When AB Dynamics needed an extremely compact actuator for the motion pack in its new guided soft target, it turned to gearing specialist Harmonic Drive UK for help
Guided soft target
One of the latest innovations in vehicle ADAS testing, developed by AB Dynamics, is the guided soft target (GST). With a total vehicle height of only 125mm, the GST is an ultra-low-profile, driverless vehicle that measures 2.8m long and 1.5m wide. The GST provides the platform on which to mount a full-sized car-body constructed from a chassis of expanded foam sections with a vinyl cover over. A metallic mesh weaved into the cover material means the GST registers the same radar signature in the approaching vehicle’s ADAS sensor. The GST and driverless vehicles are then programmed to follow guided paths via GPS to allow staged collisions to take place at speeds in excess of 50mph. The test vehicle can repeatedly crash and drive over the GST without causing any damage to either vehicle.
The brief
Directional control of the GST is achieved via an onboard motion pack, an enclosed inertial navigation device with built-in GPS correction, coupled to a steering actuator controlling the front wheels. Due to the very limited space within the GST, it became clear early on in the design process that the motion pack would need to be packaged in the same vehicle compartment as the steering actuator, meaning an actuator no more than 125mm long had to be found. At the same time, this actuator needed to provide sufficient torque to turn the wheels with the vehicle stationary, and be strong enough to withstand shocks transmitted through the steering gear during operation.
To achieve this goal, AB Dynamics turned to gearing specialist Harmonic Drive UK, a specialist in high-precision actuator technology. The company is particularly experienced in providing zero-backlash gears with high reduction ratios.
“The criteria we gave to Harmonic Drive focused on four key areas: size, weight, durability and precision,” explained Colin Martin, director of mechanical design at AB Dynamics. “The actuator needed to be compact enough to fit in front of the motion pack without forcing a change to the profile of the GST.
“This is especially challenging because the nose of the GST tapers to a sloping wedge shape at the front and the front wheels are inevitably quite far forward in the vehicle. These factors put a limit on the gearbox diameter of around 66mm. Supplying a small actuator is one thing, but being able to provide a sufficient torque output meant that an actuator with a high power density was essential.
“During use, the GST is subject to high g-forces and experiences all weather conditions, including sub-zero temperatures when test tracks may be salted. As a result, as well as shock and vibration from the repeated high-impact collisions sustained during testing, we needed the motion pack to cope with large temperatures variations as well as to withstand ingress from the elements, and corrosion from road salt.
“Finally, precision operation and lack of backlash were vital considerations for precise steering control,” concluded Martin.
Gearing up
“Having developed actuators for some of the most extreme environments on Earth and even for the Mars Rovers, we’re often asked to meet demanding specifications,” explained Graham Mackrell, managing director of Harmonic Drive UK.
“We took a standard off-the-shelf product from our CobaltLine range a high-torque-capacity component set with extended temperature range and customized it with the dimensions, torque rating and flange connectors required for use in AB Dynamics’ GST application,” said Mackrell.
The resulting actuator has a length of just 119mm, a diameter of 64mm and a weight of 1.8kg. The range itself is available with gear ratios between 50 and 160:1, offering repeatable peak torques between 23Nm and 841Nm, and a power density of up to 545Nm/kg.
Additionally, the actuator is sealed to prevent ingress, all of its external surfaces have appropriate surface treatments to withstand the corrosive environment and the gears use a specially formulated lubricating grease to provide a wide operating temperature between -40°C and 90°C. To withstand repeated shock and vibration, Harmonic Drive actuators use a low part-count, as well as the latest aerospace and military-grade materials. This combination ensures the GST can provide car manufacturers with reliable results in extreme conditions, anywhere in the world.
The final challenge was finding an actuator that was precise. The GST uses AB Dynamics’ path-following system to guide the vehicle along pre-defined paths, monitoring its position, speed and time. The controller needs to make small and accurate steering corrections to ensure the GST stays on course. Backlash in the gear could make it difficult or impossible to hold the exact path and could place excessive demand on the steering motor. This would waste battery capacity and could ultimately lead to overheating.
“Although a multistage planetary gear might have given us sufficiently low backlash and high torque capacity, the increased length of the unit would have prevented the motion pack from fitting in the same compartment, making it impractical,” explained Martin. “Harmonic Drive’s solution was smaller, had less backlash and lower mass than any other alternative.”
July 7, 2016