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Linear motor based part retriever accelerates motor production

* more than triples throughput of lamination stamping process

Baldor has developed a high performance part retriever for use at the company’s stamping plant in St. Louis, Missouri. Based on a new industrialised version of Baldor’s LSE linear motor stage, controlled by a MintDrive intelligent single axis servo drive, the retriever accelerates the manufacturing process and provides a more flexible and robust automation solution than the standard mechanical retrievers on the market.

The retriever takes motor lamination blanks out of a stamping press, and was initially developed to handle 57cm diameter parts for one of Baldor’s series of large frame size motors. Prior to installing the retriever, the lamination blanks had to be removed by hand, which meant that the press required single stroking and could only produce 12 parts per minute. It is now able to run in continuous mode, at up to 40 strokes per minute. Furthermore, the system accommodates blanks with diameters of more than 86cm, which has enabled Baldor to bring the manufacturing of laminations for three further series of large frame size motors in-house. Previously, these larger parts were produced by an outside vendor. This has enabled the company to reduce lead times significantly.

Baldor originally considered employing a mechanical retriever – these are used in other areas of the plant – but quickly decided that they were far from ideal for this particular application. Mechanical retrievers suffer from lengthy set-up times – typically 15 minutes or more – and the linkage that sets the stroke positions can be difficult to accurately adjust and is prone to breakage, which sometimes results in the carrier tray being punched by the press. Furthermore, the stamping press has a light curtain across its front to ensure operator safety; the linkage periodically interrupts the light beams, especially when the press is operating at speed, causing it to enter an emergency stop mode. The company consequently chose to develop its own, low-profile retriever to overcome these limitations.

The retriever is based on a new industrialised version of Baldor’s LSE range of enclosed linear stages, built at the company’s plant in Santa Clarita, California. Designed specifically for use in the type of hostile environment presented by stamping and metal working facilities, the new stage features wrap-around bellows and internal baffles to protect the linear motor from particles and fluids. It also incorporates a new type of linear re-circulating bearings that run longer between greasing, to minimise maintenance and system downtime.

The linear motor is a new low profile iron core design featuring a modular magnetic track, which enables designers to achieve any desired travel length simply by placing sections end-to-end. The retriever uses a pre-production model with a travel length of 1.7 metres, capable of producing 222N of continuous force, and more than 590N peak. Since the retriever was built, the motors – known as LMIC models – have entered production and there are now more than 20 standard sizes available. The industrialised linear stages incorporating these motors are also now in production, and include units with travel lengths of up to 2.5 metres and continuous force ratings in excess of 889N.

The linear stage is controlled by a 7.5A MintDrive brushless AC servo drive, which incorporates a powerful motion controller. In addition to controlling the position of the stage, the drive also handles all phase commutation for the linear motor, using feedback derived from Hall-effect sensors embedded in the motor’s coil assembly.

The stamping press crankshaft is fitted with an encoder to provide positional feedback to the controller, which uses the crank angle data to update an electronic cam; this then controls the position of the stage as the ram of the press moves. The dynamic performance is impressive. Even when stamping 86cm diameter lamination blanks for very large frame size motors, the press operates at up to 40 strokes per minute. During this time frame, the stage moves the carrier tray 1.7 metres in 0.6 seconds, pauses while the part is ejected, moves 1.7 metres out in 0.6 seconds, and pauses again while the die stamps the next part.

Baldor produced all the application software for the part retriever using the Mint Workbench PC-based development environment and the Mint motion control programming language. This combination of an easy-to-use Windows front end and a structured high-level programming language greatly simplifies software development, and allowed the prototype control program to be written in a couple of days. The language’s ‘keywords’ for various complex motion control functions include an electronic cam, which formed the basis for the program. Using the Mint drive’s integral I/O, for simplicity of use in a production environment the retriever is fitted with a selector switch to call different tables of positions for the electronic cam to follow; reconfiguring the retriever to handle laminations for a different frame size motor simply involves moving the switch and changing the carrier tray.

The retriever has been working flawlessly for the past eight months. Baldor is now planning to build a similar system to replace a mechanical retriever serving one of the other presses in its St. Louis plant.