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Motion Without Limits

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DLS Driven Linear System Catalog - 28033 System Accuracy & Repeatability Application Recommendations

A critical feature of the DLS system used in conjunction with an AC geared motor is that it does not have any position measurement or feedback, with positioning dependent upon the unit having a switch at the desired stopping or reversing position. If the unit advances to a switch, the ramp down command is issued as the switch closes, however as some time is required for the system to stop, then the carriage will overshoot the switch position by a distance which depends on the approach speed and the deceleration time. In high speed systems, this overrun can be quite substantial (10’s of mm) although it will be repeatable. If the customer has an application where improved accuracy is required from the system, it may be useful to have 2 switches next to each other. When the carriage touches the first switch, the PLC commands the drive to slow to a low “approach” speed. This means that when it contacts the second switch, the overrun is minimized. Using this (or a similar) technique it is possible to maintain system repeatability to well below 0.5mm.If the application requires even greater accuracy or more sophisticated position control, an alternative control system should be sought. In such instances, servo or stepper motor based systems driving the DLS through one of the suitable gearboxes should be considered. The accuracy will be partly determined by the motor and controller choice, but positioning to 0.1mm is achievable.There are a number of general points regarding the use of DLS units which will apply whether the customer uses a motor and drive from Bishop-Wisecarver, or makes alternative arrangements. The following points should be observed to ensure that the system works reliably and safely .With any prime mover, measures are required to prevent the motion going outside prescribed boundaries which might lead to a system crash. In intelligent systems it is normal to have 3 levels of security in this regard: software limits are programmed into the position controller; limit switches will be positioned outside the software limits, which will stop the movement when they are tripped; and physical end stops act as a final backup to safely arrest motion which has gone past the limit switches.Systems should be provided with an emergency stop facility. When using the supplied AC motor & drive, a dedicated controller input is provided for the stop function and this must be connected to common to allow the system to be run.The end stops which are supplied as standard are made from a special synthetic material which has exceptional deformation and energy absorbing properties, and these provide suitable crash protection in many applications. The end stops have a limited capacity safely to stop heavy and fast moving loads . When determining whether or not the fitted end stops are sufficient protection in a system, it should be remembered that the end stop must absorb the energy from both the moving load and the motor. In lightly loaded but fast moving applications, the motor can represent the largest load to stop , and sufficient deceleration distance must be allowed to prevent the motor rotor deceleration from over straining the belt. In applications where it is necessary to provide additional protection against high impacts at the end of the stroke, it is recommended that a separate shock absorber be fitted to the system.In many applications, the motor providing the driving force can be used to slow down the system as well as to speed it up. In some applications, notably when lifting heavy items, this arrangement may not give the required degree of safety. In such cases it may be beneficial to fit a brake to the motor . Bishop-Wisecarver can supply motors with a fitted brake on request.When determining the length of a DLS axis, it is important to allow additional stroke length beyond the anticipated length of movement. This will allow, in the event of the carriage going beyond a limit switch, a distance for the system to brake before the end stop is encountered. In many cases a length equivalent to one pulley revolution would be appropriate for the overrun zone, but this will depend very much on the application.Motor gearbox options have been selected to suit the capacity of associated DLS units. For critical applications where customers attach their own drive unit, an appropriate factor of safety must be applied to the combined system.The DLS is intended to be used as a component of a larger machine system. Depending on the application, there may be potential hazards which the designer will need to consider and address as appropriate. Any or all of the carriage, beam, belt and shafts can move at high speeds, so may need to be guarded. If using electrical prime movers or signalling, suitable precautions should be taken to eliminate hazards due to electrical shock or malfunction. The driving force is provided by a steel reinforced toothed belt engaging in a pulley - the customer should ensure that the application does not have a failure mode which could over strain the belt to the point of damage, or that such a failure mode does not pose an unacceptable hazard.If the customer requires further advice regarding the use of the DLS, please contact Bishop-Wisecarver and our trained Applications Engineers will be pleased to assist you.
© Bishop-Wisecarver Corporation 2007. All rights reserved. www.bwc.com 888.580.8272

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