Resolvers are rugged, simple rotary position transducers, they can provide high accuracy position measurement from zero speed to very high RPM.  Resolvers are wound with two rotor and two stator windings. One rotor winding is shorted, producing an electrical circuit having a single rotor winding and two stator windings which are 90 degrees apart. The effect is a rotary transformer where either stator or rotor can be used as the primary winding. The Resolver Control Transmitter configuration uses the rotor as primary, and the Resolver Control Transformer configuration uses the stator as primary. 

Today retrofitters are faced with the choice of replacing resolvers with encoders or adapting the resolver electronics to work with resolver/encoder converters.  Most of these converters excite the resolvers with the Control Transmitter type of excitation. Control Transmitters have a single phase rotor excitation E_R1-R2  and two phase stator output, E_S1-S3   and E_S2-S4 ; the  two output signals are amplitude modulated sine and cosine waveforms as defined below:

Vx = E_S1-S3 = Kx* E_R1-R2 sin q

Vy = E_S2-24 = Ky* E_R1-R2 cos q

For f = excitation frequency, E_R1-R2 = V sin w t  where (w =2p f)

So

Vx = Kx V sin w t sin q

Vy = Ky V cos w t sin q

Therefore Vx/Vy = tan q

To determine the shaft angle q requires the derivation of the inverse tangent

q = tan^-1 Vx/Vy

The electronics used with this type of resolver to calculate the inverse tangent function, where the input signals are amplitude modulated sinusoids, are complicated circuits. We will not explore here the numerous approaches which have been developed to implement the position measurement function defined above, but generally one sees the following implementation of a tracking servo:

• High accuracy sin/cos multiplier : usually two multiplying D to A converters are used to allow digital multiplication of the inputs by the sine and cosine of an angle f
• Analog subtraction of the DAC outputs to generate a signal proportional to sin (f - q )
• Analog high frequency filtering of the phase signal to reduces noise on the phase difference signal
• Phase sensitive detector, to generate an analog control signal proportional to the phase
• Integrator (integrates phase measurement to generate a signal proportional to the rate of change of input angle, which is therefore a velocity signal)
• VCO voltage controlled oscillator, follows velocity signal to generate clock pulses to a digital up/down counter
• Up/down counter, generates the digital value f to close the tracking loop

These amazing circuits can provide very high resolution rotor position data,  but generally they do so with a response time penalty which is unsuited to high speed machine tool requirements. For example, typical step response of commercial resolver/encoder converters is 8 milliseconds for a 1 degree step, and 20milliseconds for a 90 degree rotor step. 

Review of many existing machine tools shows the manufacturer originally employed Control Transformer type resolvers which have stator excitation, they did this because it provides the lowest latency position tracking, and can do so with very good resolution.  The Control Transformer type resolver is excited with two stator inputs, a reference sine and cosine input, and they produce a single rotor position output signal:

E_S1-S3= Ksin w t

E_S2-S4 = Kcos w t

E_R1-R2 = (K E_S1-S3 cos q ) – (K E_S2-S4 sin q )

  = KE sin (w t - q )

With this method, the rotor signal is a sinusoid, having the same frequency as the stator drive signals, but with a phase shift, or time delay, which is proportional to the rotor position. The rotor output signal is not amplitude modulated, it is a nice constant-amplitude signal. Detecting rotor position only requires that a timer be started at the reference signal zero-crossing, and stopped when the rotor signal zero-crossing occurs.  Using this method with 400Hz excitation, rotor position data is updated every 2.5milliseconds; using 2500Hz excitation, data updates occur at 400usec intervals. 

The most common resolver found in machine tool applications is the brushless Size 11 as manufactured by several suppliers.  The resolver manufacturers tweak the winding parameters and construction of resolvers for different applications.  The specifications for each model list their performance at one particular excitation frequency and one type of excitation, however many resolvers will work just fine when connected as either Control Transformers or Transmitters, and most can be operated over a broad range of excitation frequency, typically from 400Hz to 4000Hz.

The Vigilant Resolver-to-Encoder converter electronics use stator excitation and rotor output to get the fastest possible position measurements, and includes a number of options to best match the resolver specifications.  It emulates a 1024-line (4096 count/turn) quadrature encoder with an index/marker pulse, it can drive differential or single-ended encoder tracking counters.  Also, we have included a number of features in the design to make the retrofit job simpler.  The electronics are contained in a boxed enclosure with front panel screw terminals for simple wiring.  The box is small, it has both DIN rail mounts and panel mounts, it can be mounted close to existing resolvers.

For resolvers, the balanced impedance of the resolver cables and load is more important than the actual loading or the length of the cable, so resolvers on machine tools are usually equipped with triple twisted shielded cabling, quite often with MIL-type circular connectors.   Often, a tachometer mounted on the machine shares the cable bundle with the resolver wires.  So if the resolver wiring and connectors have been well-maintained, it makes sense to retain this cabling during the retrofit.  The Vigilant Resolver Converter enclosure front panel provides extra panel space available for mounting a connector which matches existing machine wiring. In this manner, cutting and replacing existing machine cables will not be required. Retrofitters can drill a hole in the enclosure front panel and mount the matching connector, the panel has wiring lugs on the back side to simplify the connections. In OEM quantities, Vigilant can mount and prewire the connectors, contact Vigilant Products for a quote.

• REC-1 standard version 400Hz to 8kHz excitation, 4096 counts/turn, 1X or 2X update modes, SE or RS-422 DE incremental encoder outputs, internal power supply, DIN or panel mount enclosure, screw terminals, +5V powered
• REC-2 as above, +24V powered
• REC-1V same as REC-1, with analog velocity output
• REC-2V same as REC-2, with analog velocity output