step motor..?


Thread Starter

Dominik Banaszczyk

Hello, I am looking for drive of a chain with indexing movement. Single linear move of the chain is about 10 cm. Requested torque 300Nm. I'm going to use step motor, but I am not sure if that kind of motor (400 W) is available ? Maybe someone has other solution ? Regards Dominik Banaszczyk Electronic Engineer of RCC Tel.: +48 42 225 14 30 e-mail: [email protected] Philips Lighting Pabianice S.A. ul. Partyzancka 66/72 95-200 Pabianice Poland

Charlie Griswold

Dominik, You could always buy a cam indexing unit. It is a mechanical cam box, powered by a motor, that has index and dwell time designed into it. The motor could be standard AC and run continuously. It is a robust system designed for continuous running automation. It would theoretically have greater life than starting and stopping a motor. For safety or immediate stops the system should have a slip clutch, or an electric clutch, to disengage the gear box. Camco, Furgerson and Sankyo are some manufactures of such systems. Charlie Griswold Automation Engineer SEQUENOM, INC. Automation Systems Group 11555 Sorrento Valley Rd. San Diego, Ca. 92121-1331 858-350-0345 x 497 fax 858-350-9237

Michael Griffin

At 17:00 14/03/01 -0500, Dominik Banaszczyk wrote: <clip> > I am looking for drive of a chain with indexing movement. Single >linear move of the chain is about 10 cm. Requested torque 300Nm. >I'm going to use step motor, but I am not sure if that kind of motor (400 W) >is available ? <clip> What accuracy and repeatability do you require? I've worked with several indexing chain conveyor systems, but they did not require a high degree of accuracy in the index. There are a number of different types of chain conveyor systems, and the indexing method is application dependent. The usual method was to use a DC motor driven by an inexpensive drive (e.g. KB Penta drive) with dynamic or regenerative braking. This was used on systems where parts were transported in cradles attached to a pair of parallel chains. When worked on, the parts would be picked up by a mechanism which would allow for some slight inaccuracy in the initial position. Each index position of the chain would have some sort of flag (often part of the cradle) which could be detected by a sensor which controlled the index. It was considered important to make the system insensitive to the actual index length, since the chain will stretch as it wears. The usual index length used was approximately what you have stated. These systems used either one or two position sensors. Two sensors allowed for both a "slow down position" and "stop position". When the conveyor flag reached the first sensor, the drive would go to a slower speed (briefly) before reaching the "stop" sensor. This was to provide better indexing accuracy. In many cases, this was found to be unecessary, and sufficient accuracy was obtained with just a "stop" flag. I think typical accuracy was something around +/- 2mm or better (at least the process could tolerate that much). The conveyor could be "homed" in slow speed if the machine was re-started part way through an index. The DC drives were not very expensive, and were quite well suited to the stop/start cycle involved in indexing. Brush wear has not been a significant maintenance problem as the motors do not run continuously. Another method for indexing chain conveyors involved a ratchet-like mechanism activated by a pneumatic cylinder. The index stroke was set by the cylinder stroke and the ratchet mechanism. This system obviously did not allow for chain stretch or for "homing" the chain if it was re-started part way through an index. I have seen this particular only on older machines (including one controlled by relay logic). This system can be mechanically troublesome. A third method involved a motor driven cam drive (e.g. Camco or something similar). The motor would run continuously, and the index timing was controlled by the gear box. The cam drive would have a long "dwell" (most of the cycle) which would be the period for which the chain was stopped, and the chain would index when the "dwell" was over. The gear box had an encoder which was fed back to the PLC to control the timing of the production processes. Various steps would be triggered in the machine according to the encoder count. Since the motor never stopped, the mechanisms had to be able to get out of the way whenever the chain wanted to index. This sort of mechanism was best suited to processes which were strictly time dependent. It was very difficult to integrate something whose timing may vary depending upon what was happening in the process. For some applications it seems to work fairly well, for others it is completely unsuitable. ********************** Michael Griffin London, Ont. Canada [email protected] **********************
Charlie, Yes, cam indexing unit is simply solution. But what if I need to precisely synchronize two independent machines ( feeder chain & arm conveyor ) ? Mechanical cam does not give me opportunity to steer process by momentary speed control... I guess ?. Beside, I had already S5 Simatic control system and I want to use it as a drive control. Additionaly I want to eliminate very complicated mechanical control system with many cams, clutchs and shafts... ufff. In draft version it should be two motors drive (end & front of chain, and it is necessery in process) controlled in closed speed ACS. That is why I am asking for information about step motors and similar solutions. Maybe shaft endcoder and some controller (frequency or DC controller) coupled with S5 will be good ? Regards Dominik

Charlie Griswold

Dominick, Timming of a cammed indexer is genearlly accomplished like this.... As mentioned earlier, an encoder mounted in-line with the indexer's driven shaft can count the revolutions made and tell the PLC when the "dwell" is almost over, when the line is in index (motion), and when the index is over. This data is used to sync the other devices in the line and ensure all tooling is out of the way before index. If tooling is not done with it's task as the end of dwell is approched the machine shuts down. To prevent constant shut downs, the selection of an indexer is based on the longest station cycle time. Based on this planned cycle time the gear ratio, of the indexer should put the motor rpm running at it's optimum running speed. Usually a motor speed control is included in the design to fine tune this predicted cycle time of the assembly line. On most indexers an extra drive shaft is supplied to power other stations off the same motor that powers the line. If all your automation is powered by this same shaft, timing is all mechanical. The faster you drive the line; the faster all the tooling operates. Provided everything is clamped together tight, no crashes. On these same indexers dwell and index state can be easily determined using two Proximity switches with targets. One revolution of this shaft represents one full index-dwell cycle. Two "Flags" are clamped to the indexer's extra output shaft. One switch simply warns when dwell is being approached. The other switch is triggered by the "flag" for as long as the index is occurring. This is the method I prefer and recommend for indexers. Discuss your application with an index manufacturer for more assistance. However you use an indexer, I strongly recommend the use of an over-load clutch between the indexer's output and the items it powers. Cammed indexers DO NOT STOP for anybody, or anything, until the motor is off. Good luck, Charlie Griswold Automation Engineer SEQUENOM, INC. Automation Systems Group 11555 Sorrento Valley Rd. San Diego, Ca. 92121-1331 858-350-0345 x 497 fax 858-350-9237