What to consider when selecting brakes for servomotors

Robots are gaining ground. In all important industrial sectors, the cooperation between humans and robots is becoming increasingly closer. This increases the risk potential. If, for example, a power failure occurs during a working process, the robot arm undertaking the working step must be held immediately and accurately. Therefore, it is important to permanently exclude the risk of an inadvertent descent of loads as well as unpermittedly long stopping distances already in the design phase.

Decisive for this purpose is the correct selection of safety brakes, as well as their proper integration into the overall system. Safety brakes according to the fail-safe principle are the first choice for servomotors, as these brakes are closed in de-energised condition. They provide the required braking torque even in the event of an emergency stop, a power failure or in case of an interruption in the power supply caused by cable breakage, for example. To ensure that the safety brakes also provide sufficient friction work in emergency stop situations and brake movements with a defined braking torque, a friction lining developed for this purpose with a corresponding steel counter friction surface is required. While this is common with safety brakes, permanent magnet brakes with their steel-on-steel friction combinations reach their tribological limits here.

Small brakes with high performance density

There are safety brakes for servomotors which are specially adapted to the high requirements of robotics. Users can choose between classic servo brakes in the motor, with hub and toothed rotor or so-called pad options with large inner diameter. The latter are specially designed for integration into the robot joint. But even the classic brakes can be adapted to customer specifications and integrated directly into a joint.

In the motor, servo brakes are preferably installed into the A-bearing shield, because the fixed bearing is located here and temperature expansions cannot influence the brake severely. However, brakes from renowned manufacturers can also be integrated into the B-bearing side of the motor without restriction, as here temperature expansions and bearing backlash do not influence the function and reliability of the brakes negatively. Alternatively, users can also make use of mounted brakes, which are attached to the motor in a modular fashion. High-quality servo brakes are also characterised by compact dimensions. Not only are they very lightweight, but also extremely fast when it comes to magnetic actuation. At the same time they are characterised by high performance density, and they are wear resistant.

Furthermore, the brakes impress users with their high permitted friction work during dynamic braking actions: For servo drives, normally load mass ratios (load/motor) of 3:1 or smaller are selected for the benefit of good control characteristics and high dynamics. On the brakes of renowned manufacturers, high permitted friction work and friction powers mean that load mass ratios of 30:1 and more are possible.

Short switching times over the entire service lifetime und intelligent brake monitoring

Short stopping distances are important for the safety of people and machines. The brake switching times are decisive for the braking distance, because during the free-fall time until the brake closes and the retardation takes effect, the mass additionally accelerates - possibly to such extremes that the permitted values of the brake are exceeded. Users should therefore pay attention to verified switching times which are as short as possible when selecting safety brakes - and also assure themselves that these switching times can be maintained throughout the entire lifetime of the brake.

Here, monitoring solutions are important. Previously it was not possible to monitor servo brakes due to the small air gaps. Now, however, renowned manufacturers offer intelligent systems for sensorless brake monitoring. Modules, that not only supply the brakes but also monitor them without sensors, provide data on switching times, current, voltage, resistance, power and relative attraction current. The latter permits conclusions to be drawn regarding critical brake operating conditions. Using an evaluation programme, the customer can see whether everything is running smoothly or whether there is work that needs to be done. For example, an air gap which has become too large is detected in good time. Safety becomes visible, so to speak. Maintenance becomes plannable - the system operator or manufacturer is able to undertake maintenance in a targeted manner, aligned to their working processes. Over and above this, the issue of error or warning signals via the interface also permits integration into a customer-side remote maintenance system.

100 per cent quality inspections

To guarantee that the brakes always ensure the highest level of safety in the respective application, it is important that the static and dynamic braking torques correspond to the respective customer specifications. For this purpose, it is necessary to check these two application conditions in trials. Therefore, it is highly recommended that users obtain information about the manufacturer's test field, and question whether the brakes are subjected to tests that are as realistic as possible. In addition to quality assurance measures during the construction process, a renowned manufacturer should also carry out a comprehensive final inspection. This means that every single safety brake which leaves the plant is subjected to a 100 per cent inspection following complete assembly and adjustment. At renowned manufacturers, all the determined measurement values are archived together with the corresponding serial numbers of the brake in an electronic database. Only in this way 100 per cent traceability can be ensured.

Checklist for procuring servo brakes

• Please ensure that the safety brakes are also suitable for dynamic braking actions and have been tested under realistic conditions. Please also enquire here about the testing possibilities of the manufacturer
• Make sure that the supplier implements a 100 per cent final inspection, including the automated storage of all test data. For gapless traceability, the brakes must also be labelled with a unique serial number
• Check the response times of the brake (attraction/drop-out). You can only achieve short, reliable stopping distances using a quick brake and constant switching times throughout the service lifetime
• Please make sure that the supplier provides safety characteristic values for their brakes. You need these values for your safety review
• Especially in the relatively young robotics sector, make sure to rely on manufacturers with experience in the development of lightweight brakes. Can the supplier demonstrate cooperation with renowned research institutes? Ask for a consultation to see the differences
• Take the time to view the production facilities of the supplier, especially in case of important purchases. You can convince yourself what is really going on behind the scenes during a visit to the plant

Learn more at www.mayr.co.uk.