This is the first part in a small three-part series about the re-design of the rover suspension. We’ll touch on aspects across several parts of the team, but for now I’ll introduce you to the mechanical aspects.
However, before I talk about this re-design, I feel it necessary to explain why such substantial change was needed. When we first began the design of BLUEtongue back in 2013 the team opted for a Rocker-Bogie style of suspension due to it its many benefits in traction and stability when operating in rocky environments.
Resulting from the complexity and cost attached to steerable wheels (such as swerve drives), we utilised skid steering like that you’ll find on a tank or bobcat. Unfortunately, to a significant extent, we misunderstood the physical nature of the suspension we were in the process of designing and the ramifications our choice to peruse skid steering would have. Upon initial testing, the inherent problems in the system began to make themselves known. First, the suspension was too tall and insufficiently rigid for a skid steering design. Due to this, attempts to turn the rover resulted in either the flexure of the structure or would cause the bogie to “kick”, rendering the rover immobile. You may see older photos of the rover with what we’ve called “bracing bars”.
These bars lock the bogie to the rocker, permitting limited steering capability and allowing the rover to limp around. Secondly, the rocker was too long and couldn’t fit in conventional luggage. As we’d planned from the start to flat pack the rover into our personal luggage for transit to and from the contest, we had to search long and hard to find a suitable enclosure. Thirdly, the construction order. As many undergraduates will quickly realise when they build things for the first time, build order is a very important thing to consider. Whilst in a Computer Aided Design (CAD) environment, assembly really is as easy as a few clicks. Need to mount a motor in tight spot? Sure! Try to do this in physical space where motors can’t fly through walls? Not so easy. Due to this, our assembly process was very convoluted, requiring gearboxes to be adjoined to the motors within other structures, and removed for disassembly, etc (It was a nightmare!). All in all, our first suspension iteration was an utter nightmare. Hindsight really is 20/20.
So, now that we’re on the same page as to the why, I want to introduce you to the what. Post our first presence at the European Rover Challenge in 2015, we realised the suspension was one of the key limiting factors of the BLUEtongue rover platform. With the knowledge that a fundamental redesign was needed, we got to work over the next few months. The final design is a parallel swing arm type suspension with a full rotation swerve drive. The new system was designed with a heavy focus on steering, dynamic response, assembly and transport.
As seen in the video attached below, steering is achieved through the actuation of a radially free, but axially constrained, shaft. Due to the low loads experienced and limited rotation speeds, this arrangement is achieved with radial bearings and circlips. The design originally called for the use of a swivelling hub (really just a small scale Lazy Susan) for the axial restoring force. However, during initial testing, these were negated to allow for power cabling to pass through the shaft centres. Here it quickly became evident these hubs were unnecessary. Luckily so as well, as this topside location was later used to mount analogue potentiometers for feedback once it was established that the intended locating method of relative encoders and magnetically activated homing was insufficient (Stay tuned for our next two articles for more on this). In order to drive the shaft, a DC motor with gearhead was mounted parallel, and an addition reduction gear step used to mechanically link the two. Additional problems arose from this arrangement where the torque loading during operation consistently began to “strip” the lock screw of the brass pinion gear, leading to un-actuated free rotation of the shaft. A problem easily solved through the use of thicker walled Carbon Steel (1045 for anyone interested) replacement pinion gears.