The Importance of Drive in Medical Equipment

Encoders for Motors Make a Difference

Medical and automated laboratory processes such as medical imaging or biochemical analysis increasingly require more precise and dependable machines in order to satisfy growing demands on quality, throughput, and manufacturing cost reduction. Direct drives (linear and torque motors) are gradually becoming more important in such dynamic applications with one or more motion axes. Some examples of equipment applications using these types of motors are in cancer treatment, various imaging techniques, slide digitizing, and DNA/protein analysis, to name a few. The benefits of direct drive technology are numerous, and include low wear, low maintenance, and higher productivity. However, the increase in productivity is possible only if the control, the motor, the machine frame, and the position encoder are optimally adjusted to one another. Direct drives place rigorous demands on the quality of the measuring signals coming from the position encoders. Optimum measuring signals: • reduce vibration in the machine frame, enhancing overall quality and dependability of the machine; • help provide precise constant motion, critical to many medically oriented applications, especially imaging; • stop excessive noise exposure from velocity-dependent motor sounds, aiding in quieter environments; • prevent additional heat generation, reducing thermal growth in the machine for better overall accuracy; • allows the motor to realize its maximum mechanical power Encoders for Motors Make a Difference rating, useful for larger or high speed medically oriented machines. The efficiency of a linear motor in regard to accuracy, speed stability and heat generation is therefore greatly influenced by the selection of the position encoder. Design of Direct Drives The decisive advantage of direct drive technology is the very stiff coupling of the drive to the moving component without any other mechanical transfer elements. This allows significantly higher gain in the control loop than with a conventional drive. On direct drives there is no ad - ditional encoder for measuring the speed. Both position and speed are measured by the position encoder: linear encoders for linear motors, angle encoders for torque motors. Since there is no mechanical transmission between the speed encoder and the moving unit (say a camera, x-ray source, or array of pipettes), the position encoder must have a correspondingly high resolution in order to enable exact velocity control at slow or constant traversing speeds. The velocity is calculated from the distance traversed per unit of time. This method — which is also applied to conventional axes — represents a mathematical differentiation that amplifies periodic disturbances or noise in the signal.The combination of significantly higher control loop gain, as is used particularly with direct drives, and insufficient encoder signal quality can result in a dramatic decline in drive performance. Signal Quality of Position Encoders Modern encoders feature either an incremental (non-unique counts), or an absolute (all positions have a unique value) method of position measurement. The motion path information is transformed in the encoder into two sinusoidal signals with 90° phase shift. Both methods require that the sinusoidal scanning signals be interpolated in order to attain a sufficiently high resolution for speed and position control. Inadequate scanning, contamination of the measuring standard, and insufficient signal processing can lead to a deviation from the ideal sinusoidal shape. As a consequence, during interpolation periodic position errors occur within one signal period of the encoder's output signals. This type of position error within one signal period is referred to as "interpolation error." On high-quality encoders it is typically 1% to 2% of the signal period. All encoders have some level of interpolation error. If the frequency of the interpolation error increases, the drive can no longer follow the error curve exactly, due to timing issues, and motion becomes minutely erratic. Furthermore, electric current components generated by the interpolation error cause increased motor noises and additional heating of the motor. A comparison of the effects of linear encoders with low and high interpolation error on a direct drive illustrates the significance of highquality position signals. The 8um grating period linear encoder used here generates only barely noticeable disturbances in the motor current: the motor operates normally and develops little heat (A). If at the same controller setting, the interpolation errors of the same encoder are increased through poor mounting adjustment, significant noise arises in the motor current (B). This causes an increased amount of noise and heat generated in the motor. Digital filters are often used with direct drives to smooth the position signals. However, the loss of phase-association by filtering in the a minimum, otherwise the dynamic accuracy decreases. Position encoders with optimum signal quality help to reduce the use of filters, meaning that the control bandwidth is maintained. Position Encoders for Direct Drives Linear encoders that generate a high-quality position signal with low interpolation errors are essential for optimal operation of direct drives in medically and biochemically oriented machines where accuracy and dependability are of critical importance. Encoders that use photoelectric scanning are ideally suited for this task, since very fine graduations can be scanned by this method. Encoders with optical scanning therefore play a significant role in exploiting the potential of direct drives and are clearly becoming more sought after everyday. TMD This article is supported by original research by Dr. Jens Kummetz at Dr. Johannes Heidenhain, GmbH For more information, please contact kkaufenberg@heidenhain. com