PRINT IT!

Rex Narraway checks out a new form of encoder that one day might challenge the role of traditional mechanical and optical devices

Over the past few years, there has been a trend towards using digital techniques in the design of motion control systems. This is due to their ability to provide accurate and more complex control functions. In addition to their performance in the control function, digital systems do not normally require any manual adjustment when running in the field.

Such benefits are encouraging designers to select components which are fully compatible with digital techniques: a move that is having an influence on sensor technology. In particular, mechanical encoders are now often specified for use in motion control in preference to optical versions, especially when due consideration is placed on the susceptibility of the latter to oil mist and dust. To meet these demands, Los Angeles based Spectrol Electronics is now exploiting so-called Silver-in-Glass (SIG) sensor technology in the design of its latest ruggedised encoders and potentiometers.

SIG, which first appeared in 1994, is based on thick film technology in which an ink coating of a silver and glass cermet is placed on the surface of a ceramic substrate. A high temperature glass is first fused to the ceramic substrate followed by a film of thick ink containing particles of low temperature glass and palladium silver metal which is deposited on the glass layer using a printing process. A kiln firing process induces a reaction in the two glass materials to cause a partial diffusion of the metallic conductor layer into the glass layer of the substrate, leaving a nearly flush conductor on the top surface. If necessary, the basic pattern of the layer can be laser trimmed to generate the precise dimensions of the geometry called for in the specification.

INK PATTERN SEGMENTS

Encoders based on SIG technology incorporate segments formed by the ink pattern in the same way that mechanical encoders use a pattern of segmented plated copper. These are used in conjunction with a multi-fingered, hoe-shaped wiper made from precious metal which is designed to maintain a constant and consistent pressure on the element as it travels over the segmented conductors, irrespective of wear or environmental conditions.

In addition to generating elements for encoders, the technology can be used to create other thick film functions, including fixed or variable resistors and potentiometers. It is also possible to manufacture two functions on a single substrate, which can be very cost effective compared to the use of two discrete devices, while components such as capacitors and diodes can be included on the reverse side of the basic element with feed through connection for a relatively low additional cost. More sophisticated devices like signal processors can be produced with active components on the same substrate as the encoder pattern.

The application of SIG technology provides a smooth contact path for the wiper to traverse across since the depth from the contacting surface of the film to the glass layer on the substrate measures only between 5microm to 8microm. This contrasts with 25microm to 35microm, if conventional copper elements are used.

There is also a durable interface between the wiper contact and surface material which endows other positive mechanical characteristics to the device, such as: high physical strength and dimensional stability. It is also much less susceptible to effects from contact bounce which is said to be virtually eliminated.

MUCH GREATER LONGEVITY

In combination, these features provide a very rugged device which has a capacity for handling high electrical power due to the thermal dissipation properties of the ceramic substrate, plus an ability to run in temperatures ranging from -50iC to more than +200iC. Consequently SIG encoders exhibit much greater longevity than the copper versions and are claimed to have been run through as many as 25 Megacycles before the surface shows signs of mechanical failure. They are also said to generate less electrical noise because of the lack of wear debris and have a high immunity to electrostatic discharge.

The accuracy of the switching depends on whether the application is linear or rotary. In the case of linear motion configurations, it relates directly to the movement of the shaft or slider and, generally speaking, can be held to within 0.3mm of shaft position – with tighter tolerances achievable for customised design – while resolution is typically 0.4mm. In rotary applications, accuracy is a function of the radius of the encoder or other device that is used.

The conductors contained within the pattern of segments on the ceramic substrate can be trimmed to a minimum line width of 0.2mm with a minimum space in between of 0.15mm. Hence, the number of encoder switch points can be determined for any given diameter: typically, elements using SIG technology can be manufactured in sizes ranging from 6.35mm to 127mm diameter.

Representative of the type of SIG device that is now being offered by Spectrol, is a recently-introduced 2-channel rotary incremental encoder which is claimed to have an operating life in excess of 2 million revolutions. Additional features include a highly accurate index pulse and built-in debounce circuitry and it is available with a optional seal which allows it to meet NEMA 4 or 4X requirements.

Basically, it is aimed at OEMs involved with position sensing applications in robotics, automatic surface-mount assembly equipment, automotive shift control, hospital beds or motion control in servo and stepper motors. As standard, it produces a 2-bit quadrature output signal which can be readily used by microprocessors and it can be customised for specific OEM applications.

Spectrol is also supplying a 3-track incremental encoder for use in a 19-speed, electronically-controlled power shift transmission manufactured in the USA by John Deere. The transmission, which contains a complex limit switch/detent mechanism to allow clutch-free shifting at 19 forward and 7 reverse speeds, is currently installed in the John Deere 7000 Series of agricultural tractors. The object of the encoder is to convert all data relating to position sensing into a tri-phase output which signals incremental movement and direction to the engine/transmission module.

Another SIG device is the Finite Position Potentiometer (FPP). Primarily, it is aimed at the automotive industry but the company says that, because of its durability and inherent stability up to 200iC, it is finding other industrial applications especially where dirty environments or high temperatures prevail.

UNDER-THE-BONNET USAGE

Because of the limitations of conventional contacting devices in the form of potentiometers and switches, over the last ten years automotive manufacturers have been trying to develop non-contacting sensors for under-the-bonnet use. Silicon-based designs have been tried but, as temperatures in the engine-compartment rise, these have displayed poor reliability, problems with temperature shift compensation and the generation of sporadic signals. Faced with a future responsibility of warranting emission components for up to 150,000 miles – apparently, a threefold increase over current requirements – automotive manufacturers have had to look elsewhere.

With the technology employed in the FPP, Spectrol believes that it has come up with a low cost solution. In this case, the SIG matrix on the surface uses a series of skewed individual contact bars which enable the hoe-shaped wiper to make contact with at least one conductor. The conductor bars connect to a resistance element in a series of voltage taps so that, as the wiper moves, either one or two conductor bars are engaged at any time, providing a very clean and solid voltage signal.

The output from the FPP is of staircase form, where the magnitude of the increments depend on the designed-in resolution: the more conductor bars, the smaller the step. However, the absolute output of the device will always be a series of finite positions irrespective of the size of the step.

Aside from off the shelf sensors, custom designs can also be produced.

Figure 1: A comparative look at optical, mechanical and silver-in-glass encoder technologies

Figures 2(a) & (b): The silver-in-glass encoder uses thick film technology