Electric Brakes
ROUNDUP
The next revolution in motorcycling?
KEVIN CAMERON
BOEING’S NEW 787 “DREAMLINER” commercial jet presents novelties beyond the structural carbon-fiber-reinforced plastic that makes up 50 percent of its empty weight. One that caught my eye was electrically actuated brakes—part of a worldwide movement to eliminate service-intensive hydraulic systems from aircraft. I learned that each main wheel’s stack of carbon-carbon brake discs is compressed during braking not by the familiar hydraulic cylinders but by four electrically powered actuators. As the goal of this system is to avoid the service problems of hydraulics (any leak requires removal of the wheel and brake assembly, making waiting passengers hopping mad), each of the four actuators can be field-replaced without wheel removal. This Goodrich system also remains functional with fewer than all four actuators on-line.
As I looked at pictures of the actuators, I saw that they consisted of a compact rotary electric motor driving a ballscrew jack through reduction gearing. Then, it hit me: This combo of motor, gearing and ball screw is exactly what Honda uses to drive its Combined-ABS hydraulic system. Reading on, I learned that the Goodrich brake system on the 787 is capable of ABS operation at a frequency of 12 cycles per second.
Then, I began to poke around on Google. I learned that aircraft designers are already dissatisfied with motordriven electric-brake actuation because it uses a lot of current; stopping and reversing those motors uses a lot of power, which is why we still don’t have solenoid-powered engine valves. The motor systems are also heavy, so the goals are to cut weight in half and power requirement to one-fifth to onethird. Three well-funded study programs—PAMELA-1, PAMELA-2 and PIBRAC—have already been carried out.
Then, I looked for an automotive equivalent, and I found it. There is an initiative in Europe to develop allelectric accessory systems for cars, including electric brake actuation. Can motorcycles be far behind?
An electric braking system for motorcycles would be brake-by-wire, but we have already got over any fears of such things by accepting throttle-by-wire. Incorporating ABS into an electrically actuated brake-by-wire system would be inexpensive—a natural. Any desired degree of brake boost could be provided; all problems with a “low” lever would vanish.
Why electric? The highly competitive trucking business drives a constant effort to improve fuel economy. Also, 20 years ago, Scientific American pointed out that much of the power used in industrial processes is wasted because drive motors operate at constant speed, delivering more power than actually needed. Engine-coolant pumps are a classic case, spun by V-belts at a constant ratio to engine rpm, regardless of load. This makes variable-speed electric water pumps attractive, using only enough power to provide the circulation needed at the current power level.
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Why didn’t this happen years ago? Because versatile control electronics for such motors are fairly recent. It is expected that cars and trucks of the future will likewise have electric oil pumps and electric air conditioning. Electric power steering is already with us.
How do the future electric brakeactuator programs propose to save weight and cut power requirement? Now, it gets interesting. The proposal is to use piezoelectric devices—crystals that change their dimensions when a voltage difference is applied across their faces. Such piezo drivers are already extensively used in the fast-acting common-rail diesel injection valves, which provide five or more separate spray events per firing cycle.
When CIE Managing Editor Matthew Miles and I were having dinner last year with Brembo engineer Eugenio Gandolfi, I jokingly suggested a motor that would be a disc brake operating in reverse. The tiny thermal motions of atoms in the hot disc would stick to the brake pad as they vibrated in one direction but would release as they vibrated back in the other direction, thus converting heat into mechanical power and spinning the disc. Quite impossible, of course, as the vibrations of atoms in a brake disc are random, not unidirectional as my imaginary joke motor would require.
In the PIBRAC actuator, two circular plates are pressed together. One plate is connected to the load, and the other is backed by a clever array of piezo actuators. When driven by their equally clever power supply, they vibrate their disc in such a way that each element of its surface vibrates in a circular orbit, acting just like my joke motor. With low mass and high torque, it could drive a ball screw directly to apply or cycle braking. Research is continuing to determine the wear life and other properties of such a brake-actuation motor.
I love this because it will annoy the old-timers who oppose every new technology. Relax; it could be years before we see this on motorcycles. Plenty of time to ruin more paint jobs with brake fluid spilled during attempts to bleed manual hydraulic systems.
