Motion System Design
Choosing servomotor brakes

Choosing servomotor brakes

Power-off brakes are used in servo applications to hold the driven load or to provide dynamic braking. Here are some important factors to consider before making a selection.

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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Brake selection, static

Selecting a brake to accomplish static holding is straightforward: simply choose a brake with a torque rating reasonably in excess of the worst-case holding torque required. Make sure that the brake fits into the available space and has the desired mounting configuration.

Also consider the effect of ambient temperature extremes. For example, the amount of current (amperes) that is available to disengage the brake decreases as coil temperature increases. Therefore, you may want to consult the manufacturer about this effect.

When selecting a static brake, plan for occasional dynamic stops. Engineers find that operators sometimes use a static brake dynamically, which often leads to brake failure. A reasonable approach is to select a brake with a static torque rating 50% greater than the required holding torque, thereby providing an adequate safety factor.

High vibration along the shaft axis can reduce spring force and the amount of brake torque. For this condition, ask the brake manufacturer to calculate the amount of torque reduction.

Brake selection, dynamic

Here, the brake can be used to achieve one of two objectives: stopping within a specified time, and stopping within a specified amount of travel.

Example 1 — Stop within specified time. Start the selection process by calculating the torque required to stop the system inertia within half the available time. The reason being that at this point you don’t know the brake rotor inertia or the brake response time. Therefore, the estimated torque is:

T = 0.1047 Js(dω/dt)

This formula uses different units than the earlier formula dω/dt = T. For convenience, v is expressed in rpm rather than rad/sec. All of the following formulae use rpm as well.

At this point evaluate the system drag. If possible, it should assist deceleration or stopping. If so, reduce the required brake torque accordingly:

Tb = T - Td

If counteractive overrunning torque exists during the deceleration cycle, increase the required brake torque:

Tb = T + Tb

From the suppliers catalog, choose a brake model that has 25% more than the above calculated torque Tb to allow for the brake rotor inertia. Make sure that the brake is recommended for dynamic applications and the torque listed is dynamic rather than static.

After making this preliminary selection, calculate the deceleration time using the brake rated torque and the total system inertia (including brake rotor).

dt = 0.1047Jdω/T

The deceleration time plus the brake engage time must be equal to or less than the desired stopping time. If the time to stop slightly exceeds the time required, consider improving the arc suppression circuit before evaluating a larger frame size.

Next, calculate the amount of energy the brake can absorb as described later under the section “Energy absorption.”

Example 2 — Stop within specified travel. Start by selecting a model that best fits the available space and has the desired mounting features. Then calculate the total travel, which includes travel during the armature engage time plus the travel required to decelerate the load. Referring to Figure 4, the total travel is:

S = [(t4 - t3) + (t5 - t4)/2]ω/60

If this travel is unacceptable, recalculate using the armature engage time with a Zener diode, or select another model and repeat the calculations. If the travel is satisfactory, calculate the energy absorption as described in the following section.

Energy absorption

So far, you have selected a brake based on its single cycle stopping capability. Next, verify that the brake can dissipate the kinetic energy absorbed per cycle and at the worst-case frequency in the application without generating excessive heat. To do this, first calculate the energy absorbed by the brake per cycle:

Eb per cycle = 4.6 (Jω2) 10-4 ft-lb/cycle

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If the system’s friction drag is significant compared to the brake torque, modify the energy calculation above by the ratio of brake torque to total torque:

Eb per cycle = [Tb/(Tb + Td)] × 4.6(Jω2) 10-4 ft-lb/cycle

If the braking action will occur at a rapid cyclic rate, multiply Eb per cycle by the cyclic rate, N:

Eb per minute = [Tb/(Tb + Td)]× 4.6(2) 10-4(N) ft-lb/min

To ensure that the brakes are sized properly, compare the calculated energy absorption per cycle and energy absorption per minute to the recommended values listed in the manufacturer’s catalog. Most manufacturers publish energy absorption graphs in their catalogs, such as Figure 5, to assist in the process.

Estimating life

Servomotor brakes offer years of maintenance- free service if they are properly applied. If there is any doubt about the application, request assistance from the brake manufacturer.

For brakes used in dynamic applications, the manufacturers usually provide wear life data, either estimated or gathered from tests. From this information, you can estimate the brake life in terms of the total number of braking cycles expected.

To estimate the number of cycles of maintenance-free life, divide the total allowable energy absorption (ft-lb) of the brake (specified by the brake manufacturer) by the calculated energy absorbed per cycle in your application (ft-lb/cycle).

John Mendolia is the director of technical services, API Deltran Inc., American Precision Industries Co., Amherst, New York.

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