Main spindle of a high-speed milling machine

Safe motion monitoring in high-speed milling machines Sensor-free safety relay modules monitor single- and three-phase AC motors in machine building

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, commissioning effort and costs are reduced.

HSC milling machine

High-speed milling machines from Röders TEC GmbH

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 to produce complex geometries with special surfaces for the medical technology industry, for example.

Main spindle of high-speed milling machines from Röders TEC

The top priority is operating personnel safety

The main spindle of high-speed milling machines from Röders TEC move at up to 90,000 rpm. 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 not generated until 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.

In contrast to the evaluation of external sensors, the sensor-free monitoring system can be easily integrated into our machine concept. And the solution has proven to be very robust against external influences.

Olaf Hartmann - Röders TEC, Electrical design
Sensorless PSR-MM25 zero-speed safety relay in the control cabinet of a high-speed milling machine at Röders Tec

Sensorless zero-speed monitoring as a solution

The PSR-MM25 sensor-free zero-speed safety relay is a sensorless 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. 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 do not activate until the voltage has dropped below the threshold and the subsequent delay time has elapsed.
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 an interlocking device 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.

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.

Olaf Hartmann - Röders TEC, Electrical design
Phoenix Contact Israel
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