Integrated Linear Motion in Medical Device DesignIntegrated Linear Motion in Medical Device Design
October 13, 2010
There are a number of linear motion technologies availableto machine and device designers, each with advantages and disadvantages.Medical device design in particular, in which applications typically requirehigh accuracy and repeatability, linear motion systems with belts and pulleysor rack and pinion are unlikely to meet performance specifications. Likewise, forclean and quiet operation, hydraulic and pneumatic systems are inappropriate.
Mechanical linear motiontechnologies, on the other hand, provide high performance and reliability,clean operation and low maintenance. The motion technologies in this group mostsuitable for medical device/machine applications include the linear motor,linear slide with rotary motor and stepper motor linear actuator.
A linear motor is essentiallya stepper motor where the stator has been "unrolled" so that instead ofproducing torque (rotation), it produces linear force along its length. Linearmotors allow direct coupling to the load and deliver high performance, but at acomparatively high cost. Pricing a linear motor system must include costs for acomplete stage, or enclosure, with linear bearings, limit switches, cabletrack/carrier, protective bellows and linear encoder.
Integrated Linear Motion in Medical Device Design_A
A linear slide consists of aframe-mounted carriage that travels along a lead or ball screw. A rotary motoris mounted to one end of the frame where it is coupled to the screw that itturns to produce linear motion. Advantages of this technology are accuracy andrepeatability, with support provided by the frame already in place.Disadvantages are the high cost of components and locating or designing an adapterfor the motor interface.
A stepper motor linearactuator 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 becoupled to a load using a variety of methods. Versatile, low-cost stepper motorlinear actuators are available with different shaft styles, including:
Non-Captive Shaft Style -the threaded shaft, extending throughthe motor, moves axially with the rotation of the nut integral to the motor'srotor.
External Shaft Style - thethreaded shaft, integral to the motor's rotor, rotates to move a shaft-mountednut axially.
For a complete linear motionsystem, each of the previous linear motion technologies listed requireadditional components including a compatible stand-alone controller, drive,encoder and cabling.
One technology, however, thestepper motor linear actuator, is available with all of these componentsintegrated into a single, compact linear motion product not much larger thanthe linear actuator alone. As a result, the integrated stepper motor linearactuator can significantly lower device cost and complexity.
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Integrated Stepper Motor Linear Actuator
In response toincreased demand for smaller,low-cost machines, machine designers are trending toward motion componentsolutions with reduced size and price. As a result, integrated motion controlsolutions - 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 ofcreating a device, which include:
Simplified Design Process and Machine Complexity - Eliminatingthe need to specify individual components shortens the design cycle; fewer componentsincrease 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 andinteroperability testing of individual components is eliminated when using anintegrated motion solution.
Smaller Machine Size: Compact products with reduced footprintdramatically reduce space requirements.
Increased Machine Reliability - Significantly decreasingwiring in a machine minimizes the largest source of electrical noise; reducedfield service saves time and money, and increases positive perception ofmachine quality.
Lower Machine Cost - Reducing the number of machine componentslowers costs for design, procurement and inventory; installation cost savingsinclude reduced man hours, with fewer errors requiring troubleshooting; smallermachines lower transportation costs to end-users.
Clark Hummel is managerof Applications Engineering, IMS Schneider Electric Motion USA.
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