28 The exception to this rule are multisensor inspection payloads, which are commonly used in industrial inspection and are designed as plug-and-play devices that can therefore be run at the edge of the network without affecting the robot’s core behaviour. There are three payload connectors on Keyper’s back, each being a 24 V power and signal D-Sub rectangular connector, with an additional Ethernet connector for sensor data. Actuating a leg While tighter integration has been the long-term goal in the Keyper’s engineering, the legs are detachable. Each one mounts to the body via four screws, so that in the event of damage the customer can replace the leg themselves. “It would be tempting fate to say the legs will never fail, but they can be replaced easily, minimising the system’s downtime,” Tome says. “That said, we’ve sought to maximise the ‘transparency’ in the actuation.” In this context, transparency refers to the accuracy of the output joint torque across all three actuators and the gearing that follows. “For electric motors to work well in a leg, you need a trade-off between speed and torque, but you will lose efficiency in the process, as well as the resolution with which you can control the motor’s force output,” Tome says. “That is unless you do what we did, which was to make a gearbox good enough for you to trust the estimated torque output at the robot’s feet based on extrapolations from the motor power and speed readings. “That took a lot of iterations on the gearbox’s design, the gear materials and the construction of custom test rigs for validating the efficiency of our reducers. We’d measure their performance, stress them, then collect and analyse the test data points until we were satisfied we had the most efficient gear reduction possible. “Of course, we’d get closer torque measurements by just using a torque sensor with a measuring spring or strain gauge, but we’re not doing surgery, we’re just trying to control how the leg interacts with its environment, in order to control the impedances. As people, we don’t know how much force we produce when we step, but we feel the impedance once we’ve set our foot down, and that’s what we then regulate.” In addition to its patent-pending gear system, the actuator has been designed for a balance between simplicity and robustness. All three actuators per leg are identical, and assembled in-house from 35 mechanical parts, which collectively comprise the controller board, motor and gearbox, each of which can be produced using simple processes such as injection moulding or three-axis CNC milling, avoiding the need for five-axis milling or additive printing. Although most animals’ limbs have four planes of actuation, installing three actuators has proven sufficient for giving each Keyper leg three degrees of freedom; the fourth point gives animals the ability to orient their legs in order to plant their feet carefully. In bipeds, a fifth actuator, in the ankle, is necessary for angling the foot, but for the Keyper, orientation of the leg matters little, as the ‘foot’ is essentially a rubber ball. Starting with the UGV’s hip, first there is an actuator oriented for rotating in the x axis, as this can be socketed within the body’s footprint. Just outward from that is the y axis actuator, for sidestepping motions (or abduction) while lower down but in the same plane is the knee joint. “Some robot designers put the knee joint very close to where they need the output torque, but if a heavy limb moves quickly, it adds excessive inertia to the moving mass of the robot, increasing the energy needed to stop the moving limb,” Tome says. “We want to minimise the energy needed to move the legs and increase their agility, so we’ve avoided doing that.” October/November 2023 | Uncrewed Systems Technology Each actuator contains 35 pieces, and each leg (shown here on a test bench) contains three actuators
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