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Slab resolvers: an explanation

Slab resolvers: an explanation

In this article, Mark Howard from Zettlex explains what slab resolvers are, how they work and where to use them, and discusses alternative approaches.

What is a slab resolver? Let's deal with the jargon first: a resolver is an electrical transformer used to measure angle of rotation. Most resolvers look a bit like an electric motor - with copper windings on the stator and a machined metal rotor. The inductive coupling between the transformer's windings varies according to angle. So, if we energise the resolver with an AC signal and measure the output from the transformer's windings, we get an AC electrical signal whose amplitude is proportional to angle.

The 'slab' term simply refers to the resolver's shape - flat. There is no hard and fast rule as to where traditional resolvers end and slab resolvers begin but, generally, a slab resolver's (axial) height is less than its diameter. A slab resolvers' diameter can be more than ten times its height and often has a large bore to accommodate a through shaft, hydraulic pipes, electrical connections or a slip ring.  

Confusingly, slab resolvers are also referred to as pancake resolvers. Strictly speaking, a pancake resolver is an unhoused or frameless slab resolver. However, to all intents and purposes, a pancake resolver is the same as a slab resolver and the terms are used interchangeably. You may also see or hear the term 'brushless resolver'.  This simply means that there are no electrical connections to the resolver's rotor. This means that there is no need to transfer power to the rotor via electrical brushes.

Whilst there are lots of variations on the theme, a typical resolver has three windings - a primary winding and two secondary windings. These windings are usually formed on the resolver's stationary element - the stator. The primary is used as the input for an AC drive signal and each secondary is used as pick up or receive winding. In the diagram below, the rotor is made from a material such as iron or steel and is arranged relative to the windings such that it will couple varying amounts of energy in to the secondaries depending on its angle of rotation. In the diagram, the output from the secondaries will be in the form of a sinusoid and cosinuoid. Accordingly, the ratio of signals varies in proportion to angle.

The outputs from a 'single speed' resolver are unique over each rotation - in other words angle can be calculated absolutely over 360 degrees. A two-speed resolver has outputs which are unique over 180 degrees; a three-speed resolver has outputs which are unique over 120 degrees and so on.

Where do resolvers get used?

Once upon a time, rotary electrical transformers - in all their various forms - were just about the only way to measure the angle of a continuously rotating shaft.  Nowadays, optical encoders have taken over many applications but have never taken over in those applications that require high reliability and precision operation in harsh environments. Typically, such applications are common in the heavy industrial, aerospace, defence, oil and gas sectors. Simply put, optical encoders cannot cope with muck, vibration, shock or extreme temperatures whereas resolvers will remain largely unaffected. Slab resolvers get used wherever there is a requirement for one or more of the following:
  • Large through shaft
  • Requirement for absolute position measurement
  • Integration with a large bearing
  • Space constraints that require low axial height
  • Radial mismatch in rotating and stationary parts
  • Long life in harsh physical environment
  • Continuous rotation.

Resolvers have been around since World War II and have a solid reputation for reliability. They are often the automatic choice for high-reliability and safety related applications. A common pitfall, though, is that many resolver data sheets specify that the resolution of a resolver is infinite. Whilst this is theoretically true, in practice it is daft because in most modern control systems the resolver's analogue signals will be converted to a digital signal of finite resolution. The actual resolution will be determined by the resolution of the analogue to digital conversion circuit.

This leads to a subtle but important point: to engineer a resolver based system a (separately specified and purchased) signal excitation and processing circuit is required. This means that there is a significant skills issue in using resolvers: you need to know what you are doing and typically there is no one-stop-shop.

A further issue is that slab resolvers have a reputation for being heavy, bulky and expensive. They are simply not economically viable for most mainstream industrial applications and, in general, are only regularly used in those sectors where capital cost is secondary to specification and performance. So what are the alternatives? In theory, optical encoders are the natural alternative but in practice they simply cannot cope with harsh physical environments. In the larger slab or pancake formats (typically greater than 4in diameter) optical devices are often just as costly as a slab resolver.

A new generation of device has become increasingly popular which can be thought of as a hybrid between an encoder and a resolver. Inductive encoders (or Incoders) use the same basic physics as a resolver but are less costly, lighter, more compact and more accurate.  Importantly, they are also easier to use because they only require a DC supply and output a digital signal representing absolute angle. The skills issue is eradicated because inductive encoders do not require separate electronics processing circuitry - all the necessary electronics are integrated in to the stator. This means that Incoders have all the advantages of slab resolvers but with none of their disadvantages.

Rather than the traditional resolver's copper wire windings, Incoders use printed circuit boards as their main components. As with a resolver there is a stator and a rotor but because there is no requirement for precise location of the Incoder's stator and rotor, there is no need for any bearings. There is no need to specify single speed or multi-speed devices - Incoders have a digital output with a resolution of up to 16 million steps per revolution.

Since inductive encoders use PCBs rather than wire windings, this means that they can be made with extremely high accuracy. Accuracies of better than 1 arc-minute common as are repeatabilities of better than 1 arc-second. An Incoder's basic design also means that it can be readily customised to suit a particular application's requirements.

Incoders are available in a wide range sizes up to 600mm diameter and have been used extensively in a variety of machine tools, gimbals systems, aerospace, defence and medical equipment.
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