In most servo motion control applications, a servomotor accelerates and decelerates the driven system or load. Servomotor brakes are used mainly in vertical-axis applications to statically hold the load in the absence of power. Typically, the brake is a spring-applied, power-off type, with a static torque 50% higher than required to hold the load.

On occasion, such brakes are used for dynamic braking, either as an assist to the motor, usually taking over just prior to stopping, or in emergency stop situations. Motion control designers must consider the worst-case conditions when selecting the brake. If the need to dynamically decelerate is overlooked, for example, the consequences may be premature wear, degraded performance, and possibly catastrophic failure.

Brake operation

In most servomotor applications, either static or dynamic, it is desirable for the brake to hold or decelerate the load in the absence of electrical power for reasons of safety and energy conservation. The most common power-off brake type uses compression springs to push the brake armature axially so it contacts and subsequently stops the rotor and connected load, Figure 1.

To disengage the brake, voltage is applied to the coil housing, thereby generating a magnetic field. This field attracts the armature, moving it axially to the coil housing, thereby releasing the rotor and the load. A manual release can be added to free the rotor if necessary during a power outage.

To engage the brake, voltage is removed, allowing the coil current and magnetic field to decay. When the electromagnetic pulling force (produced by the magnetic field) between coil and armature reduces to slightly below the spring force, the armature separates from the coil and engages the rotor. At this instant, the brake holds, or in a dynamic application, begins to decelerate the load.

Static applications

A typical power-off brake exhibits the electrical characteristics shown in Figure 2 during a static disengage and engage cycle. As voltage is applied to and removed from the brake coil, the current rises and decays respectively.

It is important to consider the time delay from the instant power is applied, t₁, until the rotor clamping force is removed, t₂. Only then can the servomotor accelerate the load without restriction. For this particular brake model, the time delay (armature release time) is 0.035 sec.

Equally important is the time delay from the instant when voltage is removed, t₃, until the rotor clamping force is applied, t₄. In a typical power-off brake, a silicon diode is connected parallel to the brake coil for arc suppression, resulting in a time delay (armature engage time) of 0.028 sec, Figure 3. Obviously, where the brake is used in a vertical axis, with little or no system drag, the servomotor must hold the load during this time delay, or until the brake engages.

If you need a shorter armature engage time, a Zener diode can be added to the arc-suppression circuit, Figure 3, which reduces the time from 0.028 to 0.008 sec.

Dynamic applications

Servomotor brakes in dynamic applications can achieve deceleration, Figure 4, or emergency stops. As mentioned earlier, the stopping process begins with removing voltage from the brake. As with static applications, the rate of current decay and the stopping time depends on the type of arc suppression. Adding a Zener diode reduces both the engage time and the stop time. The time to stop equals the engage time, t₄ - t3t₃ plus the deceleration time, t₅ - t₄, or t₅ - t₃.

The rate of deceleration equals the ratio of total torque to total inertia, where total torque equals brake dynamic torque plus system drag, and total inertia equals brake and motor rotor inertia plus system inertia. Accordingly, the rate of deceleration (rad/sec2) is:

dω/dt = T/J

Application information needed

To select a brake for desired performance and life, first gather the following application information:
• Braking mode required: apply when power is applied or when power is removed.
• Type of available power: ac or dc, constant voltage or constant current, magnitude, and tolerance range.
• Type of brake desired: modular, external add-on, or integral to the motor. Space available and desired mounting method.
• Ambient temperature extremes, allowable coil temperature rise, and the duty cycle.
• Brake operation: hold only, stop every cycle, or stop only in emergencies. • Maximum speed and direction of rotation.
• System drag or friction torque and inertia.
• Allowable deceleration time or number of revolutions after a stop is initiated.
• Cycle-to-cycle stopping tolerance and variation over life.

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