot vacuum, or three to four units in a chain equipped with a custom sensor package exploring a difficult to reach area or travelling inside a pipeline.

And, if such a platform is desirable enough for companies to pay thousands for (see the Minitrac below) then surely hobbyists could use it too, right?

Goals

The goals of the project tried to keep in mind the needs and abilities of people who might need to use a cheap, accessible and adaptable robotic propulsion system for a variety of tasks. People such as students, researchers and hobbyists were considered.

1. Keep it affordable (a basic version can’t cost more than $100)

2. Keep it simple (the simpler the tougher, just look at Toyota)

3. Maximise traction (torque is great but useless if you’ve no way to apply it.)

4. Keep it self-contained (drivers, batteries and all must fit within the unit’s body)

5. Modularity (individual units can easily be used and synced together)

6. Provide open source files and instructions to build the finished unit.

Specifications

Inspiration/design origins 

UNITRAC’s design is heavily inspired by a nearly identical robot called the MTT-136 built by Canadian inventor Yvon Martel (MTT, 2021). The MTT is basically the rear end of a Skidoo modified to house the engine within the track itself. I loved the idea of using every available part of a chassis (or body) for propulsion, so not a single part of the unit that faces the ground could produce drag or resistance as it is instead entirely used for traction. However, only having one track makes some tasks a little difficult … the most prominent of which is steering.

My very first design was a half sized version of the MTT, with two tracks so it could actually turn and a cable ‘follow me’ system based on that used by Precision Remotes LLC that they built for the US military. It would have been powered by electric skateboard motors and have cost in excess of $500.

It was then that I discovered the Guardian S from SARCOS.  It combines tough propulsion elements from the MTT with the snake like joints of Carnegie Mellon fame, it’s even cool enough to have magnets under the track so it can drive up walls! 

Thanks to inspiration from the Guardian I turned my attention to a smaller, and hopefully more affordable, prototype. Some further research revealed that the single track design seemed to be not only very popular but also easily buildable and could be scaled depending on how much funding was available. My plan was then to miniaturise a version of the MTT, (basically one end of the Guardian S), better suited to propelling small (25-100cm) robots. Oh wait, someone’s done that too … the Crawler Minitrac is a multi thousand dollar robotic platform built to order by yet another Canadian company, Inuktun. It's by far one of the best demonstrations that such a design is not only very capable but also very desirable. Amusingly it also looks nearly identical to my design, however I didn’t even come across these until my design was all but finished.

The Build

The first design (V1)

From the side the units were to be oblong in shape with semicircular ends (Figure 1), this would allow a single rubber track to travel around its perimeter. At the front end of each unit was to be a drive wheel slightly larger in diameter than the rest of the body to allow maximum contact with the surrounding track.

The biggest difference, between the Minitrac and the UNITRAC is the market: Minitrac units are for industry, costing thousands of dollars a piece; the UNITRAC, on the other hand, is for hobbyists and amateurs, using readily available components for a complete unit cost starting at $50. However, the UNITRAC isn’t just a knock-off.  As a completely open source project the UNITRAC is open to customisation, the parts are also available as  .step files that can be loaded straight into a program such as Fusion360 and modded to the builder’s heart’s content. The UNITRAC is very much its own thing.

Had I discovered that there was an already existing version of my product earlier I would have incorporated features to encourage and/or ease customisation. Examples being adjustable chassis length and a calculator to work out the number of teeth needed on the drive wheel for it to perfectly fit the track [it took me 3 tries to get mine right] 

The UNITRAC is primarily a mechanical/STEM project but the ideas behind its development are based around functionality in the real, human world.

Figure 1 -Side view showing the oblong shape and semicircular ends	Figure 2 - Internal view showing the bottom-set battery unit (in yellow)

Figure - 3 Drive motor (in red) with output shaft

Within the drive wheel is a drive motor, hard mounted on one side to the body with it’s output shaft mounted to a slot in the wheel. See Figure 3.

Due to the motors used and the output shaft being offset, the majority of the motor is in the lower half of the body.  While this proved frustrating to make, it had the effect of lowering the centre of gravity (COG), increasing stability and allowing for a greater application of torque. See Figure 2.

Mounted directly behind this is a compartment containing the major electronics, motor controller, microcontroller, batteries, etc. The batteries are mounted lowest, just like the motors lowering the COG.

 

Figure - 4 The first design (V1) showing white makeshift track guides

This was the prototype ‘single motor version’, evidently [as the final one looks only somewhat like it] it didn’t really work, so what was wrong with it? 

FIrstly, despite the motors having a considerable amount of torque, with no means of reducing friction, as the track turned it was pulled up against the rough 3d printed rear of the unit, this not only put strain on the motor but also on the track and the chassis itself.

Secondly, the chassis had no built-in means of keeping the track from slipping off. An early modification I made was to screw a pair of plastic guides to the chassis to prevent the track from slipping off (see Figure 4). Both of these issues were remedied in the ‘dual motor’ V2.

This does not mean it was without it’s own advantages, the plan for a rear mounted actuator (seen in the CAD models above (Figures 1-3)) would have been extremely useful and an interesting selling point along with the largely uninterrupted rear chassis allowing for plenty of mounting opportunities.

The second (and current) design (V2)

The current version fixed the issues of its predecessor, while simultaneously losing some of its advantages. Replacing the ‘solid’ rear of the original with a second motor allowed for the unit to produce more torque (in both directions) along with eliminating the issue of the track slipping or getting caught on the end of the chassis.

This posed it’s own issues, mainly the internal space, it barely fits all the needed electronics, as seen below in Figure 5. 

 

Figure - 5 Internal view of V2 showing the tight internal spacing

It also reduced the amount of mounting space, however this was remedied with a simple 3D printed part (shown in green) that can easily be modified to work in any relevant direction.

Figure 6 - External mounting attachment (in green) shown in CAD and in production

Figure 7 - Exploded view of all parts

Results

The robot works, with a custom made ‘camera trailer’ attached (see pictures below). It drives around fine and with a couple of tweaks (These are covered later on) will be plenty powerful enough to tackle whatever I throw it at. This video (https://www.youtube.com/watch?v=4vxHFcXpFy8) shows the unit fully functional and in operation.

Gallery showing the completed unit.

The completed unit undergoing field testing both with, and without the optional ‘camera trailer’

The instructable is also done, uploaded, and ready to be used, it’s linked several times throughout the document and can be found here too.

https://www.instructables.com/UNITRAC-a-Low-Profile-Robotic-Propulsion-Platform/

Issues and setbacks

1. Procrastination, wooooooo!

In terms of setbacks this was the biggest, in fact the better part of a year was spent working on other projects and occasionally coming back to designing this one. I am the kind of person that needs a serious deadline to actually accomplish anything, so I’d say at least 80% of this project was done within a week of the due date. And why this is an issue is pretty self-explanatory.

2. Steering 

As I mentioned earlier, the units can’t steer on their own. However, this isn’t that much of an issue and I have worked out several easy ways that the units can be organised to provide a very capable means of steering them:

• Tanks - one (or more) unit per side, using skid steer to move whatever you’ve mounted them to.

• Snakes - stringing units together with actuated joints allows them to move the units into a curved arrangement to initiate a turn. An example of a purpose built version of this is Tokyo Robotics institute’s Soryu V 

• Trailer or sled -  this is what the previously mentioned MTT-136 uses: something with grip on the ground that can be used as an anchor from which the track unit can be actuated. This being the method I chose for the prototype, I built a small two wheeled trailer, using chunky off road tires and a servo to pivot it. The design works a treat and can be seen in the running video at the top.

Improvements?

Nothing’s perfect, and a dodgy robot built in a week is certainly no exception! These are a few of the things that, in hindsight, I would have changed.

• Batteries 

• Problem: the cheap Lithium Ion (18650) cells I was using lack the current output for any kind of speed or serious towing with a single unit. 

• Potential solution: Swap them out for LiPo cells or find higher discharge Li-Ions, however this may mean either widening the chassis or lengthening it to fit the larger batteries.

• Wheels coming off

• Problem: the part of the drive wheels that mate with the motor outputs have worn out extremely fast and to the point that the wheels begin to slip off during operation, taking the whole track with them.

• Potential solution, use ‘couplers’ (little metal shafts that screw onto the motor outputs and hold the kit drive wheels) and somehow integrate them into the drive wheels. This would keep a robust connection between the motors and wheels at all times, along with making them easily removable by means of a single M3 bolt.

• Customizability 

• Problem: lack of built in adaptability for the chassis.

• Potential solution: design multiple versions of the drive wheels (by far the hardest part to edit) so that people would be able to change the chassis however they liked without worrying about making a gear, another option would be a ‘side plate’ that could be adjusted using screws and holes like a belt.

• Retrievability 

• Problem: if it runs out of power in  a hard to reach area it can’t be retrieved.

• Potential solution: a small eyelet on the optional mounting piece described in Figure 6 would allow the attachment of a length of cord (or even telemetry cable) with which the unit could be physically extracted from a situation which may otherwise result in complete loss of the device.

 

Conclusion 

My aim was to build a self-contained (motors, batteries and drives all within the perimeter of the track), simple and affordable propulsion system that could be adapted to a variety of applications. I also aimed to design, produce and publish open source part files and instructions made available to everyone so they could create their own versions to better suit their individual tasks. All in all, this has been accomplished. 

A combination of procrastination and distraction [a sudden desire to build an RC race car contributing largely to this] took their toll on this project’s ‘final’ product leaving it feeling incomplete and desperate for refinement.  For example, I feel it lacks inbuilt customisation options and a more robust way to keep the wheels on. Despite this, the open-source instructable is up and the single prototype unit runs well. 

Primarily this project has been a learning experience: about design, practicality and purpose, going through countless ideas, trying to find a ‘reason’ other than “I wanna build a cool robot” [this of course being the actual motivation] for what I was making. UNITRAC has a way to go and I will continue to develop and apply it into the future.