There are a number of linear motion technologies available
to machine and device designers, each with advantages and disadvantages.
Medical device design in particular, in which applications typically require
high accuracy and repeatability, linear motion systems with belts and pulleys
or rack and pinion are unlikely to meet performance specifications. Likewise, for
clean and quiet operation, hydraulic and pneumatic systems are inappropriate.
Mechanical linear motion technologies, on the other hand, provide high performance and reliability, clean operation and low maintenance. The motion technologies in this group most suitable for medical device/machine applications include the linear motor, linear slide with rotary motor and stepper motor linear actuator.
A linear motor is essentially a stepper motor where the stator has been "unrolled" so that instead of producing torque (rotation), it produces linear force along its length. Linear motors allow direct coupling to the load and deliver high performance, but at a comparatively high cost. Pricing a linear motor system must include costs for a complete stage, or enclosure, with linear bearings, limit switches, cable track/carrier, protective bellows and linear encoder.
A linear slide consists of a frame-mounted carriage that travels along a lead or ball screw. A rotary motor is mounted to one end of the frame where it is coupled to the screw that it turns to produce linear motion. Advantages of this technology are accuracy and repeatability, with support provided by the frame already in place. Disadvantages are the high cost of components and locating or designing an adapter for the motor interface.
A stepper motor linear actuator uses a threaded shaft inserted in a nut integral to the motor's rotor. Linear motion is produced by the rotation of the threaded shaft, which can be coupled to a load using a variety of methods. Versatile, low-cost stepper motor linear actuators are available with different shaft styles, including:
- Non-Captive Shaft Style - the threaded shaft, extending through the motor, moves axially with the rotation of the nut integral to the motor's rotor.
- External Shaft Style - the threaded shaft, integral to the motor's rotor, rotates to move a shaft-mounted nut axially.
For a complete linear motion system, each of the previous linear motion technologies listed require additional components including a compatible stand-alone controller, drive, encoder and cabling.
One technology, however, the stepper motor linear actuator, is available with all of these components integrated into a single, compact linear motion product not much larger than the linear actuator alone. As a result, the integrated stepper motor linear actuator can significantly lower device cost and complexity.
Integrated Stepper Motor Linear Actuator
In response to
increased demand for smaller,
low-cost machines, machine designers are trending toward motion component
solutions with reduced size and price. As a result, integrated motion control
solutions - all-in-one rotary motor with drive and/or controller and encoder -
represent a rapidly growing market segment.
Integrated motion control products, both rotary and linear versions, reduce the factors that impact the cost of creating a device, which include:
- Simplified Design Process and Machine Complexity - Eliminating the need to specify individual components shortens the design cycle; fewer components increase ease of manufacturability and decrease potential for assembly errors; reduced complexity eliminates potential failure points, e.g., fewer components, less wiring.
- Shorter Time-to-Market - Research, procurement and interoperability testing of individual components is eliminated when using an integrated motion solution.
- Smaller Machine Size: Compact products with reduced footprint dramatically reduce space requirements.
- Increased Machine Reliability - Significantly decreasing wiring in a machine minimizes the largest source of electrical noise; reduced field service saves time and money, and increases positive perception of machine quality.
- Lower Machine Cost - Reducing the number of machine components lowers costs for design, procurement and inventory; installation cost savings include reduced man hours, with fewer errors requiring troubleshooting; smaller machines lower transportation costs to end-users.
Clark Hummel is manager of Applications Engineering, IMS Schneider Electric Motion USA.