Resolvers are absolute rotary position transducers. They are very simple and robust, can be very accurate, and, in some cases, inexpensive. A wide variety of resolvers are available in many configurations and for various applications.
How Resolvers Work
Resolvers are rotary transformers with one primary winding and two secondary windings. The primary winding is generally on the rotor and the two secondary windings are on the stator. The secondary windings are arranged 90 degrees from each other such that when one is lined up with the rotor winding (full coupling) the other is at a right angle (no coupling).
The primary winding is driven with an alternating current signal at a specified voltage and frequency. The position measurement is determined by the ratio of the amplitudes of the signals on the secondary windings and their phase with respect to the primary signal.
The RMC100 Resolver Interface Module
Resolvers are commercially available in many varieties with different specifications. The primary specifications of interest as applied to the RMC are:
Frequency
The frequency of the signal driving the primary winding.
Voltage
The specified voltage of the signal driving the primary winding.
Output Voltage
The output voltage from the secondary windings or the transformation ratio between primary and secondary windings.
The RMC resolver interface card is designed to work with only a subset of all resolver types available. The Resolver Specifications topic lists the range of values compatible with the standard RMC Resolver module. In addition, a wider range of resolvers can be used by modifying components on the interface card module or by using external reference generators to drive the primary winding. Contact Delta for more details.
Determining Absolute Position
There are several conditions when the resolver axis establishes its absolute position within one turn of the resolver. These conditions are:
On power-up
After a transducer fault
After a Reset Position command
After a Set Parameters command if the Scale or Coordinate Limit parameters have changed
The axis determines its absolute position as follows.
The 16-bit value read from the resolver is added to the Count Offset
If the result is outside the range of -32768 to +32767 then 65536 is added or subtracted to force it into the range. (This is the range that can be represented by a 16 bit number.)
This value is converted to Position Units:
Position Units = Scale x Counts / 32768
This gives a value between -Scale and +Scale
The result is compared with the Coordinate Limit to determine if it is within the allowable range of Position Units. (See the Resolver Scaling topic for the definition of valid range). If it is outside the valid range then Scale x 2 is added or subtracted to bring it back within range.
Useful Commands
In addition to the typical motion commands, the following commands are useful for resolver axes, especially for resetting the position when used in rotational applications:
Zero Position/Set Target Command
See also:
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