Position measurement- Encoder

Shaft Encoders

Any transducer that generates a coded digital signal of a measured quantity is known as an encoder. Shaft encoders are used to measure angular displacements and angular velocities. High resolution, high accuracy, and digital output are some of the relative advantages of shaft encoders.

Resolution: Depends on the word size of the encoder output and the number of pulses generated per revolution of the encoder.

Accuracy:  Due to noise immunity and reliability of digital signals.

Encoder Types

Shaft encoders can be classified into two categories depending on the nature and the method of interpretation of the transducer output: (1) incremental encoders and (2) absolute encoders.

Incremental Encoder

The output of an incremental encoder is a pulse train signal, which is generated when the transducer disk rotates. The number of pulses and the number of pulse per unit time gives the measurement of angular displacement and angular velocity of the device on which the encoder disk is mounted. With an incremental encoder, displacement is obtained with respect to some reference point or marker. That is, incremental encoder giver relative position of a body wrt its initial position. The reference point can be the home position of the moving component (say, determined by a limit switch) or a reference point on the encoder disk, as indicated by a reference pulse (index pulse) generated at that location on the disk. The index pulse count determines the number of full revolutions.

Absolute Encoder

An absolute encoder has many pulse tracks on its transducer disk. When the disk of an absolute encoder rotates, several pulse trains are generated simultaneously. The number of pulse train is equal to the number of tracks on the disk. At a given instant, the magnitude of each pulse signal will be either ‘1’ (HIGH) or ‘0’ (LOW) depending on opaque and transparent segment of disk.. Hence, the set of pulse trains gives an encoded binary number at any instant. This encoded binary data gives the absolute position of the body on which the encoder is mounted. The pulse voltage can be made compatible with some digital interface logic (e.g TTL). Consequently, the direct digital readout of an angular position is possible with an absolute encoder. Absolute encoders are commonly used to measure fractions of a revolution. However, complete revolutions can be measured using an additional track, which generates an index pulse, as in the case of an incremental encoder. The same signal generation (and pick-off) mechanism may be used in both types (incremental and absolute) of transducers.

Encoder Technologies

Four techniques of transducer signal generation may be identified for shaft encoders:

1.      Optical method- we will discuss only this method in the post.

2.      Sliding contact (electrical conducting) method

3.      Magnetic saturation (reluctance) method

4.      Proximity sensor method

By far, the optical encoder is most common and cost-effective. The other three methods may be used in special circumstances, where the optical method may not be suitable (e.g., under extreme tem peratures or in the presence of dust, smoke, etc.). For a given type of encoder (incremental or absolute), the method of signal interpretation is identical for all four types of signal generation listed previously. Now we briefly describe the principle of signal generation for all four techniques and consider only the optical encoder in the context of signal interpretation and processing.

Optical Encoder

The optical encoder uses an opaque disk (coded disk) that has one or more circular tracks, with some arrangement of identical transparent windows (slits) in each track. A parallel beam of light (e.g., from a set of light-emitting diodes or LEDs) is projected to all tracks from one side of the disk. The transmitted light is picked off using a series of photosensors on the other side of the disk, which typically has one sensor for each track. This arrangement is shown in Figure a, which indicates just one track and one pick-off sensor. The light sensor could be a silicon photodiode or a phototransistor. Since the light from the source is interrupted by the opaque regions of the track, the output signal from the photosensor is a series of voltage pulses. This signal can be interpreted through edge detection or level detection to obtain the increments in the angular position and also the angular velocity of the disk.

The sensor element of such a measuring device is the encoder disk, which is coupled to the rotating object directly or through a gear mechanism. The transducer stage is the conversion of disk motion (analog) into the pulse signals, which can be coded into a digital word.

If the direction of rotation is not important, an incremental encoder disk requires only one primary track that has equally spaced and identical pick-off regions. A reference track that has just one window may be used to generate the index pulse, to initiate pulse counting for angular position measurement and to detect complete revolutions.

Note: A transparent disk with a track of opaque spots will work equally well as the encoder disk of an optical encoder. In either form, the track has a 50% duty cycle (i.e., the length of the transparent region is equal to the length of the opaque region).

Direction of Rotation

An incremental encoder generally have a second track placed at quarter-pitch separation from the first track pattern (pitch = center-to-center distance between adjacent windows) to generate a quadrature signal, which will indicate the direction of rotation.

An incremental encoder typically has the following five pinouts:

1. Ground

2. Index Channel

3. A Channel

4. +5V dc power

5. B Channel

Pins for Channel A and Channel B give the quadrature signals shown in Figure a and b, and the Index pin gives the reference pulse signal shown in Figure c. Figure 2a shows the sensor outputs (v1 and v2) when the disk rotates in the clockwise (cw) direction; and Figure 2b shows the outputs when the disk rotates in the counterclockwise (ccw) direction. Several methods can be used to determine the direction of rotation using these two quadrature signals. For example,

1.      By phase angle between the two signals

2.      By clock counts to two adjacent rising edges of the two signals

3.      By checking for rising or falling edge of one signal when the other is at high

4.       For a high-to-low transition of one signal check the next transition of the other signal

Pulsed signal output of incremental encoder. (a) CW rotation; (b) CCW rotation; (c) Index pulse

Method 1: It is clear from Figure 2a and b that in the CW rotation, v1 lags v2 by a quarter of a cycle (i.e., a phase lag of 90°) and in the CCW rotation, v1 leads v2 by a quarter of a cycle. Hence, the direction of rotation may be obtained by determining the phase difference of the two output signals, using phase detecting circuitry.

Method 2: A rising edge of a pulse can be determined by comparing successive signal levels at fixed time periods (can be done in both hardware and software). Rising-edge time can be measured using pulse counts of a high-frequency clock. Suppose that the counting (timing) begins when the v1 signal begins to rise (i.e., when a rising edge is detected). Let n1 = number of clock cycles (time) up to the time when v2 begins to rise; and n2 = number of clock cycles up to the time when v1 begins to rise again. Ten, the following logic applies:

If n₁ > n₂ – n₁àCW rotation

If n₁ < n₂ – n₁àCCW rotation

Method 3: In this case, firstly high level logic of v₂ is detected and then check if v₁ is at rising or falling edge.

If v₁ is at rising edge when v₂ is at logic high à CW rotation

If v₁ is at falling edge when v₂ is at logic high à CCW rotation

Method 4: Detect a high to low transition in signal v₁.

If the next transition in signal v₂ is Low to High à CW rotation

If the next transition in signal v₂ is High to Low à CCW rotation