For small, portable and safety-critical medical devices that can be used at home by nonprofessionals, many require exacting motor control to, for example, administer medication. The choice of a reflective motion controller can positively affect size, power, cost and accuracy considerations.
In the highly complex medical device market, device manufacturers strive to develop products that offer value to patients and practitioners as well as advantages over competitive products or solutions. For instance, due to the growing awareness of self-administered medical care benefits, people want home-managed therapy options.
Today many people attempt to watch and control their medical expenses much as they manage their energy costs. Therefore, the trend toward outpatient treatment instead of inpatient treatment to reduce high medical fees should continue as the populations of many developed countries age. Demographic trends and the need for affordable health care for a population with decreasing purchasing power will push the market to products that are portable, cost effective and user-friendly.
At present, there are various diagnostic and therapeutic devices that are readily available in the market for outpatient treatment. These include dialysis equipment, portable insulin pumps, insulin inhalers, diabetes management systems and many more. The introduction of more self-help medical devices is expected to push device manufacturers toward introducing more new and innovative product offerings.
To meet aggressive time-to-market strategies, product designers actively seek new components that support innovative ideas or future product visions. Miniature reflective encoder technology is becoming increasingly popular among portable medical device manufacturers who seek to challenge the limits of precision, power consumption, size and cost. With the introduction of reflective encoder technology, major encoder and motor manufacturers have begun to incorporate a reflective encoder into their product design and development programs (Figure 1).
Designers have begun to adopt reflective optical encoders as an answer to questions about electromagnetic interference (EMI) and precision that can lead to safety issues resulting from device failure. In addition, reflective encoders remain small, relatively inexpensive and easy to design into end products and equipment.
Optical Encoder Technology
Two types of optical encoders can be used in portable medical devices. There are transmissive optical encoders and reflective optical encoders (Figure 2). It is conceptually easier to understand the transmissive encoder first and then look at the differences and advantages of the reflective encoder. Transmissive and reflective optical encoders, such as those developed by Avago Technologies, consist of three core components. They are the emitter, detector and code wheel/code strip.
The emitter or light source consists of a lens and an LED that emits an infrared light beam. The detector is a set of photodiodes connected to a detector IC. The code wheel or code strip is an opaque material designed in a circular shape or as a straight strip. The code wheels and code strips are patterned with tracks. These tracks consist of a series of openings called “windows” and opaque areas called bars. In a reflective encoder, the windows are a reflective and the bars non-reflective.
In a transmissive encoder, the light emitter and detector are located on the opposite sides of the code wheel or strip and are positioned to face each other. The emitter functions as a light source and light travels across to the detector. This light beam will have to pass through the code wheel first, before landing on the detector. The code wheel or code strip, which is now spinning or moving in linear motion, can block the light beam from getting through except when a window is present. The windows located on the moving code wheel or code strip enable beams of light to reach the detector.
Intervals of light and shadow fall on the photodetector array. The encoder’s detector IC circuitry then picks up this signal, processes it and translates it into more recognizable outputs called channels.
The core components in a reflective optical encoder are similar to the transmissive optical encoder. However, in a reflective optical encoder the emitter and detector are on the same side of the code wheel or strip. The reflective code wheel or code strip uses materials that reflect light for the window. To create the shadow effect, the code wheel is etched and chemically treated to create an opaque, non-reflective area that acts as a bar to light transmission/reflection. The light beam is then reflected by the moving code wheel or code strip. The bars located on the reflective code wheel now absorb the light beam and prevent it from being reflected.
Reflective encoders have advantages over transmissive encoders that make them popular in motor control applications. They include: low height profile, high accuracy, surface mounting and high volume production that decreases cost. Reflective encoders attached directly to a motor housing, make a compact, integrated feedback control system possible in space-constrained equipment. A reflective code wheel /code strip is directly attached to the measuring surface. Direct monitoring of the position information increases accuracy due to the actual position measurement of the moving surface. By reducing the effects from mechanical slippage and gear backlash, overall performance is also improved. Smooth motion with real-time velocity feedback is possible with a small diameter code wheel for compact size.
Reflective encoders with two channels have two digital pulse output streams, Channels A and B (Figure 3). The rising edge of the pulses is 90 electrical degrees out of phase from each other. Direction can be determined from these two quadrature channels. For example, clockwise rotation has Channel A leading Channel B, but a counterclockwise rotation will result in Channel B leading Channel A.
A three-channel encoder is similar to the two-channel encoder but with an additional channel. This additional channel marks the index position and is referred to as channel ‘I’ and is sometimes called channel ‘Z’. A pulse occurs on this channel once for each full revolution of the code wheel. Essentially, this pulse marks a singular position of the code wheel. This is an absolute reference added to an incremental encoder.
The quadrature output signals are sent to ICs, which can perform the quadrature decoder, counter and bus interface function, or the quadrature signals can be decoded by a microcontroller and software. The microcontroller’s program controls system operation and can even issue alarms and make other decisions based on system operation or status. Usually the microcontroller also sends data for display to the patient or medical professional. Remote monitoring via the Internet is also possible for more complex systems.
The channel signals can be directly connected to a decoder IC/microcontroller, or if transmission over a long distance is required, differential line drivers can be used to send data. Some reflective encoders include the differential line drivers, which also decrease transmission errors caused by noise spikes and other interference.
Selecting the Right Motion Feedback Solution
The marketplace for portable medical devices is increasingly competitive, with a focus on product segments where each offering has a distinct advantage or differentiator against competitors or alternate technology offerings. From the feedback and reviews received, decisions about tradeoffs must be made. Prioritizing design requirements are the key in selecting affordable components that meet most critical priorities while at the same time satisfying financial objectives. The selection method for most typical portable medical devices is critically associated with a set of criteria. However, this is not presented in order of priority.
Size and weight considerations are critical to product design because the designer will have limited space and weight allowance for components. Portable medical devices that require precise mechanical position or speed data need an encoder or an electromechanical motion feedback device to translate mechanical motion to electrical output for precise position or speed tracking. A rotary encoder attached to the back of a motor turns motion into electrical signals that give position and speed information, making closed loop feedback systems with alarm capability easy. Alternatively, a linear encoder may be used with a code strip, a strip that has a series of black and white tracks on it, and have it mounted on the moving part of the system for motion tracking (Figure 4).
These ideas are not possible with conventional encoders as they cannot meet the size, weight, resolution and cost target requirements. Reflective encoders are very small, just 3 mm in length and width and are nearly weightless. Additionally, the reflective encoder is priced relatively low, while providing greater resolution at a higher accuracy than magnetic encoders.
A feedback sensor must have the resolution, frequency/speed performance and accuracy to match the system. The term resolution determines the total number of steps that represent one revolution of motion from a rotary system. Resolution is often measured as Counts per Revolution (CPR) for a rotary application or Lines per Inch (LPI) for a linear application. Higher CPR does not necessarily imply better accuracy. On the contrary, it only provides more count per revolution for your application and does not reveal details about potential cycle errors. Cycle error is the difference between shaft rotation, which causes one electrical cycle, and 1/N of a revolution.
A 6 mm-diameter housed optical encoder such as that in Figure 1 can offer at least 50 CPR pre-quadrature resolution or higher. This could be further multiplied by four times with external electronics or a microcontroller. The frequency rating of an encoder determines how fast a motor can spin without having the encoder lose count. A typical miniature DC motor is rated at around 20,000 RPM, or lower at a no-load condition, with typical applications running at around 6,000 to 10,000 RPM. At the stated motor speed, a typical 50 CPR encoder will need to have a frequency rating of at least 16.7 kHz
A typical magnetic-based encoder with interpolator has about 3-4 times higher cycle error than an optical-based encoder. Encoder technology is critical to system accuracy. For high accuracy—less than ±20 electrical degree cycle errors—optical encoder technology is the best option.
Motion control solutions constitute a large portion of the design budget because the options for precision motion control components are limited and tend to be expensive. Another concern is the mechanical mounting of the encoder device, due to the need for expertise and/or tools to assemble the encoder to the intended application. The most common practice is to rely on motor manufacturers for a complete encoder and motor assembly. However, this has often limited the solution to the magnetic-based encoder solution. With the introduction of the reflective encoder technology, engineers now have more options for their applications while ensuring that component cost remains low and accuracy high. Transmissive optical encoder suppliers often offer development and mounting kits to speed the design process. Incorporating components that consume less power can prolong battery life and give more flexibility in selecting other components. The reflective encoder’s low power consumption of less than 30 mW from a 3V supply is comparable to or lower than existing technology.
EMI issues have become more significant in recent years as more sensitive electronics have appeared in products. Cell phones, Wi-Fi and wireless computer peripherals are potential EMI sources. Ironically, many motor manufacturers continue to design with custom discrete magnetic encoder solutions that are sensitive to EMI. Optical-based encoders have better immunity against EMI interruption and are easy to adopt as manufacturers often offer mounting tools and demonstration kits.
There are many ways to assemble the reflective encoder solution into a portable medical device. The most common method mounts the reflective encoder at the back of the motor. See Figure 5 for an example of a miniature motor with an encoder. The encoder provides position and speed feedback data based on the motion (rotation) of the motor shaft. The data is transferred via two digital signals that operate in quadrature. A microcontroller or dedicated IC can decode the quadrature signals for control feedback, status display and alarm functions.
Figure 6 shows a typical geared motor with a rotary encoder. The motor acts to drive the lead screw through gears and pushes against the plunger head at programmed rates. The motion control encoder senses the motion of the motor shaft and sends the corresponding output signal to the controller, forming a closed loop system. Properly setting your overall system priority and the respective component requirement is the key to project success and fast time-to-market. The reflective encoders are ideal for medical device applications such as drug delivery devices, syringe motion control, endoscope systems and many more.
Avago Technologies
San Jose, CA.
(800) 235-0313.
[www.avagotech.com].
