Use of zero-speed safety relays at Röders TEC


Main spindle of a high-speed milling machine  

The main spindle in the milling machine enables speeds of up to 90,000 revolutions per minute

  • Röders TEC GmbH, based in Soltau, Germany, manufacturers high-speed milling machines.
  • To prevent injury to operating personnel, the required safety equipment must be impossible to open until the dangerous movement has come to a complete stop.
  • Sensor-free PSR-MM25 safety relay modules monitor single and three-phase AC motors.
  • Because additional motion sensor equipment is not needed, the startup effort and costs are reduced.

Customer profile

Company facility in Soltau-Harber  

The Soltau-Harber location is home to machine building, assembly, purchasing, and design

Röders TEC GmbH employs more than 500 people worldwide. The company started developing HSC (high-speed cutting) technology more than 30 years ago, at a time when this technology was unknown.Today, around 2,500 HSC milling machines are used in approximately 50 countries, producing complex geometries with special surfaces, for example for the medical technology industry.


Assembly hall in Soltau-Harber  

The machines are assembled in the hall, which is air-conditioned with high precision

The main spindle of the high-speed milling machines produced by Röders TEC spins at speeds of up to 90,000 revolutions per minute. To protect machine operators from the resulting dangers, the working area is protected by a movable guard. Product standard EN 12417 addresses the safety of machine tools and machining centers. It also stipulates that safety equipment must be impossible to open until the dangerous movement has completely stopped. To this end, machine operators must use safety equipment with a guard locking device in which a time control or zero-speed monitoring system is installed to ensure delayed unlocking. In comparison to a time control system, the advantage of zero-speed monitoring is that the dangerous movement is actually monitored. This is because a safe zero-speed or enable signal is only generated after shutdown, when the drive has stopped coasting. With a time control system, the safety door can be opened after a predefined length of time; theoretically, the spindle could still be moving.


PSR-MM25 zero-speed safety relay  

The measuring principle of the PSR-MM25 is suitable for connection to regulated and unregulated electric motors

The PSR-MM25 sensor-free zero-speed safety relay is a safety relay module that can be used to monitor single and three-phase AC motors, as well as DC motors. The measurement inputs of the module, which is just 12.5 mm wide, are connected directly via the motor connection cables. Standard sensors such as encoders or proximity switches, which must be additionally attached to the moving parts, are therefore unnecessary. “In contrast to the evaluation of external sensors, the sensor-free monitoring system can be easily integrated into our machine concept,” said Olaf Hartmann, who is responsible for the electronics construction of HSC machines at Röders TEC. “And this solution has proven to be very robust against external influences.”

Instead of processing the signals of movement sensors, the PSR-MM25 analyzes the remanence voltage generated in the motor windings. If the voltage induced by residual magnetization is below a variable threshold, the module signals that the motor is at a standstill by activating the safe relay output. Because the voltage threshold can be set within a range of 50 to 500 mV, zero-speed safety relays can be adapted to various motor versions. Via an additional configurable delay, the outputs of the PSR-MM25 only activate once the voltage has dropped below the threshold and the subsequent delay time has elapsed. “Depending on which production task the HSC machine is involved in, there is a large range of main spindles. Since the monitoring function of the zero-speed safety relay functions independently of the spindle type, we can use the PSR-MM25 in all applications,” explained Hartmann.

For further processing within the HSC machine, the safe zero-speed signal of the PSR-MM25 is linked to the enable signal of a key switch. The resulting signal triggers the locking solenoid of a locking mechanism so the operator can open the safety door. To prevent an unexpected startup while the safety equipment is open, the spindle’s motor contactors are disabled when the safety door is unlocked.


“Because the sensors that are typically necessary for monitoring the movement can be omitted, we can save material costs and time during construction and startup,” said Hartmann in conclusion. “Another advantage of the solution is the simple approach of the safety function: sensors do not have to be included in the calculation.” Alongside its easy integration into the drive concept, another impressive feature of the PSR-MM25 is its narrow overall width. To realize its switching tasks, Röders TEC also installs relays of the RIFLINE product family in its milling machines – the subject of another reference document.


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