Our drivetrain steps up a gear!
In order for Dullahan to be successful in the arena, a powerful drive system is critical for our robot to push other machines around and control the pace of the fight. The robot is most effective when it can bully opponents into an arena corner, and land many successive hits with the axe. Furthermore, as this is a combat machine, the drive system must be very reliable and resilient to damage - a drive train failure will likely result in the loss of the fight.
Brushed or Brushless?
The first step was to select brushed or brushless motors. Although a brushless system generally has a more desirable power to weight ratio, we selected brushed motors for superior reliability and reduced cost. Perhaps with more time to refine a brushless setup we could have gone down that route, but in consideration of time restraints, we chose brushed motors. I have always preferred robustness over raw power, and I think this choice paid off - we won both of our fights at the Fightfest due to our opponents' brushless drive systems sustaining damage in combat, while our brushed setup ran faultlessly throughout.
We paired 18v 550 motors with Banebots 26:1 planetary gearboxes. Banebots gearboxes are the toughest planetaries we could get our hands on, and the 26:1 model provided ample torque to push other robots around, whilst being compact enough for our chassis. Banebots products are usually priced well out of my budget, but thanks to our sponsorship from Labman, we were able to use them in Dullahan to great success.
Printed Power Transmission
To compensate for the weight of the motor and gearbox assembly, the front wheels are directly driven, and power is transmitted to the rear wheels via 5M HTD belts and custom 3D printed pulleys. The pulleys are printed with SLS PA12 Nylon, which provided the freedom to design very precise mounting points and keyways to interface with the rest of the drive system. The reduced weight and cost of the 3-D printed pulleys meant they had many advantages over off-the-shelf aluminium pulleys.
We used 3D People for our pulleys and they turned out very well. In consideration of the provided tolerances, we dimensioned the bore and keyway to be slightly smaller than necessary. Fine adjustments were made with a small file to produce the desired press fit.
There were concerns about the durability of 3-D printed parts in such a critical area of the robot, but the belt-pulley system of the drive worked brilliantly, and is still fully functional, even after the wheels sustained significant damage during FightFest 2021. As can be seen in the image below, the pulley diameter is designed so that the belts do not protrude out from the bottom of the robot, and are instead shielded by the baseplate and the wheels.
Wheels fit for a legend!
The wheels are 65mm diameter solid HDPE rod, cut into sections with bike tyre screwed in around the edge for traction. I have had previous experience of thinner nylon castor wheels cracking, or wheels hubs failing in large impacts, and I wanted to ensure this wouldn't be an issue on Dullahan. The rear wheels were bored out, and two roller bearings were pressed in to eliminate the need for a separate hub. Unfortunately as I do not own a lathe and could not access my University's equipment during lockdown, we couldn't manufacture these parts ourselves. The absolute legend, Felix from Team Impact, stepped in and gave us a hand here with some fantastic craftsmanship. These are by far the most durable wheels I have designed for a combat robot, and I hope to improve this design even further for future builds.
How does it perform?
Running on a 5s LiPo and 85mm diameter wheels, we estimated that in ideal conditions on a steel floor, Dullahan would reach a top speed of 9.8 mph, in 0.69s. While real-world values will be slightly less than this, this drive setup is quick, manoeuvrable and very robust with plenty of torque - a huge step forward from my previous builds in terms of performance and engineering design. I am curious to learn how lower ratio gearboxes would perform - a 16:1 ratio is a popular choice for banebots gearboxes for a high top speed, but I was not convinced this system would have the torque to reach that top speed over the distance of the arena. We could definitely consider this for future builds, or as I mentioned above, attempt a brushless system with more testing.
Nonetheless, I am delighted with how our chosen setup ran at the FightFest. I am sure it will continue to perform well in this machine for many events to come!