Medical Device Link.

Originally Published EMDM January/February 2003

Product Update

Two Micro Linear motors form an x-y stage that provides fast, repetitive, high-accuracy positioning of glass slides for white blood cell imaging. A digital camera photographs white blood cells on the slides (Click for larger image).

Low-inertia motors and integrated motion control systems enhance medical and diagnostic devices.

How difficult is it to achieve short-distance moves in high-accuracy motor-driven applications? “It’s not difficult at all,” says Dean Crumlish, senior applications engineer at Copley Controls Corp. (Canton, MA, USA), “provided, of course, that time is not an issue.” Stepping in rapid succession through a sequence of precise x and y coordinates only a few centimeters apart becomes increasingly difficult as speed and precision demands rise. Positioning tasks in biomedical applications are especially challenging. Travel is scaled down to millimeters, accuracy tightened to microns, and positioning time is counted in milliseconds.

Traditional instruments for positioning slides, test tubes, and microarrays have depended on leadscrew mechanisms. “Today’s demand for higher throughput—which translates into higher acceleration, speed, and accuracy—has run into mechanical limitations,” says Crumlish. Friction and rotational inertia slow down the process at the expense of both accuracy and cost. 

Enter Copley’s direct-drive Micro Linear motors, which reportedly achieve fast throughput and high reliability in a low-cost package. Their design is very simple. It consists of two basic parts, the thrust rod and the forcer. The thrust rod contains high-intensity permanent magnets. The load-carrying forcer is supported by low-friction bearings and incorporates electrical drive coils in a lightweight polymer. The forcer surrounds the thrust rod, and is propelled along the thrust rod’s length by digitally controlled magnetic fields. Tools for holding slides and test tubes are mounted directly onto the forcer.

Micro Linear motors from Copley Controls Corp. can be provided as system components (top) or a fully assembled positioning stage with integral bearings and a position encoder (bottom).

The motor’s tubular design features a 0.016-in. symmetrical air gap between the fixed and moving components. This gap introduces a very significant application benefit: it obviates the need for exact alignment between the forcer and the thrust rod. By relaxing the force/rod concentricity specifications, the gap simplifies the motor’s integration into high-performance positioning stages. Sinusoidal coil excitation and magnetic symmetry of the tubular design also ensure a constant drive force throughout the forcer travel. The motor’s tubular construction also simplifies cooling, as the heat radiates from the thruster’s full circumference.

The motors are currently used in a blood cell– imaging application. Two motors form an x-y stage that positions blood-coated slides beneath the lens of a high-resolution camera. The equipment captures images of white blood cells that measure 10–25 µm in diameter. Positioning accuracy in such applications is 1 µm. Typically, the equipment takes 50–200 blood cell photographs from a few square centimetres of slide surface. The Micro Linear motors complete repetitive 1-mm x-y movements at 1-µm accuracy within 25 milliseconds. 

Small Size, High Torque

Speed and torque are among the typical measurements that define a motor’s performance. In many medical device applications such as IV pumps, x-ray equipment, peristaltic pumps, and blood analyzers, size is also an issue. Specific requirements such as shaft length, shaft diameter, mounting configuration, and motor size are addressed in each application. “With our medical customers, the general trend is ‘smaller is better,’” says Bob Parente, manager, applications engineering, at Intelligent Motion Systems Inc. (Marlborough, CT, USA). 

A motor with an integrated drive from Intelligent Motion Systems Inc. offers cost benefits.

Among the obvious advantages of a compact motor size is space economy. “If a smaller motor can produce the same required torque as a larger motor for a given application, then the overall machine size can be reduced,” according to Parente. “You don’t need as big a factory, as big a room, as big an operating area to have the equipment mounted.” 

The MDrive 14, supplied by Intelligent Motion Systems Inc., is a compact and powerful system that “features an onboard drive to reduce installation costs and assembly time,” says Parente. All it needs is power and the same signals that a stand-alone drive and motor would need. Integrated electronics eliminate the need to run motor cabling through the machine. “This allows the power cabinet—the enclosure that contains all the electronics that operate the machine—to be smaller too.” 

This design offers many cost benefits, according to Parente. “First of all, buying one MDrive motor costs less than getting a separate motor and drive. Also, you don’t have to pay for a high-current motor cable, the larger cabinet that would be needed to house the separate drive, and the labor costs to install the drive.” Eliminating the high-current motor cable has the additional advantage of reducing the electrical noise that is caused when a motor and driver are in different places.

Maxon Motor AG

Threaded spindles that convert rotary to linear motion are made from zirconium dioxide. Developed by Maxon Motor AG (Sachseln, Switzerland), the spindles made from this high-performance ceramic are said to have several advantages compared with conventional steel spindles or recirculating ball screws. The material exhibits no slip-stick effect and is more wear resistant than steel. Ceramic spindles are less costly than linear ball screws and allow greater design freedom. Spindles measuring 2 to 10 mm diam at various pitches can be economically produced, reports the firm. The product is suited for use in high-temperature or magnetically inductive settings, such as MRI systems. Because of the material’s biocompatibility, other medical applications may be envisaged. 

The company recently added to its current catalogue a mechanically commutated dc motor. The RE 30 is 30 mm in diameter and 68 mm long, and has two shaft ends and a nominal output of 60 W. The core of the motor is a patented lightweight ironless rotor, the System maxon. The motor features low mass inertia and quick acceleration. It is suitable for applications in which the drive is required to provide significant power in a limited amount of space, such as analysis equipment or robots. The modular system can also provide a sensor for detecting speed and position.

ACS Tech80 B.V.

A control module combines an advanced multiaxis motion controller and integrated digital servo amplifiers. The SpiiPlus CM, offered by ACS Tech80 (Oud-Beijerland, Netherlands), features an integrated design that makes it suitable for stage manufacturers, systems integrators, and OEMs. The main engine is a Pentium-compatible motion-processing unit. Real-time control is achieved by means of a second-generation servo processor (SPii) developed by the firm. The SPii has a 120-MHz RISC processor core and peripherals that meet the needs of demanding motion control applications, all contained within a single chip. The standard module is fitted with a PIV filter and controls automatic feed-forward velocity and feed-forward acceleration. Other features include a notch and second-order low-pass filter, digital current control, and automatic sinusoidal commutation on all axes at 20 kHz. The capabilities of the SPiiPlus controller include multiaxis interpolation, inverse kinematics, PV spine motion, tight trajectories, and velocity accuracy and stability. 

The integrated drives are software-configurable for both brushless and brushed servomotors. Operating on supply voltages from single- or three-phase 110–380 V ac or 24–375 V dc, the SPiiPlus CM universal digital drivers output current levels of 5 A continuous and 10 A peak, or 10 A continuous and 20 A peak. In addition, they have pulse and direction outputs that allow three of the total axes to be open-loop steppers. 

Nippon Pulse Motor Company, Ltd.

An advanced PC/104 four-axis motion control card is suited for use with stepper motors or digital servomotors. The NPMC6045A-4104 from Nippon Pulse Motor Company, Ltd. (Tokyo), incorporates a PCL6045A motion control chip as part of its compact design. The motor features a maximum output frequency of 6.5 Mpps. It can obtain trapezoidal and constant-speed motion profiles, and also perform linear and S-curve acceleration and deceleration, and circular and linear interpolation. A 28-bit up and down counter provides incremental encoder feedback. The NPMC6045A-4104 has output power of 5 V dc ± 5% and external power input of 24 V dc ± 10%. It is suitable for operation in temperatures ranging from 0� to 50�C and can withstand 80% humidity. 

“The control card is suitable for a variety of medical applications,” says Shuichi Eddie Hagimoto, head of the international export department. “It can be used in blood testing, genome analysis, and other applications where multiaxis control is required.” 

Nippon Pulse Motor offers a variety of controllers, drivers, and motors for stepper motor applications. 

Reliance Gear Company, Ltd. 

Flexible shaft couplings feature a patented design that provides accurate motion transfer and flexibility. Reli-a-Flex couplings, supplied by Reliance Gear Company, Ltd. (Lepton, W Yorks, UK), provide kinematic transfer of motion with high torsional stiffness, zero backlash, and constant velocity.

The couplings’ slit pattern gives them very high torsional stiffness—comparable to bellows-type couplings—while still providing low bearing loads and accurate motion transmission. It also reduces stress in the flexing elements as the couplings rotate. In applications that have more-stringent accuracy requirements, the couplings can help to minimize rotation errors.

The products are made from low-inertia 7075-T6 aluminium and have been fully tested. A life expectancy of more than 50 million cycles can be readily achieved.

Intelligent Motion Systems Inc.

A company’s motor technology combines a high-torque stepping motor with electronics in a small and powerful package. Developed by Intelligent Motion Systems Inc. (Marlborough, CT, USA), the MDrive 14 comprises a NEMA size-14 high-torque stepping motor with an onboard half/full step drive. The compact system measures 1.7 ¥ 1.8 ¥ 1.4 in. and weighs 5 oz. Optically isolated logic inputs accept 5-to-24- V-dc signals. Setup parameters include motor resolution, motor direction with respect to the direction input, and run and hold currents. These settings may be changed on the fly or downloaded and stored in nonvolatile memory with the use of a simple graphical user interface, eliminating the need for external switches or resistors. Parameters are changed via an SPI port. Operating voltage ranges from 12 to 48 V dc. 

The MDrive line includes a single-shaft stand-alone device, a dual-shaft rotary motor with either an optical encoder or a control knob for manual positioning, and a linear actuator version. Interface connections are accomplished with the use of a 12-position keyed locking pin and receptacle. The motors can be used for a variety of medical applications such as dispensing, packaging of light materials, and orientation of microscope stages.

Haydon Switch & Instrument Inc. 

A size-14 captive linear actuator features engineering thermoplastics in the rotor drive nut and a stainless-steel acme leadscrew. This patented design ensures quiet and maintenance-free operation. The actuator is the latest addition to the Haydon Switch & Instrument Inc. (Waterbury, CT, USA) hybrid motor line. Available in resolutions ranging from 0.000125 to 0.002 in. per step, the new motor delivers thrusts of up to 25 lb without compromising long life or cost. 

The captive linear actuator is designed with an internal antirotation mechanism that makes it suitable for applications in which external antirotation methods would be difficult or impossible to employ. The motor is also suited for applications that require precise positioning and rapid motion. In addition to standard configurations, custom designs are also available. 

Copley Controls Inc.

Low-inertia motors offer increased accuracy in biomedical applications. The Micro Linear motors from Copley Controls Inc. (Canton, MA, USA) are available with a peak force from 19 to 45 N and continuous force from 3.1 to 8.7 N. Peak acceleration ranges from 15 to 25 m/sec and peak velocity achieves 3 m/sec. The units’ 11-mm-diam thrust rod can be from 62 to 510 mm long. The polymer-constructed forcer, which encapsulates the electrical drive coils, comes in lengths from 36 to 113 mm. The forcer is 34 mm wide and 25 mm high. 

Loads mount directly onto the forcer, requiring no additional mounting hardware. The motors operate from dc power 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.

SMAC Europe Ltd.

High-resolution actuators offer medical device manufacturers precision positioning and high speed. Provided by SMAC Europe Ltd. (Horsham,W Sussex, UK), the actuators feature low friction and zero backlash. The linear motors’ direct drive provides repeatability and long life, along with high-speed acceleration from minimal moving mass. The integrated encoders achieve resolutions from 5 µm to 10 nm. 

The company offers a range of voice coil–driven actuators, grippers, and x-y stages that are used in a variety of applications, such as stent and catheter manufacture, nebulizer and syringe driver verification, and materials testing. 

Honeywell Sensing and Control

A sensor provides low offset drift in temperatures from –40� to 85�C. Available from Honeywell Sensing and Control (Motherwell, Lancs, UK), the new CSN-series magnetoresistive current sensor uses an application-specific integrated circuit to provide stable, repeatable, and accurate current measurement. The sensor operates from a 5-V unipolar supply and has an accessible internal 2.5-V voltage reference. It can operate from either an internal or external voltage reference, allowing many sensors to be used without offset imbalance. Three primary pins enable the sensor to be configured for different measuring ranges. While designed for the motion control market, the current sensor is sufficiently flexible to be used in many other applications, including frequency convertors and power supply systems. 

Minimotor S.A.

Brushless dc servomotors are available in a variety of speed profiles and feature reverse-operation, start/stop, and stand-alone positioning modes. The new BS series from Minimotor S.A. (Croglio, Switzerland) outwardly resembles the company’s B series: the motors measure 44 mm diam ¥ 90 mm long and integrate a proprietary skew-wound high-performance neodymium magnet. What sets apart the BS-series motors is the way in which the three-part windings of the coil are wired.

BS motors have star-connected windings in lieu of the delta-connected windings that are used in the B series. The new configuration yields lower motor currents and a higher voltage. When the BS motors are used with a 44/1 series planetary gearhead, they can achieve 16 N•m in continuous operation and as much as 20 N•m in intermittent operation. 

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