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Test system gets straight to the point

14 September 2012

Actuators that switch railway points are clearly mechanisms that are highly safety-critical. Proof that these actuators meet the requirements of exacting railway standards is one of the main functions of a new test system designed by QM Systems with some motion technology help from Lenze.

QM Systems' managing director, Nick Field insists that his company's success is built on ground breaking innovation, claiming that rare ability to deliver fully integrated systems that combine mechanical, electronic and software components. The company is relatively young, starting out in the 1980s to serve the high-integrity electronics test market. More recently it has established an automation division that specialises in the design and manufacture of systems that are often deployed in harsh or safety critical environments. Today the company operates from two sites, one in Aldershot, Hampshire and the other at Holt Heath, Worcestershire. 

Point changers perform the simple task of moving railway tracks, but must do so under high dynamic forces, to the highest reliability and with comprehensive safety monitoring. They operate from a 110V dc supply via a ‘throw bar’ that exerts a force of around 3kN to move the rails. The sideways axial movement is created by a dc motor driven ball screw, and position is monitored by safety relays.

QM’s challenge was to present a method of testing that would be credible to a conservative and highly safety conscious worldwide market. Clearly, it had to be accurate, and the measurements recorded and reported with precision. Moreover, the test process had to be faster than the usual 1.5 hours, while providing a step change in safety with the minimum of manual handling. The system had to be capable of testing trackside and in-track changer designs both old and new.

The test procedure
The new test system connects to point changers that are brought in on wheeled trolleys These are accurately located on dowel pins, followed by a mechanical connection to the throw bar and an electrical connection to the motor. The necessary external resistance to the throw bar is provided by a motorised ball screw linear actuator, carefully controlled to deliver a programmed stroke/force profile.

The points changer is zeroed and then driven to the closed position. Stroke measurement is via an encoder and the stroke defines the changer model for the test report. The operator manually adjusts the torque limiting clutch in the changer to trip at a force that is dialled up on one of the two touch screen operator stations, running National Instruments test software applications . The electrical function of the points changer is tested automatically, including the operation of the safety limit switches

The principal drive component supplied by Lenze was the linear actuator, a Servomech unit featuring a ball screw mechanism that allows continuous operation with low backlash. Dynamically rated at above 10kN and capable of linear speeds of up to 78mm/s, the actuator is driven by an asynchronous ac motor with incremental encoder and blower, suitable for continuous operation at low speed.

In turn, the motor is controlled by a Lenze 9400 series servo inverter, which is capable of delivering controlled torque and force at zero speed. QM also introduced a high-accuracy load cell between the linear actuator and a bar that simulates the moving rail, the latter being supported on linear bearings to avoid side loads on the load cell.

According to Nick Field, a key factor in the success of this system was the development of a complex control algorithm, which is used to simulate a rail. It is possible to programme any load up to 25kN although 4kN is often the simulated load used. The drive runs in servo mode and, thanks to some particularly clever PID control loop programming, the actuator is made to act as a constant force spring, opposing the force of the throw bar and moving in either direction through a stroke of up to 150mm.

Under test the points changer starts from standstill and moves through the stroke in three seconds whilst the actuator delivers a constant resistive force. An early problem solved by the QM team was the unpredictable nature of the mechanics - largely overcome by the flexibility of the drive software. The PID control loop was fine tuned using function blocks available in Lenze’s ‘L-force Engineer’ software

The machine provides test results in the form of a two page data sheet that includes the measured forces, the settings for the torque limiting clutch and all the electrical characteristics including flash, bond and continuity. These results give visual evidence of quality and greatly assist the successful marketing of points changers by their manufacturers.

However, the QM test system has achieved much more than this. A typical test interval has been reduced from 1.5 hours to just 30 minutes, and only one operator is required instead of the previous two. Comprehensive guarding with light beams and safety interlocks, and no requirement for heavy lifting provides an additional safety bonus for the operators.

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