Stepping up without a Lead

Solving difficulties with repetitive short-distance positioning, quietly

Rapid, short distance, high accuracy moves are notoriously difficult to achieve. Precision positioning and scanning applications in medical/biotech instrumentation typically require stepping in rapid succession through a sequence of precise X and Y coordinates only a few centimeters, or even a few score microns, apart. The task becomes increasingly difficult as speed and precision demands rise. Numerous mechanisms can complete short distance movements if time is not an issue — likewise, high accuracy devices. The technical challenges rise exponentially with need for simultaneous rapidity, accuracy, and short travel distance. Here is an analogy to illustrate the difficulty:

Consider the imaginary task of moving a large loaded truck in precise 6-in. increments. To overcome the vehicle’s inertia and static friction, the driver will press the “pedal to the metal.” Once the vehicle is moving, accelerator pressure is slackened and the driver’s foot shifts to the brake. However, vehicle momentum and the high engine speed will tend to carry the truck beyond the required 6 in. before the driver can brake hard. Such vehicle control tasks are obviously difficult, and are especially difficult if many short distance movements are required in rapid succession.

The positioning task is no less challenging when travel is scaled down to millimeters, accuracy tightened to microns, and positioning time shrunk to milliseconds. For example, in a white blood-cell imaging application, an X-Y stage positions blood coated slides beneath the lens of a high-res camera. The equipment captures images of white blood cells measuring 10-25 microns dia., requires 1 micron positioning accuracy. Typically, the equipment takes 50 to 200 blood cell photographs from a few cm2 of slide surface, requiring exceptional accuracy and speed.

Lead screws

Traditionally, instrumentation for positioning slides, test tube and micro arrays has depended on lead screw mechanisms. Today’s demands for higher throughput, which translates into faster acceleration and speed, as well as stop-on-a-micron accuracy, run into fundamental lead screw limitations.

The lead screw stage reveals the barriers to lead screw responsiveness. For a start, the drive motor and lead screw itself imposes substantial rotational inertia, which limits acceleration and responsiveness for short distance moves. The lead screw stage also has six bearings, which introduce speed-limiting friction. Worse, the lead screw itself, in driving against the nut on the load bearing platform, adds further friction, and more problematic, the potential for stick/slip that complicates starting and precise stopping. Finally, backlash between the lead screw and drive nut adds to system imprecision, especially when long term wear degrades lead screw integrity.

In quiet lab/hospital environments, lead screw positioning mechanisms are considered noisy. The lead screw itself needs to be greased, thereby introducing maintenance and sanitization issues. In raising lead screw rpm to maximize random access positioning, lead screws tend to spin off their lubricant, complicating maintenance efforts. Overall, given the number of parts and lead screw wear, downtime and long-term reliability suffer significantly in comparison with the linear motor’s excellent uptime record.

Shedding performance-limiting barriers

In contrast, the low inertia, direct drive Micro Linear motor from Copley Controls Corporation, Canton, MA, is suited for making the repetitive short distance moves that typically occur in high-resolution blood cell imaging, genome research and other medical applications. The Micro Linear motor consists of two basic parts: thrust rod and forcer. The thrust rod (see image) contains high intensity permanent magnets (A), and the load carrying forcer (B), whose electrical drive coils are encapsulated in an advanced lightweight polymer. The forcer, supported by low friction bearings, surrounds the thrust rod and is propelled along the thrust rod’s length by digitally controlled magnetic fields.

Polymer construction aids this motor in achieving the light weight and low inertia that are critical for fast response. An elegant tubular construction, with 0.016-in. symmetrical air gap between fixed and moving components; in contrast to the lead screw’s friction producing drive mechanism, there is no contact — hence no friction and wear — between thrust rod and forcer. The motor’s electrically energized magnetic fields develop a powerful drive force. Lightweight construction and freedom from friction and backlash permit the rapid acceleration, fast travel, and resonance free stopping necessary for fast repetitive positioning. The new motors can complete repetitive 1 mm X-Y movements with 1-micron accuracy in 25 milliseconds. Cost is comparable to lead screw drives.

These motors are available with peak force from 19 to 45N and continuous force from 3.1 to 8.7N. Peak acceleration ranges from 15 to 25 m/sec2 and peak velocity goes to 3 m/sec. The motor’s light weight, low inertia, and freedom from friction provide inherent advantages over lead screw mechanisms in repetitive short distance positioning applications. Thrust rod lengths from 62 to 510 mm are offered, with thrust rod diameter of 11 mm. The polymer forcer, which encapsulates the motor’s electrical drive coils, is available in lengths from 36 to 113 mm. Forcer width is 34 mm, height 25 mm. Loads mount directly onto the forcer, no additional mounting hardware is required. The motors operate from 75 VDC supplies. They are designed for sinusoidal commutation and incorporate built-in Hall sensors to assist commutation. A PTC temperature sensor signals the drive amplifier to shut down to prevent overheating.

The motor’s tubular, contact-free construction offers various advantages. Absence of friction, coupled with the forcer’s lightweight, yield acceleration almost an order-of-magnitude better than any comparable lead screw (to 290N). More subtle, but vitally significant for rapid starting and stopping, there are no stick/slip concerns to complicate servo control and braking. With only one set of bearings, versus six bearings and lead screw friction, the linear motor unites high speed and agility with better acceleration. Lastly, unlike lead screw positioners, the linear motor has no mechanism to create accuracy-degrading backlash.

The 0.016 in. air gap between forcer and thrust rod introduces a very significant application benefit. It obviates the need for exact alignment between forcer and thrust rod. By relaxing the force/rod concentricity specifications, the air gap simplifies integration of Micro Linear motors into high performance positioning stages. Sinusoidal coil excitation and magnetic symmetry of the tubular design also ensure constant drive force throughout forcer travel. The motor’s tubular construction simplifies cooling because heat is radiated from the thruster’s full circumference. Loads — in the form of tools for holding slides, test tubes and so forth — mount directly onto the forcer. No additional mounting hardware is required.

—SG