A resolver whose axial height is small compared to its diameter, can be referred to as either a frameless resolver, a slab resolver or a pancake resolver. Strictly speaking ‘frameless’ simply means that the resolver’s housing has been eradicated but many engineers will use the term frameless when referring to a resolver with low height and big diameter.
Fig 1 – a frameless resolver with low axial height and large diameter
Nowadays, most resolvers are brushless rather than brushed, but are all based on transformer principles. In other words, they are inductive angle sensors. As the position of a resolver’s rotor varies relative to its stator, the electromagnetic coupling between the rotor and stator varies. This can be seen as the resolver’s output signals vary relative to the excitation or input signal.
Some resolvers are termed ‘single speed’, ‘two speed’, ‘four speed’ etc. This refers to the number of times that the resolver’s output uniquely varies over 1 revolution. A single speed resolver’s output is unique over 1 rev; a two speed resolver’s output is unique over any 180 degrees within 1 rev; a four speed resolver’s output is unique over any 90 degrees within 1 rev and so on.
Resolvers have an excellent track record in safety related applications – notably in civil aerospace. There are various reasons including non-contact operation; insensitivity to environmental conditions and ‘friendly’ failure modes.
In many safety related or safety critical applications, the most dangerous type of failure is not one which results in no output signal but rather one which produces a credible but wrong output signal. As an example, consider aircraft aileron control – a wrong but credible angle measurement could have catastrophic consequences. Given the construction and operation of a resolver, the probability of a failure which produces a credible but wrong signal is vanishingly small.
Resolvers will be the first option for many older generation engineers in the aerospace, defense, oil & gas sectors because they know the resolvers’ track record as well as the corresponding AC analogue electronics. Some knowledge of analogue electronics is required to specify and select the electronics required to power a resolver and decode its signals in to a digital format. Younger generation engineers are more familiar with digital electronics so they will struggle to specify a suitable resolver.
The perceived complexity of analogue electronics has been a significant factor in the demise of resolvers since the 1990s. This has led to a gradual reduction in the number of companies producing resolvers and an increase in the number of companies producing optical encoders. Perhaps surprisingly, this reduction in availability of resolvers has led to a disproportionate increase in their price and of large diameter resolvers in particular. Lead times of >6 months are not uncommon for high specification resolvers in the aerospace and defence sectors.
Advantages: Reliability, robust, wide environmental operating envelope, accuracy (in multi-speed arrangements).
Disadvantages: High cost, heavy, bulky, complicated electrical interface, limited availability, tight installation tolerances.