Tandem encoders give two axes and one power input

‘Tandem Encoder’ is a new term but one which is increasingly being used by mechanical and electrical designers. Mark Howard of Zettlex* explains what a tandem encoder is, how it works, its technical features and where it is best used.

The term ‘tandem encoder’ refers to an encoder with at least two measurement axes but only one power input and one data output. In other words, two physically independent encoders but with a single electrical interface.

These encoders are increasingly being used in applications with one or more of these factors - cost pressure, tight space constraints, weight limits or requirements for high reliability.

For most position encoders, the single largest cost element is their electronics and the tandem approach spreads the cost of a single set of electronics across two (or more) encoders.

The result is that the cost, weight and volume per sensor are reduced.

Another important factor is the use of slip rings in the host equipment. If slip rings are being used to energise the encoders, it is often the case that a tandem encoder is applicable.

This is because the cost, size and complexity of a slip ring are directly proportional to the number of its contacts. Eradicating slip ring connections to a second encoder can be a major advantage for the tandem encoder approach.

A typical arrangement for a tandem encoder is shown in Figure 1. If we consider the system as a gimbal mounted camera, we can see that axis #1 is the elevation axis and axis #2 is the azimuth axis.

Only a single set of connections to both encoders is required via the slip ring.

Figure 1:  Tandem encoder schematic.

Of course, not all position sensing technologies are suited to tandem encoders. The tandem encoder approach is only applicable when the encoder electronics can be displaced away from the actual sensing point.

The tandem encoder approach cannot be applied if electronics are required at the sensing point. Optical and magnetic devices both require electronic devices adjacent to the sensing point.

The tandem approach is, however, suited to capacitive or inductive sensing technologies, where the electronics required for sensor operation can be displaced away from the sensing point.

Furthermore, the electronics in capacitive or inductive techniques can be arranged so that they can be multiplexed across two or more sensors.

While capacitive devices can be made very accurately, they are best suited to applications with tightly controlled operating or storage environments such as offices or laboratories.

This is because most capacitive devices suffer from significant temperature or humidity drift as well as serious reliability problems if foreign matter - most notably water in the form of condensation or ice - is adjacent to the capacitor’s plates.

Capacitive sensing devices find it impossible to differentiate between the capacitive effects caused by water or humidity and the effects caused by relative displacement of the sensor’s principal components.

The potential applications for tandem encoders include: CCTV and security cameras; electro-optic and infrared gimbals; remotely controlled weapons systems; rotary joints and gimbals; actuator servos and motor encoders; robotic arms and CNC machine tools; test and calibration equipment; antenna pointing devices and range finders; and packaging and laboratory automation.

Since most of these applications involve arduous operating or storage conditions then it is of little surprise that tandem encoders have become mainly associated with inductive techniques.

Such techniques are ideally suited to arrangements where the energisation and signal processing are carried out remotely from the position sensing.

Traditionally, such inductive detectors have used transformer constructions in the form of accurately wound wire spools. The basic principle is that as a passive, magnetically permeable element, such as a rotor or a rod, moves, it changes the electromagnetic coupling between at least one primary winding and one or more secondary windings.

The energy that inductively couples in to the secondary windings is directly proportional to the displacement of the rod or rotor relative to the primary windings.

All windings must be wound accurately to achieve accurate position measurement, and to achieve strong electrical signals, lots of wires are needed. This makes traditional inductive position sensors bulky, heavy and expensive.

Traditional inductive position sensors are usually referred to as resolvers, synchros, linearly or rotationally variable differential transformers (LVDTs and RVDTs).

Now there is a new generation of inductive encoders that is particularly suitable to the tandem encoder approach. These inductive encoders use the same fundamental physics as their traditional counterparts but rather than the traditional transformer or wire spool constructions, they use printed circuits as their main components.

This means the coils can be produced from etched copper or printed on substrates such as polyester film, paper, epoxy laminates or ceramic. Such printed constructions can be made more accurately than windings.

Hence a far greater measurement performance is attainable at less cost, bulk and weight - while maintaining the inherent stability and robustness. The approach also allows the principle components of the inductive position sensors to be installed with relatively relaxed tolerances.

Not only does this help to minimise costs of both sensor and host equipment, it also enables the principal components to be encapsulated.

This enables the sensors to withstand very harsh local environments such as long-term immersion, extreme shock, vibration or the effects of explosive gaseous or dust-laden environments.

Electromagnetic noise susceptibility is often cited as a concern by engineers who are considering next generation inductive position sensors. The concern is misplaced given that resolvers have been used for many years within the harsh electromagnetic environments of motor enclosures for commutation, speed and position control.

The new generation of inductive encoders is produced in either incremental or absolute forms and output digital data. This means that a tandem encoder can produce a digital data stream, with a single data stream carrying data for both axes.

*Zettlex is a British company that designs and manufactures sensors; supplies sensor components and integrated circuits. The company offers bespoke sensor design and development for specific customer applications.