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Positioning Methods for Overhead Cranes and Monorails


Application

Final positioning is a critical aspect of crane and monorail systems. The positioning method selected affects the duty cycle and final spotting accuracy of the system.

Problem
With operator or limit switch assisted positioning, the stopping sequence determines the positioning accuracy. If you reach a selected location and you use the electric brake to stop, then the final position can vary due to brake wear, wheel slippage and inertia due to load variance from no load to full load.In the past, spotting accuracies of +/- two inches were typical without a secondary fine positioning method. In many cases, where greater accuracies were required, operators would first stop the system by using the electric brake, and then release the brake and mechanically position the carrier with an auxiliary mechanical device 'grabbing' or 'pinning' (shot-pin) the carrier in final position. Another method uses two multiple limit switches or sensors and targets to provide a slowdown and stop method of positioning. This positioning method provides greater accuracy than using a single limit, but the reduced travel speed results in slightly higher cycle times.

Solution
Counter/Limit Switch
A more accurate and sophisticated approach to positioning is to use a multi-speed stopping method. A series of limit switches or other inputs can indicate the location of the crane or monorail. In the simplest case, the crane or monorail travels at a high speed until it reaches a preset count or position prior to the final location. At this point speed is reduced or a deceleration program is initiated. As it approaches the final stop position, location inputs further reduce the speed until a position just before or at the final stop. The final positioning accuracy is now dependent upon the sophistication of the crane drive.

With today's IMPULSE® AFD technology it is possible to obtain higher speeds with controlled acceleration/deceleration to suit the variable travel distances.
Previously, the positioning inputs were set to allow acceleration/deceleration from the highest speed obtainable. If the unit traveled only a short distance, it sometimes was only accelerating or decelerating, and never did reach full speed. Using today's AFD controllers, PLCs, microprocessors or a combination of these technologies, it is possible to optimize the acceleration/deceleration rates and speed for the distance to be traveled. In this way we have been able to utilize higher travel speeds over the longer distances and still retain positioning accuracy. This results in faster cycle times and increased throughput.

To further reduce cycle time, bridge, hoist and trolley motions can be programmed to operate at the same time when it is safe to do so. Multi-axis control is available to synchronize these various crane and monorail motions. This can be done using a PLC, PC or a microprocessor based system designed for this operation. Multi axis controllers can be programmed or self-taught to allow location and positioning of a crane or monorail anywhere within a volume or plane of service.

Encoders
Encoders coupled to the motor, running off a drive or idler wheel, or driven by a separate tire riding on the crane rail or monorail track, provide another method for determining bridge, hoist and trolley location. The encoder signal is used in a similar manner to the counter/limit switch method in that revolutions or increments of a revolution are counted, and the controller determines position from this information. In the case of a trolley or bridge, if the wheel 'slips' the count is not accurate. To correct this, additional targets can be added at intervals along the rail which trip a realignment limit switch, correcting any deviation.

Two types of encoders are used for crane location. An incremental encoder has a pulsed signal output similar to a limit switch output. The controller counts the number of pulses to determine the location from a zero point. The controller must determine direction and location. An absolute encoder has a coded address output. This address allows the controller to determine the address location from a zero point. Direction of travel (up, down, forward, reverse) can be determined by the increasing or decreasing of the address. Generally, incremental encoders are used in most crane applications. Absolute encoders are more expensive but provide more information and are often used in hoisting applications where location and speed sensing is critical.

Distance Sensors
Rather than using counting or address positioning, there are several stand alone and packaged distance sensing methods available using a sonic, infrared laser. The stand alone devices must be connected to a PLC or PC for calculations and control. Packaged systems contain their own microprocessor or software to allow them to determine the location from a fixed point. This location information must be interfaced to a PLC or PC so that information is useful to the rest of the control system.

Laser sensors transmit a modulated red laser beam which is reflected off of a special reflective target. They operate in the same means as photoelectric sensors and are available in the retro reflective and proximity modes. With the retro reflective arrangement ranges of four inches to 1500 ft. are available. These retro reflective devices use two laser diodes (one is the sender and the second is the receiver). At the receiver, the device amplifies and digitizes the return signal and sends it to a microcomputer. The unit computes the distance by comparing the phase of the outgoing laser beam with the phase of the returning signal. These units can determine the distance the target is from the sender within +/- 1 mm (.040") resolution. Laser light is in the visible light spectrum so these units are affected in the same way as visible light photoelectric sensors.

A self contained total control system contains a modulated infrared LED. A transmitter sends a light beam to a reflective target and the distance is computed based on the time it takes the transmitted light to return. An on-board microprocessor compares the phase of the transmitted light with the phase of the reflected light and calculates the distance with a resolution of 1 mm (0.040 inch). These systems often have ranges exceeding 2,000 ft. and positional repeatability of +/- 0.40 inch. Outputs can be coupled to a PLC or PC.

Result
The important thing is to select the best method to suit the application requirements. Will the device meet the spotting accuracies and duty cycle requirements? What is the environment that the device has to operate in? Is it to operate in snow, rain, dust, fog, smoke, steam, high heat, cold, etc.? Is vibration a problem? Can it be interfaced with the controller? Will it respond quickly enough? Will it be properly maintained? Is it cost effective? Magnetek can help discern the proper and most cost effective positioning method required for your individual application needs.
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