By combining patented digital Silver-in-Glass® and analog conductive plastic technologies in a single environmentally sealed unibody package, Spectrol Electronics' DigiSense™ provides thousands of application-specific multifunction outputs (Figure 1). The conductive plastic potentiometric output furnishes the analog signal and absolute position, while the Silver-in-Glass digital logic output provides limit-switch functions, logic signals, and zone logic.
Analog and digital circuits are completely isolated, providing redundant signal capability in the same package. Sensor users are offered complete flexibility to design the element to match their functional requirements. They can also select optional shaft configurations, output terminals, electrical angle and logic switch points.
The unique sensor can be used as an actuator, feedback position device, or both. When used as an actuator, DigiSense sets the speed, height, and position of a particular mechanical action. Used as a feedback sensor, DigiSense transduces the mechanical angle into a voltage signal to complete a control loop. In most closed-loop control circuits, there are both actuator and feedback demands.
Thick-film Silver-in-Glass technology not only adds physical strength to the sensor design but also minimizes the step-height differential between the conductor and the insulator surface. Instead of a "bump-up" contact intersection on the order of 25-35 microns, as found in designs utilizing composite
board with copper foil etching, contacts must only "climb up" between 5 and 8 microns at each contact point as the shaft is rotated (Figure 2). The relatively flat, smooth wiping surface helps eliminate wear, electrical noise, and dimensional instability, which can cause switching inaccuracies. Single-track sensors utilizing Silver-in-Glass technology have a current capacity up to 100 mA. Rotational life exceeds 25 million cycles. Problems associated with vibration, shock, and contact bounce are counteracted by multifingered precious-metal "hoe"-shaped contacts.
The process involves the deposition of a smooth thick-film glass over the entire surface of an alumina ceramic substrate (Figure 3). After the glass layer has been fused to the substrate, a conductor pattern of palladium silver is printed on top of the glass. During kiln firing, the silver conductor sinks into the glass surface, forming a very smooth switching surface.
Conductive plastic technology, patented by Spectrol, is a process formulated to survive the 300 million dither cycles that the Ford Motor Company set as its benchmark for most under-the-hood automotive sensors. Temperature range is -40 to 150 °C. The process provides an output signal that is very smooth, exhibits extremely low noise, and has virtually infinite resolution.