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Linear actuator helps to test railway points

11 April, 2013

The Servomech actuator used for the QM application has a dynamic rating of more than 10kN and a linear speed of 78mm/s. The device, supplied by Lenze, incorporates a ballscrew mechanism which allows continuous operation and low backlash. The actuator is fitted with an asynchronous AC motor with an incremental encoder and blower, allowing continuous operation at low speeds. The motor is driven by a Lenze servo inverter which can deliver controlled torque and force at zero speed. QM is using a load cell between the actuator and a bar which simulates the moving rail, fitted on linear bearings to avoid side loads to the load cell.

“The key factor to the success of the test system is a complex control algorithm that is used to simulate a rail,” explains QM Systems’ managing director, Nick Field. “It is possible to program any load up to 25KN, although 4kN is often the simulated load used. The drive runs in servo mode and particularly clever programming of the PID control loop has been applied.

“Effectively, this makes the actuator into a constant-force spring opposing the force of the throw bar – a spring that can move in either direction through a stroke of up to 150mm,” he continues. “Under test, the points-changer starts from standstill and moves through the stroke in three seconds, while the actuator delivers a constant resistive force.”

According to Field, “one of the problems we solved was the unpredictable nature of the mechanics. This was overcome by the flexibility of the drive software”. The PID loop was fine-tuned using the function blocks in Lenze’s L-force Engineer software.

The test system gives results in the form of a two-page datasheet that includes the measured forces, the settings for the torque-limiting clutch, and all the electrical characteristics.

As a result of the easy handling, the QM system has cut testing times from 1½ hours to 30 minutes. Only one operator is needed, instead of the previous two. Comprehensive guarding with light beams and safety interlocks, together with elimination of heavy lifting, ensures the welfare of the operator. The success of the machine has opened the door for further similar test systems worldwide.

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