CLUTCHES, BRAKES & COUPLINGS FEATURE


Considering brakes for industrial robots I


ndustrial robots require brakes for two primary reasons. Firstly as a holding brake to hold the


robot arm or mechanism in place; and secondly to dynamically stop and hold position in the event of power loss. When selecting a power-off brake for these types of applications, permanent magnet brakes or spring applied brakes are suitable. Permanent magnet (PM) brake technology is


often seen as the attractive option because it offers higher torque density (torque per diameter) compared to spring applied brake technology, as well as zero-backlash performance. Spring applied brake technology, however, is the only true failsafe mechanically-engaged solution available. A failsafe solution is often non-negotiable where safety is critical – such as an AGV or forklift truck operating in a production plant, or a medical boom in an operating theatre, for example. Other key factors to consider include:


• The torque required to stop or maintain the static position of a robot. • Backlash – this is defined as the amount by which the shaft will rotate while the brake is holding. The level of acceptable backlash depends on the application. For high precision, high accuracy applications, permanent magnet (PM) brakes offer zero backlash performance, with examples including semiconductor fabrication and medical device manufacturing. For most other industrial applications, spring-applied brakes will


perform effectively and at a reasonable cost. • Bore diameter – brakes for robotic arms need through-holes to allow the passage of cables, lasers and fibre optics. Typically, PM brakes provide the largest bore diameters, followed by spring-applied tooth brakes. • 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 – power-off brakes are typically used as safety brakes that engage when power fails, so response time is particularly important. PM brakes tend to have the faster response times. • Resolution – robots provide highly controlled motion and so resolution is always important. Both PM brakes and spring-applied brakes offer infinite resolution. • Cost – spring-applied friction brakes tend to be the most economical solution, offering effective performance in a wide range of applications. PM friction brakes are next in terms of cost, followed by spring-applied tooth brakes – the most costly due to the precision manufacturing required for the tooth interface. With robotic arms and mechatronic systems


becoming smaller, low profile or ‘pancake’ style brake technology is often needed. KEB has


recognised the increasing demand for compact spring applied brakes and has successfully designed custom low profile brakes. While functionally identical to the standard spring applied brake, these achieve the same rated torque in approximately two-thirds the thickness, which allows for highly compressed drive system braking in robotic joints, AGV’s, forklift trucks, etc. Magnetic simulation software can also be used to design custom brakes. Not all robotic applications require the


brake to be incorporated into the robot arm, which means the size of the brake itself is less critical. In some cases, such as torque motors or hollow shaft motors, the brakes will have to be mounted separately. In medical units such as a surgical operating arm, the brake can also act as the clutch. Other features can, however, be added to brakes


used in compact robotic applications. These include: reduced holding voltage; cable assembly and connectors; and custom coil voltages.


KEB (UK) www.keb.co.uk


/ DESIGNSOLUTIONS


DESIGN SOLUTIONS | OCTOBER 2020


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