FEATURE GEARS & GEARBOXES
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
Vehicle crash testing developments
To improve road safety, car manufacturers are creating advanced driver assistance systems (ADAS). So, when developing test technology for the next generation of cars, Anthony Best Dynamics turned to gearing specialist Harmonic Drive UK
T
here are currently over 72 million cars worldwide. In 2014, 268,527 vehicles
were involved in reported accidents in the UK alone, according to the National Road Traffic Survey. Although this is a 42% reduction on ten years previously, it still resulted in 1,775 deaths. Advanced driver assistance systems (ADAS) are therefore being developed to take control of a vehicle during an emergency and take evasive action to prevent a collision. ADAS technologies include features that use radar to monitor cross-traffic, apply emergency brakes, warn drivers of forward collisions and lane departure, as well as provide information on blind spots when merging on slip roads and even automatic high beam support during night-time driving. Before rolling out final production
models, car manufacturers are required to put new vehicles through their paces in hundreds of hours of driven and driverless testing routines on closed test tracks. To accomplish this, they turn to specialist vehicle testing companies such as Anthony Best Dynamics.
AN INNOVATION In recent years, car manufacturers have used driverless setups with robots operating the vehicle controls to stage near-miss scenarios that test the evasive action of onboard ADAS sensors. Here, the paths of two vehicles are defined such that a high speed collision is only just avoided and the efficacy of the ADAS system can be observed without expensive test vehicles being destroyed or damaged. One of the latest innovations, developed by AB Dynamics, is the use of a guided
22 SEPTEMBER 2016 | DESIGN SOLUTIONS
soft target (GST) – a driverless vehicle that measures 2.8m long, 1.5m wide and 125mm high. It 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.
MEETING THE CHALLENGE Directional control of the GST is achieved via an on-board motion pack, an enclosed inertial navigation device with built-in GPS correction, coupled to a steering actuator controlling the front wheels. 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. This also needed to provide sufficient torque to turn the wheels with the vehicle stationary. To achieve this goal, AB Dynamics
turned to gearing specialist Harmonic Drive UK. Colin Martin, director of mechanical design at AB Dynamics, said: “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
The GST (guided soft target) and driverless vehicles are
programmed to follow guided paths via GPS to allow staged collisions to take place at speeds in excess of 50mph
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 temperature variations as well as to withstand ingress from the elements, and corrosion from road salt.” So, the company selected the CobaltLine range - a high torque capacity component set with extended temperature range - and customised it with the dimensions, torque rating and flange connectors required. The resulting actuator measures in at
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 23 and 841Nm, and a power density of up to 545Nm/kg. It is also sealed to prevent ingress, with all its external surfaces featuring appropriate surface treatments to withstand the corrosive environment. The gears also use a specially formulated lubricating grease to provide an operating temperature between -40 and 90˚ C. To withstand repeated shock and vibration, the actuators use a low part-count, as well as the latest aerospace and military-grade materials. 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, which 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, and had less backlash and lower mass than any other alternative.”
Harmonic Drive T: 01785 245190
http://harmonicdrive.de/en/home/
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