Replacing Pneumatic Cylinders with Moving Coil Linear Motors for High-Speed Bottle Rejection

State-of-the-art electric actuators today are linear motors. These are direct-drive devices, as opposed to ball screw drives. Direct drive, low moving mass, combined with very low-friction linear guides greatly increases speed, cycle repeatability, and cycle life when compared to air cylinders or ball screws. A case in point is the application of moving coil actuators to eject top-heavy bottles on a conveyor.

A leading global supplier of inspection and rejection systems for bottles and cans was struggling with the inherent variability of pneumatic rejection systems at higher line speeds. Their application involved ejecting lightweight bottles when the weight was NOK (Not OK). The bottles were top-heavy and needed to gently slide over approximately 100 mm, then retract quickly. With pneumatics, the process initially appeared adequate; however, the following day all diverted bottles began tipping over due to inconsistencies in the pneumatic cylinder’s speed. This is why they came to us.

Initially, the customer used an air cylinder. The system was tuned to a reject speed to which the bottles stayed upright. This tuning was performed manually. Speed controls were manually adjusted and relied on a fixed orifice and back pressure, both of which were affected by pressure variations from the regulator. The higher the reject speed, the faster the conveyor could run. The problem became evident the next day, when a combination of piston seal stiction and pressure variation caused bottles to fall over. Air cylinders typically vary in the time required to move a given distance by ±15 msec. As a result, the conveyor had to be slowed down, although uncertainty remained because the full speed tolerance range was unknown.

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A linear motor with low moving mass and very low static breakaway friction presents a different picture.

A number of tests were conducted:

• Speed variation over time: A speed of 280 mm/sec was used. Measured variation was ±1 msec.
• Stability testing: A wooden object with approximately the same size and mass as the bottle was placed on a surface with friction similar to the conveyor belt. By incrementing speed in 5 mm/sec steps, testing showed that at 280 mm/sec the bottle moved across the surface without wobbling. At 290 mm/sec, wobble was introduced, and at 300 mm/sec the bottle fell over.

A linear motor is driven by a controller amplifier, allowing extremely precise speed settings. Real-time encoder feedback provides accurate information on actual velocity. The guesswork associated with setting up pneumatic systems is eliminated, resulting in improved and repeatable performance.

Switching to moving coil actuators made the application a success. Virtually no cycle-to-cycle speed variation allowed the customer to achieve their targeted maximum line speed—something that could not be accomplished using an air cylinder. Air is compressible; electrons are not.