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Electrical Actuator Braking Systems

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abhishek sengupta

on 3 October 2012

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Transcript of Electrical Actuator Braking Systems

Electrical Braking System Brakes: A General Introduction A brake is a mechanical device which inhibits motion.
The principle of braking in road vehicles involves the conversion of kinetic energy into thermal energy (heat). When stepping on the brakes, the
driver commands a stopping force several times powerful as the force that puts the car in motion and dissipates the associated kinetic energy as heat. Conventional Braking Systems
Types 1.Disc brakes use a clamping action to produce friction between the “rotor” and the “pads”mounted in the “caliper” attached to the suspension members . Disc brakes work using the same basic principle as the brakes on a bicycle: as the caliper pinches the wheel with pads on both sides,it slows the vehicle . 2. Drum brakes consist of a heavy flat-topped cylinder, which is

sandwiched between the wheel rim and the wheel hub (see Figure 4.3). The

inside surface of the drum is acted upon by the linings of the brake shoes.

When the brakes are applied, the brake shoes are forced into contact with the

inside surface of the brake drum to slow the rotation of the wheels . Electromagnetic Brake System:
Braking action primarily due to friction Why Electrical Braking Systems over Mechanical Braking systems? 1.Electrical Brake is smooth.

2.Electrical Braking is more economical than mechanical Braking (In mechanical Braking,due to excessive wear on brake drum,liner etc it needs frequent and costly replacement)

3.Mechanical Braking produces metal dust,which can damage bearings.There are no such problems in electrical Braking. Permanent magnet type :

The Main Parts are –
The brake shoes, which are connected to two levers
The spiral springs, which are connected to the two levers.
The electromagnetic solenoid, which is connected to the two levers

Electromagnetic brakes are usually found on machines like cranes and lifts and they work as back-up barking systems .These brakes only work when the electricity supply stops to make sure that for example the load carried by the crane or the lift that moves up and down in building, doesn’t fall. If the load that the crane or the lift was carrying fell, it could cause serious damage to property as well as serious injuries or even death to people underneath the load or lift .The braking action of an electromagnetic brake works as follows:

When the electricity is flowing, the electromagnetic solenoid used magnetic force to pull the two levers towards it in towards it which keep the springs attached to the top levers open.

The open spring keeps the brake shoes, which are positioned on each side of a shaft ,away from the shaft while is turning.
When the electricity stops flowing, the electromagnetic solenoid stops working. With magnetic force gone, the spiral springs close this pushed the brake shoes against the shaft to stop it from turning. 2.Particle Brake: Magnetic particle brakes are unique in their design from other electro-mechanical brakes because of the wide operating torque range available. Like an electro-mechanical brake, torque to voltage is almost linear; however, in a magnetic particle brake, torque can be controlled very accurately (within the operating RPM range of the unit). This makes these units ideally suited for tension control applications, such as wire winding, foil, film, and tape tension control. Because of their fast response, they can also be used in high cycle applications, such as magnetic card readers, sorting machines and labeling equipment.
Magnetic particles (very similar to iron filings) are located in the powder cavity. When electricity is applied to the coil, the resulting magnetic flux tries to bind the particles together, almost like a magnetic particle slush. As the electric current is increased, the binding of the particles becomes stronger. The brake rotor passes through these bound particles. The output of the housing is rigidly attached to some portion of the machine. As the particles start to bind together, a resistant force is created on the rotor, slowing, and eventually stopping the output shaft.
When electricity is removed from the brake, the input is free to turn with the shaft. Since magnetic particle powder is in the cavity, all magnetic particle units have some type of minimum drag associated with them Eddy Current Brake System:
Braking action due to direct use of Magnetic force Classifications Anti Lock Braking System:
Automobile safety syste utilising sensors 1.Speed Sensors
The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.

There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:
•In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
•In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.
•In position three, the valve releases some of the pressure from the brake.

Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.

The controller is a computer in the car. It watches the speed sensors and controls the valves. Construction of eddy-current electro brake as shown in the picture below, has a notched disc(rotor) which is driven by a prime mover(such as engine, etc.) and magnetic poles(stators) are located outside of it with a gap. The coil which excites the magnetic pole is wound in circumference direction. When a current runs through exciting coil, a magnetic flux loop is formed around the exciting coil through stators and a rotor. The rotation of rotor produces density difference, then eddy-current goes to stator. The electromagnetic force applies in opposite of the rotational direction by the product of this eddy-current and Vector of magnetic flux and it becomes brake. An anti-lock braking system (ABS) is an automobile safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up (that is, ceasing rotation) and therefore avoiding skidding. The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second. 1.Four-channel, four-sensor ABS 1.Four-channel, four-sensor ABS
This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.
2.Three-channel, three-sensor ABS
This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle.
This sys¬tem provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness.
3.One-channel, one-sensor ABS
This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle.
This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness.
This system is easy to identify. Usually there will be one brake line going through a T-fitting to both rear wheels. You can locate the speed sensor by looking for an electrical connection near the differential on the rear-axle housing.¬ Disadvantages:
1.The Choice of motor is limited. It has to have suitable braking characteristics.
2.Electrical Braking, the driving motor operates as a generator during the period of braking and motor ceases to operate as a generator at standstill so that although an electric brake can almost stop a machine or load, but it cannot hold it stationary therefore a friction brake is required in addition.
3.It is expensive ,high initial cost, the special motors make it a costly application. video Video
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