Know Your Position: New Technology In Automation Sensors
Netzer transmit/receive read head over a modulating active linear stator strip with coarse/fine pattern shown.
There are a number of technologies used to create linear and angular position sensors. Potentiometers, differential transformers, inductive transducers, as well as magnetostrictive, magnetoresistive, Hall effect, capacitive, and encoder based sensors (among others), all serve the industrial market according to the specific requirements of the applications. Each of these technologies has a set of operating, environmental, and economic parameters that makes it particularly appropriate for a set of uses. Because of their inherent digital nature, great stability, and potential for very fine measurement over relatively large measuring ranges, encoder based sensors have particularly become the instrument of choice in robotics and machining applications. Encoders are also very widely used for high speed motor or simple multi-turn rotating shaft situations where an incremental output is adequate but directional information as provided by quadrature may be needed. Encoders are either incremental or absolute.
Absolute, Incremental, Or Electric Capacitive?
Absolute encoders are able to power-off and power-on, and still “know” where they are, while incremental encoders only know where they are relative to where they started. In a power-off/power-on situation, the incremental encoder has to be “homed” before it can be used again so that it is able to re-establish its relative position.
Exploded view of two-part rotary Electric Encoder where top plate transmits/receives and bottom plate modulates the field.
Into this mix comes the “electric capacitive” encoder, a new technology commercialized by Netzer Precision Motion Sensors, Ltd. of Misgav, Israel. This insurgent technology delivers the extreme accuracy of the optical encoder in a very small package, while operating in severe environments at an economical cost comparable to magnetic encoders. Invented by Yishay Netzer, formerly of Israel’s Rafael Armament Development Authority and a holder of over twenty international patents in sensors and related electronics, the capacitive Electric Encoder® was first created to provide extremely reliable rotary position accuracy for fire control and reconnaissance operations in defense/aerospace applications.
The severe requirements for these applications included rotary measurement tolerance to .001 degrees, low weight, very low section width, up to 7” shaft acceptance, high shock and vibration resistance, extreme operating temperatures and frequent temperature cycling, low power consumption, high tolerance to condensation and contaminants, and no magnetic signature or sensitivity to EMI/RFI or magnetic fields. The company that followed the invention, Netzer Precision Motion Sensors, has created a multitude of both linear and rotary sensor forms based upon its technology.
How They Work
Netzer’s non-contact position sensor technology relies upon the basic principle of the variance of RF signal attenuation as a function of capacitance. Within a Netzer three-part rotary Electric Encoder, a dielectric rotor rotates (in synch with the shaft motion) between two stationary printed boards. One board serves as a space/time modulated electric-field transmitter, and the other as a field receiver. The rotor interaction with the electric field is influenced by the shaft rotation angle. The received, integrated field is converted to a proportional current which is processed to provide DC output signals, proportional to the Sine and Cosine of the shaft angle.
The Netzer two-part rotary Electric Encoder comprises a fixed stator board that includes field transmitting and receiving plates on its bottom side, and processing electronics on its top side. The bottom side faces the rotor, which in turn modulates and redirects the field. The received integrated field is converted to a proportional current which is processed to provide DC output signals proportional to the Sine and Cosine of the shaft angle.
A cutaway view of linear Electric Encoder with a passive “cable-less” spoiler gliding between active transmitter and receiver plates.
All Netzer Electric Encoders employ a dual, coarse/fine code pattern that enables extraordinarily fine resolution and accuracy even in a very inexpensive rotary device. The basic theory behind this coarse/fine scheme is that if the total measuring range of a sensor device can be broken up into a series of small, equal, distinctly identifiable segments, the incremental scale of each small segment can be much finer than it would be if that same incremental scale were set over the entire measuring range.
A three-part rotary Electric Encoder with a dialectric rotor altering capacitance between the transmitter and the receiver plates.
A Look To The Future
Relying on the interaction between the absolute position or displacement of an object and a space/time modulated electric field, capacitive encoder based position sensing has the potential to bring extremely high accuracy to applications where cost and environmental barriers have been limiting.