Servo motors perform a variety of tasks in the medical arena, such as smoothly moving a patient into position or rapidly packaging pills or have the robot arm position an operating tool with pinpoint accuracy. These features and others are very important in typical medical applications – smooth operation, pinpoint accuracy.
Tools for growth
According to the Motion Control Association (MCA), the servo/motion control market is growing by about 10% per year. MCA has developed several initiatives, including ‘Motion Control Online’ – a global resource for motion information that has been getting over 20,000 hits per month; and a ‘Motion Control Statistics’ programme for members – a report providing accurate and reliable marketing data. The association aims to promote the use and understanding of servo motor technologies through its analysis of the market.
MCA shipments for the ‘total market’ represent a conservative estimate of the ‘actual market’, given these represent only reporting company figures. MCA is in a growth mode and more companies are adding to that reporting data regularly.
The growth in the servo market is led by product innovations. This trend is confirmed by the fact that equipment with higher technical expertise is thriving, whereas less technically demanding equipment suffers with competition from low-cost regions.
It is noted that for the OEM and designer, development tools which are available play a big part in speeding up time-to-market. For example, auto-tuning and other Windows-based tools play a big part in simplifying servo/motion configuration. Auto-tuning conveniently sets up current, velocity and position control loops to make adjustment easier and faster. The Windows-based tools allow many different parameters to be viewed on a software oscilloscope thus monitoring key machine parameters. Other application advancements such as low pass and notch filters allow the designer to eliminate any machine resonances.
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Additionally, there are fieldbuses such as Ethernet Powerlink which is a software interface providing a proven open-standard, real-time deterministic solution for many applications.
Many applications today are looking at using absolute positioning feedback devices providing 19-bit resolution simply because it provides advantages of improved control, greater positional accuracy and most important – absolute position data.
Servo technology controls speed and direction, and more importantly, the position of a load. Servos provide advantages of very fast and accurate positioning, higher peak torques, wider speed ranges and controllability compared to other motor technologies.
But what allows the servo motor to accomplish this, what makes the servo motor different from other motors? The answer is in the servo motor’s design, its use of a feedback device, and how it is manufactured. These provide the servo motor with its many advantages. The advantages allow the servo motor, and especially the brushless motor, to make way for advancements in equipment – less maintenance, quiet operation, smaller package, less weight, faster positioning, longer life, higher torque and power output.
Servo motors have been designed specifically to have smaller diameters, while maintaining the same output HP or torque. For example, a typical one horsepower AC induction motor would be 7.6in (19cm) in diameter, whereas a SCR rated motor would be 4.7in (12cm), and a brushless servo motor is smaller still at 3.5in (9cm).
Smaller motor diameters mean smaller rotors, which reduce the rotor mass, or inertia. This allows for faster acceleration and faster positioning. Faster operation reflects directly to the bottom line of an application – for example, how fast white blood cells are counted, or how many pill containers are packaged per hour.
Servo motors are available in a wide range of diameters, from less than 1in (25mm) to over 9in (225mm).
Servo motors are used with a feedback device which allows the ‘loop to be closed’. Examples of feedback devices include: tachometer, resolver and encoder. A tachometer provides speed information. Resolvers and encoders provide speed information as well as position information.
As indicated, feedback devices coupled with servo motors provide a ‘closed loop’ package. First of all, in an ‘open loop’, the drive command goes to the motor – there is no data or information coming back to the control to indicate motor speed or position. In a closed loop, the drive command goes out, and a feedback signal comes back. This feedback signal provides the means to monitor or measure the procedure (what the motor is doing) and it provides the process to make corrections.
The servo motor, with an appropriate feedback device in a closed loop system, therefore provides for greater accuracy. This accuracy reveals itself in the quality of the end result – for example, how smoothly the X-ray table moves the patient, how accurately the scanner is positioned, or how accurately the medical robot positions instruments.
Manufacturing and materials
In a servo motor, slightly different material is used in the design, and a slightly different winding technique is used in manufacturing, compared to a conventional motor.
Materials are selected to allow servo motor operation at elevated winding temperatures if necessary. Using material in the design which is rated at higher temperatures improves the product life and reliability. This also improves the quality of the equipment into which it is designed.
Materials used in the design of servo motors meet UL, CSA and IEC requirements. Additionally, in manufacturing, when winding a servo motor, more wire is fitted within the stator steel laminations. This allows the servo motor to provide extra torque and improved efficiency. This allows usage of a smaller, lighter motor, which will deliver higher performance for equipment such as medical beds and blood centrifuges.
Stainless steel servo motors and gearheads
If equipment requires stainless steel components, servo motors are available with this feature. However, many equipment designers may not be aware of this beneficial feature. The housing, end-caps and shaft are all stainless steel; these motors employ FDA-compliant shaft seal and H1 bearing grease; and the nameplate is laser etched. Stainless steel servo motors are protected against various environments; they are durable and are easy to clean.
Additionally, some medical applications require gearing. There are servo rated gearheads available in the market and there are stainless steel gearheads available to compliment the stainless steel servo motors. Servo rated gearheads offer low backlash – on the order of 10 arc-min and lower – which provides accuracy for the equipment and precision in the application.
Servo motor technology is advancing into power ranges and entering fields traditionally served by fluid power drives. Efficiency and energy saving is the prime driver. Servos are available up to 20HP (15Kw) and this performance may be extended up to the range of 30HP (22Kw) with blower cooling options.
Servo technology is also being used to replace smaller stepping motors in numerous applications. Improved productivity rate, part quality and accuracy are the driving factors for this trend. Servos are used in packaging, labelling, registration and trimmers.
Another significant trend has been extending servo/motion productivity enhancements into environments with cleanliness requirements, which are now being served with the stainless steel servomotor.
Servos provide numerous improvements/advantages over typical, traditional motors. They are designed to respond quickly for improving the process, whether sorting or packaging pills, or filling containers; servos provide accuracy in systems, whether positioning lasers, positioning operating robots, or spinning blood centrifuges; servos are designed to provide more torque and higher efficiencies for equipment, whether counting white blood cells, pumping blood or operating centrifuges. Servos represent the best long-term equipment investment.