When zero torque is applied the signals from the two resolvers show zero phase shift. As torque is applied, the phase of one output appears to shift relative to the other. Accordingly, the phase shift is directly proportional to applied torque. Using a multi-speed resolver with a high number of cycles (e.g.128) only a small amount of twist is required to produce a significant phase shift. In other words, it’s a highly sensitive technique and suitable for measuring twists of <1 degree or even <0.1 degrees. This means that the shaft need not necessary be long. Indeed the length of shaft needed for this approach can be <25mm. This can be achieved using a deliberately flexible shaft or by arranging the resolvers concentrically – one inside the other – and connecting the inner and outer parts of the shaft using a (very) stiff torsion spring.
Unlike strain gauges, resolvers are famously robust, reliable and accurate – that’s why they get chosen for all the tough jobs in aerospace, military, oil and gas equipment. Since they are non-contact devices there’s no need for any slip-rings or radio frequency signal transportation.
So, why has this technique fallen out of fashion? Perhaps one reason is that resolvers have also fallen out of fashion. Pancake or slab resolvers (flat with a big hole in the middle) are the ideal shape for measuring torque but they are notoriously expensive. Furthermore, specifying a resolver’s drive and processing electronics can be tricky. Since modern engineers are mostly familiar with digital electronics, they are perhaps reluctant to get to grips with analogue electronics and measuring phase shift of AC signals.
New generation inductive sensors
Nowadays, resolvers are increasingly being replaced by their more modern replacements – inductive encoders or ‘incoders’. Incoders operate using the same inductive principles as a resolver but use printed circuits rather than the bulky and expensive wire wound transformer constructions. This is important in minimising the incoder’s bulk, weight and cost whilst maximising measurement performance. Incoders also offer the simple and easy to use electrical interface:- DC power in and serial data out. Since incoders are based on the same fundamental physics as a resolver they offer the same kind of operational advantages – high precision, reliable measurement in harsh environments. What’s more, they are the perfect form factor for angle measurement – flat with a big hole in the middle. This allows the shaft to pass through the middle of the incoder’s stator with the rotor attaching directly to the rotating shaft. This eradicates the need for slip rings in the same way as resolvers.