IncOders as an alternative to Slab Resolvers

4 March 2012 This Technical Article was published in March 2012 and describes slab resolvers and IncOders. Everything you always wanted to know about slab resolvers but were afraid to ask…. The Chief Engineer says “We could use a slab resolver for that” and you say: “That’s a great idea boss” but you are actually thinking: “If I ask him what a slab resolver is I’m going to look like an idiot”. Sound familiar? If so, this article is for you – it explains what slab resolvers are; how they work; where to use them and suggests some lower cost options. What’s a Slab Resolver? Let’s deal with he jargon first. After all, we engineers love jargon because it helps us to differentiate between engineers and normal people. 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 energize 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. zettlex inductive encoders replace slab resolvers Slab resolvers are ideal where there a large through shaft is necessary 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 >10 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. How do they work? 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 below, 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. zettlex inductive encoder as alternative to slab resolver A resolver is an electrical transformer used to measure angle of rotations The outputs from a ‘single speed’ resolver are unique over each rotation – in other words angle can be calculated absolutely over 360o. A two-speed resolver has outputs which are unique over 180o; a three-speed resolver has outputs which are unique over 120oand so on. Typically, most resolvers are 1, 2, 4, 8, 16 or 32 speed. Where do they 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