Energy and Timing Advantages of Highly Non-Linear EMB Actuation 2013-01-2067
With linear actuated brakes the actuation force (or torque) rises linearly from 0 to the full actuation force at full braking. This means that the actuation must be designed for the rare case of full-braking. The parts must be designed for this peak load (e.g. motor, gear) and the transmission ratio is determined by the full-braking actuation torque, which causes the highest transmission ratio and hence determines slow actuation dynamic.
Ideally the actuation should make the fastest travel at low normal force and turn to slow movement and high force at the highest pad force. Mathematically the torque transmission ratio should optimally be an exact representation of the actuation characteristics (actuation torque over actuation movement), creating the highest torque-transmission ratio at highest force and the fastest movement at low pad force.
This highly non-linear actuation characteristics means that the actuator motor is always running on constant load (although the pad pressing force changes dramatically). The resulting actuation timing is the fastest possible, because the actuation is as fast as possible at low force. The actuator electro motor is running at constant-load and hence can be operated in best efficiency, making the electrical power consumption the lowest possible.
How can the optimal, variable transmission ratio (that is mathematically given by the actuation characteristics) be achieved in reality at the Vienna-Engineering EMB (Electro Mechanical Brake)? Firstly the VE-EMB has a non-linear actuation due to the principle of the eccentrics, which are turned to press the pad to the disc. Secondly the VE-EMB uses an approx. 90° angle-range at the eccentrics, making it easy to insert a non-linear gear between eccentrics and the motor gear. It is shown how the behavior can be optimized to the mathematical needs and how the characteristics can cover a wide range of changing transmission ratio.
It is also discussed how the optimized non-linearity also assists to turn the brake back to “released” in the power-off case: At high actuation force the VE EMB always automatically rolls back to released by the high internal force and its efficiency of 90% and higher. At low force any EMB must overcome the motor cogging (“snapping” and friction) to roll back to released at power-off. The highly non-linear actuation of the VE-EMB makes it far easier, because at low force the transmission ratio turns to “fast pad movement”, assisting in turning the motor from the side of the pad-force.
The VE-EMB Simulator is a detailed (and calibrated) representation of the brake in software, making it easy to accurately study improvements. It also can be used to study linear actuation on the same brake with the same motor. So this work also discusses timing and energy advantages by optimizations in the VE EMB and it also compares it to linear actuated brakes (e.g. screws, electro-over-hydraulic). By these optimizations an extremely simple EMB with the smallest possible electro motor can be shown.