2012

Motion Control and System Engineering Considerations

Motion systems with low-ratio drives or direct drive operation have a better survival rate, in general, for manual back driving that occurs when an operator manually clears jams in a system. Many high-ratio gear trains needed to provide hightorque output from a fast, low-torque motor can be easily damaged when manually back driven, causing either immediate — or worse — slightly delayed failures or intermittent operation.

Figure 2 shows a portion of a bank of ten positive displacement pumps used to package contact lenses. These pumps take advantage of the capabilities of direct-drive hybrid servo operation in a high-stiction environment.

Lead-screw-based pumps and linear actuators generally have a critical speed rating in the range of 500 to 2000 RPM, which allows direct driving with a hybrid servo while utilizing its optimal performance range. Direct drive operation saves the cost and maintenance of adding gearheads, while reducing size and weight. Direct drive operation with high-torque hybrid servomotors can be effective in de-capping and recapping sample tubes, as the torque is easier to measure at the motor without the added effects of a gearhead. Direct drive systems eliminate gear backlash and wear, and typically function better in highstiction environments such as pumps with sliding seals. Lack of overshoot, backlash, and ringing on pumps can be especially important to produce repeatable results.

Summary

Consider the system engineering approach to design as a means to divide and conquer a complex system. Proper partitioning and robust interfaces can greatly simplify subsystem testing and system integration at the design stage, in production, and in the field. Proper selection of motor types as well as controllers can significantly speed the design and optimize the final product.

This article was written by Don Labriola P.E., President of QuickSilver Controls, Inc., Covina, CA. For more information, visit http://info.hotims.com/40430-320.