Brake: What to consider

1 Outline

The following information applies to the maxon motor brakes diameter 20 to 44 mm (AB 20, AB 28, AB 32, AB 41 and AB 44). Please consider also the information and specifications in the maxon catalog. 

2 Holding Brakes

All brakes listed in the catalog are holding brakes with the following characteristics:

• They maintain the load in position and inhibit the stationary, deactivated motor from drifting.
• They are not designed to slow down a moving load – this is the task of the servo controller.
• They normally brake when not powered. Thus, in the event of a power failure, the brake will be activated and the load will be kept in position. This allows to save energy and avoids to re-initialize the encoder (so called “Homing”).

3 Permanent magnet Brakes

The two most commonly used holding brakes are either permanent magnet brakes or spring brakes. The brakes listed in the maxon catalog are permanent magnet brakes with the following advantages in comparison to spring brakes:

• Higher brake torque
• Design without backlash between motor and brake disk
• No friction contact when "lifted", thus no friction losses
• Low-noise due to no friction

In addition to that the latest generation of permanent magnet brakes (AB 20, AB 32 and AB 44) have the advantage of an extended ambient temperature range of -40...+100°C. This cannot be reached with conventional permanent magnet brakes (see explanation below).

3.1 Overview of the maxon Brakes

3.2 Conventional design of permanent magnet brakes (valid for AB 28, AB 41)

The brake is activated if the coil is not powered. The braking force is produced by the permanent magnet (Figure 1, left).

Figure 1: Brake conventional design (left non-powered, right powered)

The brake will be released by powering the coil which produces an opposing magnetic field, which “neutralizes” the field of the permanent magnet (Figure 1, right). The brake disk will be lifted off the magnetic return by the springs and the brake is free running.

However, this design has a significant disadvantage (Figure 2): If the magnetic field in the coil gets too big, an unwanted “drop back” of the brake disk will occur.

This can be the case at low ambient temperatures, because then the resistance of the coil decreases. Since the brake is usually supplied with a constant voltage, the current increases and the electrically induced magnetic field increases as well. Once this is strong enough, the brake disk will be attracted to the magnetic return again!

Figure 2: Drop back of the brake at low temperatures (conventional design)

3.3 Improved design of the permanent magnet brakes (valid for AB 20, AB 32, AB 44):

The principle of the improved design is similar to the conventional design. But the limited temperature range is extended by an improved arrangement of the permanent magnet (Figure 3, left).

Here the coil no longer "neutralizes" the magnetic field produced by the permanent magnet, but it “redirects” it into the coil’s magnetic return (Figure 3, right). This is also the case at low temperatures. Thus with this improved design, much lower temperatures would be required to provoke an unwanted “drop back” of the brake.

Figure 3: Brake optimized design (left non-powered, right powered)

4 Lowering the holding voltage

On some of the holding brakes it is possible to lower the holding voltage after deactivating/switching the brake. This leads to less power consumption in order to reduce the heat losses. Basically, this may be applied on the AB 28, AB 32, AB 41 and AB 44. But, maxon recommends:

• not to reduce the voltage below 60% of the nominal voltage (this corresponds to 14.4 VDC at 24 VDC nominal voltage) and
• to verify the behavior of the brake in the final application, since also external factors (such as vibration, acceleration, temperature, etc.) may provoke a “drop back” of the brake disks.

The AB 20 is only conditionally suited for lowering the holding voltage since the air gap is very narrow. Lowering the holding voltage must also here be tested and verified in the final application.

5 Brakes and Encoder

The maxon holding brakes are mounted directly on the back of the motor. If a brake and an encoder is required, this means that the encoder is mounted on the back of the brake. Only AEDL and HED_ 5540 Encoder are suitable. Please check the maxon catalog for details.

Figure 4: Symbol picture of a gear-motor-brake-encoder combination