When the patient is on the operating table and when highly sensitive and expensive equipment is being used, the accuracy and reliability of the rotary switches, coded switches, and encoders in the equipment's interfaces must be accurate and reliable beyond reproach. Poor performance, intermittency or failure of any kind in a mission-critical medical application can be disastrous.

Also, there are ergonomic design issues to address: positive tactile feedback, or not, and smooth and even rotation are among them

Rotational life

Rotational life helps a designer estimate how long a switch or encoder will last so long as “apples-to-apples” comparisons are followed. When looking at switching cycle specifications from various rotary switch manufacturers, designers must be certain that they understand what each manufacturer defines as a “cycle”. One cycle may be considered rotating the switch once from the first position through the last position. The cycle may otherwise be defined as rotating the switch from the first position to the last and then back again. Obviously, these are two vastly different numbers and cannot be compared side-by-side fairly. For mechanical encoders, one 360° rotation of the shaft is usually considered a cycle. It's important to note that a rotational life rating of 10,000 cycles can mean the encoder will perform within specifications for a minimum of this many turns. Other manufacturers will have this number represent the life expectancy of the product, where degradation and performance loss may be noticed much sooner. It's analogous to a doctor saying you may live to be 70 versus you will live to be at least 70 years old.

Inside story

Designers should closely review the components used in the internal design structure of switches for their medical devices. Using inexpensive encoders in applications where performance is critical is a recipe for disaster. Selecting an inexpensive switch or encoder often results in metal shavings getting into the contact chambers, broken end-stops and/or indexing springs, or failing contacts. When the indexing mechanism isn't designed properly, particles from the mechanism can get into the contacts if the two systems share the same chamber. The debris can create opens or shorts in one or multiple positions of the switch. Inexpensive contact designs can also create premature mechanical wearing of the wiper and/or contacts, causing the switch to become intermittent. In the case of encoders, the contact surfaces can wear unevenly, causing the software to not be able to properly decipher the outputted information. There are many other ways that a poorly designed or inexpensive switch can fail, but one should note that rotary switches and coded switches usually will fail mechanically before they fail electrically. To prevent many of these problems, only switches with a reputation of reliability should be designed-in for mission-critical applications. A highly reliable switch or encoder should have heavier gold flash on the contacts; a brass, stainless steel or solid aluminum shaft; and Teflon, polybutylene (PBTB) or other quality plastic sub-assemblies. See Figure 2 for a close-up of the inner parts of a switch.

Regular wear and tear aside, switches and encoders often see a lot of exaggerated abuse. In emergency situations, a boost of adrenaline can create a forceful crank, damaging the switch and thereby disabling the diagnostic device. It's possible that the end stop of the switch may be broken and the switch no longer stops in the last position, turning around and around in your hand. Or, the indexing spring can break, locking the switch in one position permanently. It is also common for the switch to be the point of impact when a piece of gear is struck by an external force or dropped by the user. One consideration is to employ a design that protects panel control from impact. Regardless, it is best if the panel controls utilize stainless steel shafts, a supportive bushing design and robust internal shaft reinforcement. Along with quality components, stronger metals, and a thick gold flash on the contacts, these design concepts can reduce the chances of failures in the field.

Tactile feedbacK and ergonomics

Tactile feedback is another important design consideration. In general, a precise tactile feel is a strong indicator of a well-engineered indexing mechanism. A switch should have an audible click from position to position that you can be felt. In fact, with a solid tactile feedback, the user can change settings by touch (rather than sight) — important in emergencies and where light conditions may not be optimal. An indexing mechanism that is mushy can also be more difficult to get set to an exact position easily. Turning fast, the correct position can be overshot, forcing the user to have to turn the shaft back in the opposite direction to find the correct setting. The lack of a good, definitive feel will usually require that the user take some extra, (precious) time to visually confirm the position of the switch. Some users will even resort to slightly rocking the shaft back and forth to confirm the proper detent positioning, trying to ensure that the internal, electrical contacts are being mated securely. Obviously in critical medical applications where lives may be at stake, this situation is unacceptable.

Ergonomics are also a key part of the switch design. The knob on the switch should be shaped to easily turn, even with sweaty (or bloody) hands. “Soft-touch” knobs can be incorporated that provide a firm rubberized material for a softer feel and easy to grip surface. The shape of the knob can also provide aesthetics and save space. Bell or top-hat shaped knobs have a wider base (which is better for markings and to cover the panel nut) and a thinner center to turn. This allows space for fingers in compact medical devices. They should incorporate finger grooves for slip-proof turning action for both ergonomic and aesthetic value.

So far, we've been discussing switches that require human interaction. Switches that do not require the human touch are gaining popularity in medical devices.

No-touch switches

Infrared and capacitive switches incorporate a no-touch design. Capacitive switches detect changes in capacitance between the switch-sensing surface and the air space directly in front of it. Infrared switches work on a principle of comparing internally generated light to light reflected back to a receiver. There are several benefits for medical applications for both of these types including sanitation, reliability, and robotics. See Figure 3 for various capacitive and level-sensing switches. Figures 1a and 1b show an infrared switch in the process of activation. A second activation can trigger a separate function, such as locking the switch in the active state. Also, these types of switches can be programmed for activation once at a certain distance and a second time at a closer (or farther) distance.

These switch designs can be adjusted for distance of activation and length of time of activation. With one setting, the switch can act as a “latching push button” switch, and with another setting the switch can act as a “momentary push button switch”. In the OR, switches can be activated without being touched. Devices can be turned on and off, frequencies changed, camera zooms adjusted, etc, by bringing the hand within a set distance (1 to 2 inches for example) Clearly, precautions can be incorporated for each application to prevent accidental activation.

Infrared switches reduce the influence of ambient light via an internal compensating circuit, preventing false switching. Optional LEDs can be incorporated to show change-of-state when activated. Infrared or capacitive switches can be encased in plastic or glass for easy cleaning of the end-device without damaging the switch. Often, the solvents commonly used for medical device cleaning and sterilization can seep into switches and encoders, risking corrosion and affecting its inner components. A no-touch switch behind glass prevents this problem. No-touch switches also do not have any mechanical components, so mechanical wear is not an issue. The switches can also be sealed to IP68 for waterproofing. No turning of electro-mechanical contacts means there is no wear on the switches' components, so they can last 1,000,000 cycles.

Level-sensing capacitive switches also can precisely monitor the level of various liquids, chemicals, etc. When a fluid bag or medical device liquid is too low or too full, the sensing switch's output could signal a nurse or technician to drain or add more fluid.

Whether physically touched or not, switches and encoders are critical components in medical devices. The right choice helps ensure an effective user interface and device reliability.

Get Your ‘Edge’

The Medical Edge e-newsletter is your source for the latest on medical engineering and medical devices.

For your FREE subscription, visit http://medicaldesign.com/subscribe/newsletters/