How does the encoder measure position, speed, and angular acceleration

In the electric drive system, it is the core component used to measure the motor speed and rotor position. In the photoelectric encoder, its shaft is connected with the shaft under test, and the shaft under test drives its shaft to rotate, and then the measured physical quantity is converted into a binary code or a series of pulses.

As far as the photoelectric encoder is concerned, there are three types of subdivision, including incremental encoder, absolute encoder, and hybrid encoder. Among them, incremental encoders are mostly used for shaft speed measurement, absolute encoders are mostly used for measurement of shaft space position, and hybrid encoders are actually a combination of incremental encoders and absolute encoders, and the rear end is set Into the processing chip. So in fact, these three types of encoders have the function of measuring the rotor speed and spatial position.

How to realize the measurement of physical quantity? This must start from the principle. So let's talk about the working principle of incremental encoder.

The above figure is the structure diagram of the incremental encoder. The measured shaft drives its shaft to rotate so that the photoelectric code wheel rotates. The key part is the photoelectric code disc. There are radiating slits with equal pitch engraved on its periphery, and there are two sets of transparent detection slits. The two sets of transparent detection slits are staggered by 1/4 pitch, so two The output signal of the photoelectric converter is 90 degrees out of phase. When working, the discriminating disc does not move, only the main code disc and its rotating shaft rotate together with the rotating shaft to be measured, so that the emitted light source is projected onto the master disc and discriminating disc.

If the opaque area of the master code disc is aligned with the transparent slit of the discriminator disc, the projected light is completely blocked, and the voltage signal output by the encoder is the smallest. If the transparent area of the main code disc is aligned with the transparent slit of the discriminator disc, all the projected light is transmitted through, and the voltage signal output by the encoder is the maximum at this time.

Therefore, every time the main code wheel of the incremental encoder rotates for a tick mark cycle, it outputs an approximate sine wave voltage signal, and the phase difference of the output voltage signals of the two sets of photoelectric transducers is 90 degrees.

Look at the figure below, the output waveform of the incremental encoder does not mean that there are two sets of photoelectric transducer output signals? A and B in the figure are the two sets of output voltage signals, which belong to two orthogonal pulses. Z in the figure is a zero pulse. Its function is to correct the number of pulses generated by the encoder per revolution, control the error within each revolution, and avoid the accumulation of errors.

Differentiate the rotation direction of the motor rotor, and judge whether the motor rotor is rotating forward or backward according to the phases of the two pulse signals of A and B. However, incremental encoders have advantages and disadvantages. The advantages are easy miniaturization, simple structure, and fast response. The disadvantage is that it is easy to lose data after power failure, and it is easy to accumulate errors.

In summary, an incremental encoder can be used to detect the speed of the motor rotor and the initial position of the rotor.

For example, the speed measurement; according to the motor speed expression n=60·m/T·N, it can be known that as long as the number of output pulses N per revolution of the photoelectric encoder is known, the motor rotor speed can be known.