Direct Drive Offers Performance,

Direct drive technology is increasingly being used for both linear and rotary motion in medical device applications. Direct drive technology often substantially improves the productivity of medical devices due to higher stiffness and lower settling time; the time required to scan a patient in an imaging application can be reduced by 5% to 10% or more. Direct coupling between the motor and the load greatly reduces the number of components in the system, which improves reliability and reduces maintenance requirements. Direct drive technology takes advantage of today's electronic control technology to increase safety and to guard against overtravel and other malfunctions.

APPLICATION CHALLENGES IN MEDICAL DEVICES

Medical devices provide some of the most critical and challenging motion control applications; there are numerous types of minimally invasive surgical procedures that require the manipulation of wire guides and interventional catheters through a patient's body. The required linear and rotary movements are typically performed by linear and rotary Direct drive linear motors offer improved productivity in medical device applications. actuators consisting of motors and mechanical transmissions that provide the combination of speed and accuracy that is required to perform procedures quickly, while minimizing the risk to the patient.

Medical imaging devices represent another category of medical devices that frequently require multiple drive axes. A typical computerized tomography (CT) scanner has a gantry assembly that carries the x-ray beam and detection equipment, as well as linear and rotary drives for manipulating the beam in the scan circle. The CT scanner also has a patient support system driven by additional linear drives. Some medical imaging machines have both CT and positron emission tomography (PET) scanners, where the position of each scanner is controlled by multiple linear and rotary axes.

Frameless DDR motors.

These and other motion control components that are used in medical devices must meet tough requirements, safety being the most critical requirement for every medical device application. Reliability and maintainability are also important because failures often result in care interruptions. Motion control performance and accuracy can have a major effect on the productivity of medical applications by affecting the speed with which procedures can be completed. This affects the cost of care and availability of healthcare resources. Finally, the current budgetary pressures faced by all sectors of the health care industry mean that both acquisition and operating cost are very significant.

ELIMINATING THE PROBLEMS CAUSED BY THE TRANSMISSION
Until now, the majority of medical applications have used conventional motion control technologies consisting of electric motors and mechanical transmission systems. Transmissions perform two main roles, converting rotary to linear motion, and providing a mechanical advantage. But, mechanical transmissions introduce compliance, or lack of rigidity, between the motor and the load. Compliance reduces the accuracy of the system by allowing the transmission to wind up, especially when the motor is under load and the controller relies upon the motor sensor for position feedback. Mechanical transmissions can also generate considerable mechanical noise, produce roughness in operation, and increase the need for system maintenance

The most effective way to address these problems is to remove the mechanical transmission. Direct drive technology couples the motor directly to the load, eliminating all mechanical transmission components. By coupling the motor directly to the load, direct drive utilizes the interaction of a series of coils with a highflux magnetic field to produce the desired motion. Instead of being in conventional packages; however, these motor elements are incorporated into the bodies of linear or rotary actuators, eliminating the need for the drive mechanism.

ROTARY DIRECT DRIVE CONFIGURATIONS
Direct drive technology is available in a variety of rotary and linear configurations. Frameless direct drive rotary systems were the first direct drive technology, and are comprised of a separate rotor and stator without bearings, housings or feedback devices. These components are intended as a kit to be designed into and become a direct part of the machine itself. If the system operates as a closed loop servo, the feedback device must also be designed into the machine. An electronic drive amplifier runs the motor and manages the feedback device. Direct drive rotary motors are designed to deliver much higher levels of torque than conventional motors at lower rotational speeds.

Housed direct drive rotary (DDR) systems integrate the rotor, stator and factory aligned feedback within a housing that includes precision bearings. Housed DDR systems are best suited to applications where the load is designed to ride on the motor's bearings. These applications offer fast and easy installation, a pre-aligned feedback device, and built-in bearings. On the other hand, they are not ideal for applications that already have bearings because the motor must be coupled to the load, or multiple sets of bearings must be aligned, which is a complex and timeconsuming task.

Cartridge DDR systems provide an integrated full-frame motor without bearings; they simplify the process of using direct drive technology in applications that already have bearings. Consisting of a rotor, stator and factory-aligned, high-resolution feedback device, cartridge DDR systems use the machine's bearings to support the rotor, saving space and design time. A compression coupling connects the rotor to the load, and these units can typically be installed and running in less than 30 min.

LINEAR DIRECT DRIVE CONFIGURATIONS
A direct drive linear system is essentially a rotary servomotor that has been rolled out flat. Permanent magnet rotary motors have two main components, the stator, which consists of the primary coils and the rotor, comprised of the secondary coils, or permanent magnets. In direct drive linear (DDL) technology, the rotor is rolled out to become the magnet track, or magnet way, and the primary coils of the rotary motor are rolled out flat to become the coil assembly, also known as the forcer. The basic electromagnetic principles are identical to those of a rotary motor.

There are two main types of linear motor systems, ironcore and ironless. Ironcore motors have coils that are wound on silicon steel laminations to maximize the generated force with a single sided magnetway. The high thrust forces within ironcore motors make them ideal for accelerating and moving high masses and maintaining stiffness. Ironless motors have no iron or slots for the coils to be wound on. The modular magnet ways consist of a double row of magnets to maximize the generated thrust force and to provide a flux return path for the magnetic circuit, meaning these motors provide zero cogging, a very light mass, and no attractive forces between the coil assembly and the magnet way. These characteristics are ideal for applications requiring very low bearing friction, high acceleration of lighter loads, and for maintaining constant velocity even at very low speeds.

All brushless motors require feedback for commutation. DDR motors utilize a resolver or encoder mounted on the rear of the motor, or Hall effect devices mounted integrally in the coil windings. Linear motors typically use digital or linear Hall effect devices that enable the drive electronics to commutate linear motors in a manner analogous to rotary motors. Sinusoidal drive electronics provide sinusoidal drive currents to the motor for the best constant force and velocity performance.

DDR and DDL technologies connect the load directly to the motor and eliminate the shaft and shaft coupling, as well as the gearbox and belts in the case of a rotary drive, and leadscrew in the case of a linear drive. This reduces compliance to the point that it does not even have to be considered by the designer. By placing the feedback device directly on the load, direct drive minimizes error in position measurements.

With linear motors, designers should keep the center-of-mass of the load as near as possible to over the center-ofmass of the motor to avoid torsion on the system during periods of high acceleration. The encoder should be located as closely as possible to the motorways to reduce compliance between the encoder and the motor. On both linear and rotary direct drives, the load should be as rigid as possible. For example, the shafts connecting an inertial load to a DDR motor should be kept as short as possible.

DIRECT DRIVES VERSUS CONVENTIONAL DRIVES

Direct drive technology eliminates a number of components compared to conventional systems, such as shafts, shaft couplings, bearing blocks, motor mounts, gears, belts, pulleys, etc. Direct drive technology eliminates the potential for wear and failure of all these extraneous components, and therefore provides more reliable operation. Decreasing the parts count greatly reduces downtime of direct drive solutions as compared to conventional solutions.

Direct drive rotary motors also improve reliability and patient comfort by eliminating the primary sources of noise and vibration. The motors used to power traditional rotary and linear transmission operate at such high speeds that they must be geared down to drive the load. The interactions between the gear teeth, and the belts, and the pulleys, as well as the bearings in these transmissions, are the primary source of noise in most drives. Direct drive technology eliminates common sources of noise. The simple construction of DDR and DDL motors allows stiffer frames that reduce problems with resonant modes, as well as the need for periodic tweaking to reduce vibration.

SUBSTANTIAL PERFORMANCE ADVANTAGES
Direct drive technology also offers substantial performance improvements that can provide medical device manufacturers with a significant competitive advantage. Since there are no wear surfaces in direct drives (with the exception of housed DDR systems) and the limitations of mechanical transmissions are eliminated, very high and very low speeds are easily attained. Application speeds of greater than 5 m/s (200 in./s) or less than 1 M/s (0.00004 in./s) can usually be achieved. In comparison, mechanical transmissions such as ball screws are commonly limited to linear speeds of 0.5 to 0.7 m/s (20 to 30 in./s) to avoid resonances and wear.

The elimination of backlash and near elimination of compliance means that direct drive motors are capable of very high accelerations. Limited only by the system bearings, accelerations of 3g to 5g are typical for larger motors and accelerations of smaller motors can easily exceed 10g. Direct drive motors also have excellent constant velocity characteristics, typically less than +/-0.01% speed variation. Settling time is significantly lower compared to conventional drives because of reduced system compliance and higher position accuracy.

The result is that direct drive solutions bring the payload to position substantially faster than conventional drives. There are limits in how fast a patient can be transported due to comfort issues and the speed with which the detection system can be moved. It is typically possible to reduce the total time required to perform a procedure by about 5% to10% or more. This level of improvement can provide substantial benefits to health care providers, who are constrained by third-party payers in what they can charge for a procedure, making much more productive use of expensive equipment and skilled labor.

TOTAL COST OF OWNERSHIP
Over the past decade, cost has become an increasingly important consideration in medical device design. In the past, the higher cost of direct drive technology was a barrier to its use in the medical device industry.

The cost of components for a direct drive system is typically lower because so few components are required. On the other hand, direct drive technology requires additional design and integration costs in many applications because of the need to design the motor into the medical device. Design and integration are usually non-recurring expenses, while component savings extend through the life of the product, and into additional versions and spin-offs as well. The simpler design of direct drives also increases reliability and reduces maintenance, providing further reductions in total cost of ownership. Direct Drive is a high performance technology and often not appropriate for very simple applications where precision motion is not required, such as positioning a dental chair.

Direct drive technology provides the benefits of a brushless servomotor solution while eliminating the drawbacks created by compliant couplings, gearbox backlash, and gearbox and belt maintenance. Direct drive motors provide high reliability, eliminating the transmission, higher performance through higher velocity and lower settling time, and reductions in total cost of ownership because fewer components are needed, which means less failures and lower maintenance. Direct drive motors provide no compromise in critical patient safety considerations. Various direct drive options are offered in a wide range of configurations that handle virtually any rotary or linear control application. As a result, medical devices that utilize direct drive systems provide the ultimate in performance, with zero maintenance operation.