Using reflective encoder technology to enable portable medical device designs

Over the past few years, many semiconductor companies have gradually focused on the medical device market. Emerging application trends in the medical field have increasingly demanded higher technology, and designers have begun to develop highly competitive products for various needs. For example, the demand for home medical devices has grown very rapidly. This is in line with two trends: the public's growing awareness of the advantages of self-managed medical care and the lower cost of outpatient treatment compared to inpatient treatment. These trends are continuing to drive healthcare companies to develop products that are more convenient to carry, less expensive, and simpler to use.

Currently, there are many different diagnostic and therapeutic devices in the outpatient treatment market, such as dialysis equipment, portable insulin pumps, insulin inhalers, and diabetes management systems. The emergence of these self-service medical devices has also prompted equipment manufacturers to launch innovative products.

Therefore, product designers are actively looking for new components and products that can support innovative thinking and future medical product development trends and meet the rapid market development strategy of products. In particular, miniature reflective encoder technology has become an effective helper for portable medical device manufacturers to break through the existing accuracy, power consumption, size and cost limitations. These enhanced motion control feedback devices can surpass existing encoder technologies such as magnetic induction technology, bringing lower costs and lower power consumption.

This article will compare reflective encoder technology to existing technologies and provide several examples of end applications where reflective encoders are particularly suitable. This article will also explain why designers are beginning to use reflective optical encoders to solve electromagnetic interference (EMI) and accuracy issues that may cause safety issues. With advantages such as small size, relatively low cost and easy design-in, reflective encoders provide a successful solution for a wide range of medical device applications.

Avago Technologies AEDR-8500 reflective optical encoder.

Finding the right motion feedback solution

The portable medical device market is becoming increasingly competitive as customers must be provided with products that offer a unique value over their competitors. The design process typically begins with a clear list of product requirements based on market needs and circumstances. In most cases, trade-offs must be made when deciding on the best components to meet all design requirements at a reasonable cost. Prioritizing design requirements will be key when selecting components that are inexpensive, meet cost targets, and meet the most important priorities.

Most typical selection criteria for portable medical devices usually relate to the following priorities:

Weight and Size: Classifying a medical device as portable means that the device can be easily carried anywhere. Weight and size specifications are critical to portable product designs because designers have space and weight constraints on components. Portable medical device designs that require precise mechanical positioning may require encoders or electromechanical motion feedback devices to convert mechanical motion into electrical output for precise position tracking. The most common approach is to add a rotary encoder to the back of a selected motor and have the motor drive a series of mechanical systems. Another approach is to use a linear encoder with a yardstick or a belt with a series of black and white tracks and mount them on the moving parts of the system for motion tracking. However, these approaches are not necessarily effective unless engineers find encoder products that meet size, weight, resolution and cost targets.

A reflective encoder that is particularly suitable for portable medical device applications is the AEDR-8500 from Avago. Its length and width are less than 4 cm and 3.4 cm respectively, and the package weight is almost negligible. In addition to meeting the stringent requirements of weight and size, this encoder can also provide robust performance at a relatively low cost compared to other magnetic technologies.

Resolution, Frequency, and Accuracy: Another important set of criteria for selecting the right motion feedback sensor for a design is whether the device can provide good resolution, frequency, and accuracy. Resolution is an important parameter that determines the total number of steps required to complete one revolution of a rotary system. Resolution is usually expressed in counts per revolution (CPR) for rotary applications and lines per inch (LPI) for linear applications. A higher CPR does not necessarily mean a better design accuracy - it simply provides information about more counts per revolution for the application, without any detailed information about potential periodic errors.

The final output resolution of a rotary encoder is determined by the count density of the encoder module and the size of the matching media or code disc. The relationship between the encoder output resolution in CPR, the code disc size in optical radius or Rop, and the encoder count density in LPI is as follows:

LPI=CPR/2*π*Rop (all units must be consistent during the calculation)

The optical encoder with a 6mm diameter package can provide a minimum resolution of 50CPR or higher before quadrature calculation. This resolution can be further increased by 4 times by external electronic circuits to obtain the output after quadrature calculation.

Figure 1: A reflective encoder mounted behind a motor.

The frequency rating of the encoder determines the maximum motor speed that can be reached without missing encoder counts. Typical micro DC motors are rated at 20,000 RPM or lower under no-load conditions, with typical applications running in the 6,000-10,000 RPM range. At rated motor speed, a typical 50CPR encoder will need to have a frequency rating of at least 16.7kHz or higher.

The best cyclic error of a typical magnetic encoder system with an interpolator is about 3-4 times higher than a comparable optical system. It is very important to choose the right encoder technology that meets the product design requirements. If a higher accuracy output is required (for example, less than ±20 mechanical degrees of error or lower), then optical encoder technology is arguably the best choice on the market.

Cost: Since the main reason why medical device manufacturers focus on the autonomous portable device market is to save medical expenses, designers are often faced with the challenge of finding suitable components at a lower cost. The best case scenario is to find a matching product priced at the consumer market level, rather than a high-cost component for the medical or industrial market.

Motion control solutions often represent a very high share of the design budget due to the high cost and lack of choice of precision motion components. Mechanical mounting of encoders is also an issue due to a lack of expertise and tools to install encoders for the intended application. The most common approach is to use encoder solutions provided by the motor manufacturer, but this approach often limits the choices to magnetic encoder technologies on the market. Reflective encoder technology provides engineers with more options in finding the right product for the application while ensuring lower cost and higher accuracy.

Power consumption and electromagnetic interference (EMI): When designing battery-powered portable medical devices, engineers usually try to keep the power consumption of all components as low as possible. Using components with lower power consumption can extend the battery life and provide greater flexibility in the selection of other components. The Avago AEDR-8500 reflective encoder mentioned above consumes less than 75mW when operating at 5V, which is comparable to or even lower than other competing technologies.

Another consideration in selecting the right motion feedback solution is electromagnetic interference (EMI) immunity. EMI issues have become more important in recent years due to the increasing use of sensitive electronic devices in devices and the increasing number of EMI-related equipment failures. EMI issues can come from the increased use of wireless communication devices such as mobile phones, WiFi, and RF transmitters. In addition, most motor manufacturers design encoder solutions based on custom discrete magnetic technology and ignore the related issues that may cause EMI. Optical encoder technology provides a good alternative to magnetic technology in combating EMI issues because it can provide better EMI immunity at a similar price.

Application

Engineers have several options when implementing a reflective encoder solution in a portable medical device design. The most common approach is to use an optical-based reflective encoder solution mounted behind the motor (Figure 2). This encoder can provide feedback based on the movement of the motor shaft.

Figure 2: A micromotor equipped with an encoder.

Figure 3 shows a typical gear motor application using a rotary encoder. The motor drives the lead screw through the gears and pushes the piston head at a pre-programmed rate. The motion control encoder captures the motor's motion information and transmits the corresponding output signal to the controller to form a closed-loop system.

Figure 3: Volumetric dispenser.

Alternatively, a reflective encoder module or yardstick can be mounted on the moving mechanism to track motion, as shown in Figure 4. Common medical applications using reflective encoders include drug delivery, syringe motion control, and endoscopy systems.

Figure 4: Linear motion tracking.

Conclusion

Finding the right and low-cost motion encoder solution for portable medical designs is a key challenge for product designers. Correctly setting overall system priorities and relative component requirements is key to project success and fast time to market. For many medical applications, reflective encoders offer the advantage of providing high performance at a relatively low price.