QUADRATURE ENCODER
Rotary
encoder is a sensor attached to a rotating object (such as a shaft or
motor) to measure rotation. By measuring rotation your robot can do
things such as determine any displacement, velocity, acceleration, or
the angle of a rotating sensor.
1.0 Introduction
 |
A
typical rotary incremental encoder consists of a light-emitting diode
(LED), a disk, and a light detector on the opposite side of the disk
(see next figure). The disk, which is mounted on the rotating shaft, has
patterns of opaque and transparent sectors coded into the disk. As the
disk rotates, the opaque segments block the light and, where the glass
is clear, light is allowed to pass. This generates square-wave pulses,
which can then be interpreted into position or motion. These pulses can
be read by microcontroller as part of a PID feedback cont System to determine translation to distance, rotational velocity, and/or angle
of a moving robot or robot part. For instance, if you have a wheel
rotating, and you want to measure the time it takes to rotate exactly 40
degrees, or if you want to know when you have traveled X distance, you
can use an rotary encoder. The encoder will be fixed on your robot, and
the mechanical part (the encoder wheel) will rotate with the wheel.
Since the output of an encoder is a square wave, you can then count the
pulses if you hook up this signal to a digital counter or
microcontroller. Knowing the distance/angle between each pulse, and the
time from start to finish, you can easily determine position or angle or
velocity or whatever. Encoders are necessary for making robot arms, and very useful for acceleration control of heavier robots. They are also commonly used in robot for maze navigation.
Rotary Incremental Encoder Basic working model
Rotary
Encoders usually offer 100 to 6,000 segments per revolution. This means
the encoder can provide 3.6 deg of resolution for 100 segments and 0.06
deg of resolution for 6,000 segments. Linear encoders work under the
same principle as rotary encoders except that instead of a rotating
disk, there is a stationary opaque strip with transparent slits along
its surface, and the LED-detector assembly is attached to the moving
body.
2.0 Quadrature Encoder
An
encoder with one set of pulses is sometime not sufficient because it
cannot indicate the direction of rotation. Using two code tracks with
sectors positioned 90 degree out of phase (see next figure); the two
output channels of the quadrature encoder indicate both position and
direction of rotation. For example, if A leads B, the disk is rotating
in a clockwise direction. If B leads A, the disk is rotating in a
counter-clockwise direction. Therefore, by monitoring both the number of
pulses and the relative phase of signals A and B, the microcontroller
can track both the position and direction of rotation. In addition, some
quadrature encoders include a third output channel – called a zero or
reference signal – which supplies a single pulse per revolution. This
single pulse can be used for precise determination of a reference
position. This signal is called the Z-Terminal or the index in most of
encoder. A typical, ideal quadrature signal looks like this:
Quadrature Encoder Output
With
incremental encoders, you can measure only changes in position (from
which you can determine velocity and acceleration), but it is not
possible to determine the absolute position of an object. Another type
of encoder, called an absolute encoder, is capable of determining the
absolute position of an object. Its function is similar to position
feedback using variable resistor (analog output), the only differences
are that it can be rotated in 360 degree and digital output. This type
of encoder has alternating opaque and transparent segments like the
incremental encoder, but the absolute encoder uses multiple groups of
segments that form concentric circles on the encoder wheel like a
bull’s-eye on a target or dartboard. The concentric circles start in the
middle of the encoder wheel and, as the rings go out toward the outside
of the ring, each of them has doubled the number of segments than the
previous inner ring.
To
make encoder measurements, you need a basic electronic component called
a counter. Based on its several inputs, a basic counter emits a value
that represents the number of edges (low to high or high to low
transitions in the waveform) counted. Most of the Microchip PICs have
this peripheral; normally Timer 0 or Timer 1 is used as external input
counter. External interrupt pins (INT) can also be used for counting the
pulse; the rising edge (low to high) or falling edge (high to low) is
configurable. Once the edges are counted, the next thing you need to
take care is how those values are converted to position, further to
speed and etc. The process by which edge counts are converted to
position depends on the type of encoding used. There are three basic
types of encoding, X1, X2, and X4.
2.1 1X Encoding
You
will be able to see the signals shown in the next figure if we are
scanning from left to right ; and reverse the direction or scan from
right to left on previous figure. This is a quadrature cycle and the
resulting increments and decrements for X1 encoding. When channel A
leads channel B, the increment occurs on the rising edge of channel A.
When channel B leads channel A, the decrement occurs on the falling edge
of channel A.
1X Encoding
2.2 2X Encoding
A
shortcoming of the previous method is that the count frequency is the
same as the frequency of channel A. Thus, an encoder is said to have a
resolution of 500 pulses per revolution (ppr) does exactly that. We can
do better by using both edges of Channel A. This is not too hard to
arrange in hardware but this uses up valuable board space. The equality
test described just now works just as well if we are detecting falling
edges. Thus we can use the same routine for both rising and falling
edges and detect twice as many transitions. With 2X decoding our 500 ppr
encoder can generate 1000 pulses per revolution.
2X Encoding
2.3 4X Encoding
It
is possible to do even better if we examine the edges of both channel A
and channel B. There are four edges for each phase of channel A and it
is possible to get 2000 pulses per revolution from our 500 ppr encoder.
4X Encoding
When
I cut out the middle part of this signals which shown in the next
figure, we can see clearly that the two bit encoder field (A, B) is Gray Code Encoded. Only one of the two bits changes for any given state transition.
4X Encoding State Transition
Furthermore,
we can tell whether the wheel is turning clockwise or counter-clockwise
based on the state transitions, which are mutually exclusive for the
two directions, as shown in the table below.
4X Encoding State Transition Table
Most engineers will be more comfortable with the table above representing a state transition diagram, as shown in figure below.
4X Encoding State Transition Diagram
If
you have a microcontroller with the ability to generate interrupt form
external source, it is pretty simple to get the count we want. But at
least it must has two external interrupt pins for 4X encoding, let’s say
we use PIC18F4520 (3 external interrupt pins). Channel A is connected
to the INT1/RB1 pin and channel B is connected to the INT0/RB0 pin. The
sense of the interrupt is changed after each interrupt so that the
routine responds alternately to rising and falling edges. On each
interrupt, after determining the current state, we can get the direction
by checking back the previous state and Count Value will be increased
or decreased. Listing below is the example interrupt routine for
PIC18F4520 and the sequence is based on the previous state transition
diagram.
4X Encoding Interrupt Routine Listing
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