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Linear motor-based part retriever

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article image Operates at up to 40 strokes a minute.

AUSTRALIAN Baldor has developed a part retriever for use at the company's stamping plant in St. Louis, Missouri. It is based on an 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 standard mechanical retrievers. It takes motor lamination blanks out of a stamping press and was developed to handle 57cm diameter parts on large-frame motors. Before the retriever was installed the lamination blanks had to be removed by hand. The press required single stroking and could only produce 12 parts a minute. It is now able to run in continuous mode, at up to 40 strokes a minute. The system accommodates blanks with diameters of more than 86cm, allowing Baldor to bring the manufacturing of laminations for three further series of large frame motors in-house. These larger parts were previously produced by an outside vendor.

Mechanical retrievers suffer from typical setup times of 15 minutes. The linkage that sets the stroke positions can be difficult to accurately adjust and is prone to breakage. This sometimes causes the carrier tray to be punched by the press. 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 developed its own low-profile retriever to overcome these limitations.

Baldor’s retriever is designed 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 recirculating bearings that run longer between greasing, to minimise maintenance and system downtime.

The linear motor is a low-profile iron core design with a modular magnetic track, enabling designers to achieve any travel length by placing sections end to end. The retriever uses a preproduction model with a travel length of 1.7m. It is capable of producing 222N of continuous force and more than 590N peak. Since the retriever was built, the motors 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 2.5m 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 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 in the motor's coil assembly.

The stamping press crankshaft is fitted with an encoder to provide positional feedback to the controller. The controller uses the crank angle data to update an electronic cam, this controls the position of the stage as the ram of the press moves.

When stamping 86cm diameter lamination blanks for very large frame motors the press operates at up to 40 strokes a minute. During this time the stage moves the carrier tray 1.7m in 0.6 seconds, pauses while the part is ejected, moves 1.7m 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. The system combines a Windows front end and a structured high-level programming language. It allowed the prototype control program to be written in a few days. The language's keywords for various complex motion control functions include an electronic cam, which formed the basis for the program.

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.

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