Overview
Rotary transformers and optical encoders are among the most widely used position sensors in servo applications.
Servo Systems
Also called follow systems, servo systems are feedback control systems that accurately follow or reproduce a process. They are automatic control systems that make controlled outputs such as position, orientation, or state follow arbitrary changes of the target setpoint.
Servo motor control modes
Servo motors typically support three control modes:
- Speed control - analog: Rotation speed is controlled by an analog input or by the frequency of input pulses.
- Torque control - analog: Torque setpoint is changed by adjusting an analog value or by communication that writes the corresponding parameter.
- Position control - pulse: Rotation speed is determined by the frequency of externally applied pulses, and rotation angle is determined by pulse count. Some servos also accept direct assignment of speed and displacement via communication.
Servo control loops
Servo control is generally implemented as three nested closed-loop negative feedback PID controllers. From inner to outer these are the current loop, the speed loop, and the position loop.
Signal types and relationships
- Analog signals: continuous in time and amplitude, for example voltage or current. After sampling and quantization they become digital signals.
- Digital signals: discrete in time and amplitude, represented by binary values. Digital signals are formed by 0 and 1 and are often encoded into structured formats.
- Pulse signals: characterized by instantaneous transitions in voltage or current. After quantization, regular pulse sequences are represented digitally. If a signal jumps from 0 to a fixed level and remains, it is a switch signal.
- Switch signals: binary state signals with two states, 0 or 1, reflecting on/off conditions and used as inputs or outputs.
Rotary Transformer
Also called a rotary variable transformer, it is a sensor whose output voltages vary with rotor angle. The excitation winding is driven by an AC voltage at a specified frequency; the amplitude of the output winding voltages depends on the rotor angle. The output relationship may be sinusoidal (sine and cosine), proportional, or approximately linear within a limited angular range.
Reluctance-type rotary transformer
- Output winding voltages vary as sine and cosine functions of rotor angle.
- Excitation and output windings are mounted in stator slots with different winding arrangements.
- The two output phases are 90 degrees apart, and their amplitudes change with angle as sine and cosine functions.
- With the rotor rotating, the fixed stator windings form a transformer whose coupling coefficient depends on angle. By processing the two output signals, the angular position of the motor can be obtained.
Input-output voltage relationships
Let the rotor angle be θ, excitation frequency be f, excitation amplitude be Ea, and time be t.
Excitation input voltage: E(r1-r2) = Ea*Sin(2π*f*t)
Let the transfer ratio be K and the rotor deviation angle be θ.
Output winding voltages:
E(s1-s3) = K*Ea*Sin(2π*f*t)*Cosθ
E(s2-s4) = K*Ea*Sin(2π*f*t)*Sinθ
The transformation ratio of a rotary transformer is defined as the ratio of the fundamental component of the maximum no-load output voltage to the fundamental component of the excitation voltage under specified excitation conditions.
Optical Encoder
Optical encoders operate by photoelectric conversion. They convert the mechanical displacement of a rotating output shaft into pulses or digital signals.
Typical components: code disc and photodetection assembly.
Working principle: The code disc is coaxial with the motor so that motor rotation drives the disc. The photodetection assembly outputs pulse signals generated by the disc; speed is calculated from these pulses.
Types are classified by the encoding method and output format: incremental, absolute, and hybrid.
Incremental encoder
- Working principle: photoelectric conversion.
- Outputs three groups of square-wave pulses: A, B, and Z. A and B are quadrature signals phase-shifted by 90 degrees to determine rotation direction. The Z pulse provides one reference pulse per revolution for indexing.
Absolute encoder
- Working principle: photoelectric conversion.
- Outputs a digital code. The code disc has multiple concentric tracks composed of alternating transparent and opaque sectors. The number of tracks equals the number of binary bits. Light sources and photodetectors are placed on either side of the disc; the set of detected binary values indicates the disc position.
Summary
- Optical encoders output digital signals directly, making processing straightforward and resolution easy to increase. Their disadvantages include lower resistance to shock, limited high-temperature tolerance, and susceptibility to optical contamination and electromagnetic interference.
- Rotary transformers offer advantages such as resistance to shock, high-temperature tolerance, resistance to oil contamination, high reliability, and long service life. Their disadvantage is that they output modulated analog signals, which require more complex decoding.
