MOTORS, DRIVES & CONTROLS
Industrial robots still require brakes
BRAKE DESIGN OPTIONS
Mark Checkley presents the factors to consider when specifying brakes for robotic applications
drive operation – an approach that largely eliminates the need for clutches, but that still requires brakes. T ere are two primary reasons why
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brakes are required for industrial robot applications. First, the brake is used as a holding brake. In normal operation, the brake is engaged at zero speed and used to hold the robot arm or mechanism in place. T e second purpose of the brake is to dynamically stop and hold position in the event of power loss to avoid harming nearby operators or equipment. When selecting a power-off brake for these types of applications, permanent magnet brakes or spring applied brakes are suitable.
IS SAFETY CRITICAL? In many robotic applications, permanent
magnet (PM) brake technology is attractive because it off ers higher torque density (torque per diameter) compared to spring applied brake technology. PM brakes also off er zero-backlash performance. Spring applied brake technology, however, is the only true failsafe mechanically engaged solution available. In applications where safety is critical, a failsafe solution is often non-negotiable. Some examples include an AGV or forklift truck operating in a
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ndustrial robots have a key role to play in manufacturing, assembly and packaging operations. Most robots today incorporate servo motors for direct
production plant with employees walking around, or a medical boom in an operating theatre that is holding expensive and dangerous medical equipment in close proximity to patients and doctors, or a robotic pick-and-place arm on an assembly line. T e requirements for industrial robotic applications vary widely. Key factors to consider include torque (this is the torque required to stop or maintain the static position of a robot. T is depends on weight, structure, load and velocity). Backlash is another key factor: this is
defi ned as the amount by which the shaft will rotate while the brake is holding. T is lost motion depends on the hub used to connect the shaft to the brake. T e hub must provide suffi cient clearance to enable the armature to move axially along the shaft. As a result, a spring-applied brake will always have some degree of backlash. T e level of acceptable backlash depends on the application. For high precision, high accuracy applications, permanent magnet (PM) brakes off er zero backlash performance. Examples include semiconductor fabrication and medical device manufacturing. For most other industrial applications, spring-applied brakes will perform eff ectively and at a reasonable cost.
Bore diameter is also worth considering. Brakes for robotic arms need through-holes
to allow the passage of cables, lasers and fi bre optics. Typically, PM brakes provide the largest bore diameters, followed by spring- applied tooth brakes. Another key factor is torque-to-diameter
ratio. Robots are normally constrained by the space they operate in, which means the torque-to-diameter ratio is a key consideration. In general, OEMs should look for the highest possible ratio, particularly for the brakes used at the base of the robot arm. Response time must also be considered.
Power-off brakes are typically used as safety brakes that engage when power fails. In this case, response time is particularly important. PM brakes tend to have the faster response times and so are often the preferred choice in safety-critical applications.
Another factor to consider is that of
resolution. Robots provide highly controlled motion and so resolution is always important. Both PM brakes and spring- applied brakes off er infi nite resolution. Cost is the fi nal factor to consider.
Spring-applied friction brakes tend to be the most economical solution, off ering eff ective performance in a wide range of applications. PM friction brakes are next in terms of cost, followed by spring-applied tooth brakes (which are the most costly due to the precision manufacturing required for the tooth interface).
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