Author:
Petr Kotajný, KZV, s.r.o.
The main product of Commercial Railway Research, Ltd. (KZV), a company engaged in the development and manufacturing of diagnostic equipment for railway superstructure, is a system named KrabTM. It is a trolley designed for continuous measurement of track geometry and features evaluation software. This device was developed not only for domestic Czech railways but also for export abroad. Like any measuring equipment, Krab requires regular calibrations. Thus, the developed calibration stand meets the technical requirements.
The Krab Trolley
During the measurement, the Krab trolley (Figure 1) is usually manually pushed along the track, though it can also be pulled or pushed by a two-way vehicle. The measurements of track geometry include an assessment of track gauge, alignment, top, cant and twist (change in cant) of the track. Krab is a chord measuring system, i.e., top and alignment of the track is measured by the entrenchment of an asymmetric chord. The entrenchment of the alignment and top is measured only on a single rail. The situation on the other rail belt is calculated using gauge and cant signal. For a more accurate measurement of cant, Krab is equipped with an auxiliary twist arm. The measurement is performed with a final sampling interval of 0.25 m. The data is collected using a rugged PDA with Windows Mobile OS. Readings are then transferred to a PC and further evaluated.
Figure 1. The Krab Trolley on the Track
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Stand Construction
The idea for a robotic calibration stand arose from the need to smoothen and refine the existing calibration method of Krab trolleys (and similar trolleys) as all the equipment in use should be calibrated annually. During
development, however, an interesting idea appeared to also simulate trolleys on the track along with calibration.
The Krab trolley is equipped with six sensors. One of them is a distance sensor, which can be simulated electronically. Therefore, it was sufficient to equip the stand with five motor drives.
The stand frame is composed of the upper part on which the trolley stands, and the lower part, which lies on the ground. Both parts are connected with joints. This connection allows the upper part to tilt with respect to the ground and thus to set the cant.
Because trolleys are produced in gauges ranging from 750mm to 1,668 mm, a corresponding mechanism allowing this range is used on the stand. Variation of the top and alignment is provided by a mechanism with two degrees of freedom. Movement of the twist arm is provided by a special mechanism.
All moving mechanisms are implemented using Berger-Lahr hybrid stepper motors. The stepper motor allows the tester to set up to 10,000 steps per revolution, but for use with the stand, 1,000 steps per revolution is sufficient thanks to the transmission. The conversion of the rotation-to-linear movement is provided by a belt drive and ball screws.
Control Hardware
For motion control, the PCI-7356 card from National Instruments (NI) is used. This card can control up to six stepper and DC motors, which can be plugged into the PCI slot of a desktop PC. The card also has eight analogue inputs and 64 digital inputs/outputs. The card also allows connecting a feedback input. In addition, the card allows onboard programming, so it can run application programs for motion control in time without using the real-time operating system. Used with the stand, the card works in a step/dir mode. This means that for every card, the engine generates two signals: step and direction. These signals are processed by a Berger-Lahr SD3-15 drive. Based on the step/dir signals, a converter generates a voltage signal in the form of three mutually shifted curves similar to a sine wave. These signals form a stator rotating magnetic field. The SD3-15 drives are powered by two BKE DC 24V switching power supplies.
As used, the stepper motors and motion control card are equipped with a step counter, so there is no need to use feedback sensors. Control is performed in an open loop. After turning on the stand, it starts running an
initialization program, which sends all the drives to an exact position on their limit switches, from which a specific number of steps is counted. Thus, the stand establishes its starting position. The practice has shown that this technique works very well, except that one must consistently ensure the position of the limit switches.
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LabVIEW
The PCI-7356 can be programmed easily in the LabVIEW graphical environment, which includes, inter alia, the NI motion controller with many features specialized for NI motion-control cards. Two basic programs were created in LabVIEW: calibration and simulation programs.
Calibration Program
The calibration program works in two modes. The first mode is the control of individual axes, which are intended for manual calibration using the Krab PDA so the movement to the desired position will be defined by the user. The second mode is automatic calibration (Figure 2), which takes place in the following steps: first, the stand is put in preprogrammed positions; next, the positions are programmed to form a closed loop and at each location, data is retrieved from Krab sensors and stored on the PC; after the cycle is complete, the calibration constants are calculated on the basis of the stored data; and last, the calibration protocol in MS Excel is generated in one click using ActiveX.
Figure 2. Automatic Calibration
Simulation Software
While running the simulation program, the Krab trolley is in acquisition mode. The stand performs the signals measured in the track or artificially generated signals. The Krab trolley evaluates and records the stand behaviour.
After the simulation is finished, verification can be done comparing the recorded and performed signals. This will reveal any errors in the tested trolley.
The simulation software program is running in two parallel loops. This is not a problem because LabVIEW manages parallelism. The program uses PCI-7356 card with a buffer, to which data is added at the time, when the number of samples sinks below a set limit. Since the stand is equipped with five drives, the last axis serves to
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generate an incremental rotary encoder signal with the help of two D-type flip-flop circuits.
Conclusion
The calibration and simulation of the stand (Figure 3) was already tested and proven during numerous previous calibrations and simulations. The hybrid stepper motors used seem to be very precise. They do not lose step and do not suffer from unwanted resonance, as with motors with permanent magnets. Perhaps the only disadvantage of these motors is that when disconnected from their power sources, they hold for a moment and lose their
position. This deficiency was solved by using the motor brake. Using the LabVIEW development environment and a large quantity of ready-made features, one can easily and quickly program control applications. The advantage of the solution is also the ability to easily and quickly add new features, which is expected in the future.
Figure 3. Calibration and Simulation of the Stand
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