SPECIAL SUPPLEMENT Machine Safety
Best ways to specify brakes for robotic applications
Most robots today require brakes, says Mark Checkley of KEB T
here are two primary reasons why brakes are required for industrial robot applications: First, it 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. Its second purpose 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 this type application, permanent magnet brakes or spring-applied brakes are suitable. In many robotic applications, permanent magnet (PM) brake technology is attractive since it off ers higher torque density (torque per diameter) compared with 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 production plant with employees walking around, a medical boom in an operating theatre that holds expensive and dangerous medical equipment in close proximity to people, or a robotic pick-and- place arm on an assembly line. The requirements for industrial robotic applications vary widely, but key factors include: • Torque – what’s required to stop or maintain the robot’s static position, and it depends on weight, structure, load and velocity. • Backlash – defi ned as the shaft’s rotation while the brake is holding. This motion depends on the hub used to connect the shaft to the brake. The hub must provide suffi cient clearance to enable the armature to move axially alongside the shaft. As a result, a spring-applied brake will always have some degree of backlash. The level of acceptable backlash depends on the application. • Bore diameter – brakes for robotic arms need throughholes to allow the passage of cables, lasers and fi bre optics. Typically, PM
32 February 2021 | Automation
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. 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.
• Resolution – robots provide highly-
controlled motion, hence resolution is always important. Both PM brakes and spring- applied brakes off er infi nite resolution. • Cost – 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 – the most costly due to the precision manufacturing required for the tooth interface.
Low profile brake designs Recent advancements have enabled robotic arms and mechatronic systems to shrink in size. This reduction in footprint often requires low profi le or ‘pancake’ style brake technology. KEB has recognised the increasing demand for compact spring- applied brakes and has successfully designed custom low-profi le brakes. This style of brake is functionally identical to the standard spring-applied brake but achieves the same rated torque in approximately two-thirds the thickness, which allows for highly-compressed drive system braking in robotic joints, AGVs, forklift trucks, and so on. Magnetic simulation software can also be used to design custom brakes that meet customer-specifi c design constraints. 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. However, size of the brake is still important for all applications.
In medical units such as a surgical operating arm, the brake can also act as the clutch. The brake holds the robot arm in place, but if the maximum torque limit is reached, the unit can be moved (i.e. pushed out of the way) and still held safely. Often, the friction material can aff ect or control this, which is a good safety point.
Common brake design options Other features can be added to brakes used in compact robotic applications, including: • Reduced holding voltage – after
being released at nominal voltage, some manufacturers of spring-applied brakes can drop to a holding voltage of 50% nominal. This is very advantageous for an AGV, forklift truck or other mobile robotic application where the power supply is a battery, as power consumption is signifi cantly lowered. This will also reduce heat generated from the brake coil. It is typically achieved with PWM (pulse width modulation) controlled by the customer. • Cable assembly and connectors – some suppliers can provide brakes complete with heat-shrink tubing, wire jackets, pig- tailed leads, cable labels, and more, along with connectors and plugs. This eliminates the need for customers to add cable assembly to their processes and provides a plug-and-play solution.
• Custom coil voltages – some batteries and control systems operate at voltages outside the common standard off erings (24, 48, 105, 205VDC). Some suppliers can design and wind a coil to suit any desired voltage.
CONTACT:
KEB
www.keb.co.uk
automationmagazine.co.uk
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