Servo Motor Encoder Technology Introduction

Photoelectric Encoder Application Circuits

Photoelectric Encoder is a sensor that converts angular displacement or linear displacement into electrical signals. It consists of a code disk with a series of transparent and opaque sectors, a light-emitting device, and a light-receiving device.

Photoelectric encoders are devices that convert angular displacement or linear displacement into electrical signals. They are widely used in various fields due to their high precision and reliability. 

Application of EPC-755A Photoelectric Encoder in a Car Driving Simulator

The EPC-755A photoelectric encoder is chosen for its excellent performance in angle measurement and displacement measurement, with strong anti-interference ability and stable output pulse signals. In the development of a car driving simulator, this encoder is used to measure the rotation angle of the steering wheel.

The output circuit of the EPC-755A is of the collector-open type, with a resolution of 360 pulses per revolution. Considering that the steering wheel can rotate bidirectionally (clockwise and counterclockwise), phase discrimination of the encoder's output signals is necessary for accurate counting.

The phase discrimination and bidirectional counting circuit utilize one D flip-flop and two NAND gates. When the encoder rotates clockwise, the output waveform of channel A leads that of channel B by 90 degrees, causing the D flip-flop to output a high-level signal (waveform W1) and a low-level signal (waveform W2).

This opens the upper NAND gate, allowing counting pulses to pass through (waveform W3) to the count-up input (CU) of the bidirectional counter (74LS193). Meanwhile, the lower NAND gate is closed, outputting a high-level signal (waveform W4). Conversely, when the encoder rotates counterclockwise, the output waveform of channel A lags behind that of channel B by 90 degrees, causing the D flip-flop to output low and high levels for W1 and W2, respectively. This closes the upper NAND gate and opens the lower one, directing counting pulses to the count-down input (CD) of the counter.

The steering wheel has a maximum rotation angle of two and a half turns in both directions. With an encoder resolution of 360 pulses per turn, the maximum output pulse number is 900. The counting circuit consists of three 74LS193 chips, initialized to 2048 (800H) upon system power -up. This setup allows the count range to be 2048 to 2948 for clockwise rotation and 2048 to 1148 for counterclockwise rotation. The data output (DO~D11) of the counting circuit is sent to the data processing circuit.

Application of Photoelectric Encoders in Gravity Measuring Instruments

An incremental encoder is a sensor that outputs signals in the form of pulses. Its code disk is much simpler in structure compared to that of an absolute encoder and can achieve higher resolution. Typically, it only requires three tracks. These tracks, however, no longer carry the same meaning as those in an absolute encoder; instead, they are used to generate counting pulses.

The outer and middle tracks of its code disk have an equal number of evenly distributed transparent and opaque sectors (gratings), but the sectors of the two tracks are staggered by half a sector. As the code disk rotates, it outputs A-phase and B-phase pulse signals with a phase difference of 90°, as well as a pulse signal generated by a third track with only one transparent slit (which serves as the reference position of the code disk, providing an initial zero-position signal for the counting system).

The rotational direction can be determined by the phase relationship (leading or lagging) between the A and B output signals. As illustrated in Figure 3(a), when the code disk rotates forward, the pulse waveform of the A track leads that of the B track by π/2, whereas when it rotates backward, the A track pulse lags behind the B track pulse by π/2. Figure 3(b) depicts an actual circuit where the positive pulse generated by a monostable multivibrator triggered by the falling edge of the A-track shaped wave is ANDed with the B-track shaped wave. When the code disk rotates forward, only forward pulses are output, and conversely, only reverse pulses are output when it rotates backward.

Therefore, an incremental encoder determines the rotational direction and relative angular displacement of the code disk based on the output pulse source and pulse counting. Typically, if an encoder has N output signals (tracks) with a phase difference of π/N, the number of countable pulses is 2N times the number of gratings.

In this case, N equals 2. A drawback of the circuit in Figure 3 is that it may sometimes generate erroneous counting pulses, leading to errors. This occurs when one of the signals is in a "high" or "low" state while the other is in transition between "high" and "low," even though the code disk has not undergone any displacement, resulting in unidirectional output pulses. For instance, this can happen when the code disk experiences jitter or is manually aligned to a position (as will be seen in gravimeter measurements).

Basic Waveforms and Circuitry of Incremental Photoelectric Encoders: 

Figure 4 illustrates a four-frequency quadrature subdivision circuit that not only prevents erroneous pulses but also enhances resolution. In this circuit, D-type flip-flops with memory functionality and a clock generation circuit are employed. As shown in Figure 4, each track has two D flip-flops connected in series.

Consequently, during the interval between clock pulses, the two Q outputs (for example, pins 2 and 7 of the 74LS175 corresponding to track B) retain the input states of the previous two clock periods. If these two states are identical, it indicates no change occurred during the clock interval; otherwise, the direction of change can be determined based on their relationship, thereby generating either a "forward" or "reverse" output pulse.

When a track oscillates between "high" and "low" states due to vibration, it will alternately produce "forward" and "reverse" pulses. These pulses can be canceled out when taking the algebraic sum of the two counters (this point will also be relevant when discussing instrument readings below).

Therefore, the frequency of the clock generator should be greater than the possible maximum value of the vibration frequency. As can also be seen from Figure 4, within the period of an original pulse signal, four counting pulses are obtained. For instance, an encoder originally producing 1000 pulses per revolution can generate 4000 pulses after four-frequency quadrature subdivision, resulting in a resolution of 0.09°.

Therefore, an angular displacement measurement system can be formed by simply adding subdivision and counting circuits (note that the 74159 is a 4-to-16 decoder).

Issues and Improvement Measures in Applications

Issues Encountered

 • Mechanical vibrations can cause displacement or misalignment of the emitter or receiver, affecting reliable signal reception.

 • Environmental factors such as high temperature and humidity can alter or damage electronic components within the photoelectric detection device.

 • Electromagnetic interference from the production site can distort the output waveform of the photoelectric detection device, leading to system errors.

Improvement Measures

The installation method of the photoelectric encoder has been modified. Instead of being mounted on the motor housing, a fixed bracket is fabricated on the motor's foundation to independently install the photoelectric encoder. After adopting this installation method, vibration measurements with a vibrometer show that the vibration speed has been reduced to 1.2 mm/s.

The transmission medium for the output signals of the photoelectric detection device has been reasonably selected, with twisted-pair shielded cables replacing ordinary shielded cables. Twisted-pair shielded cables possess two significant technical characteristics. Firstly, they offer strong protection against electromagnetic interference (EMI) because the interference currents induced by spatial electromagnetic fields on the cables can cancel each other out. Secondly, after twisting, the distance between the two wires is very small, and their distances to interfering lines are essentially equal. Additionally, the distributed capacitance between the two wires and the shielding net is also nearly identical, which further enhances the suppression of common-mode interference.

PLC software is utilized for monitoring or intervention. During the dummy bar delivery process in continuous casting production, the photoelectric detection device is required to generate timed electrical signals that correspond to different stages of the entire process, as illustrated in Figure 5.

Before the dummy bar delivery process is initiated, photoelectric signal 1 is "1".

After the dummy bar delivery process is started, in Phase A, the roller table is activated, and the dummy bar is fed upwards. When the dummy bar blocks the infrared light emitted by the photoelectric device, the photoelectric signal becomes "0". When the infrared light passes through the two small circular holes in the middle of the dummy bar, the photoelectric device emits signals 2 and 3, both of which are "1".

In Phase B of the dummy bar delivery process, the photoelectric signal is "0", the roller table stops, and the upward feeding of the dummy bar is paused. The 10th sector of the fan-shaped segment is pressed down, and the withdrawal straightener and "Synchronization 1" are activated, allowing the dummy bar to continue feeding upwards.

In Phase C of the dummy bar delivery process, the dummy bar continues to feed upwards and no longer blocks the infrared light. Photoelectric signal 4 becomes "1", activating "Synchronization 2" and halting "Synchronization 1". The dummy bar continues to feed upwards. At this point, the working process of the photoelectric device is completed.

The output signals of the PLC program are then input into the PLC's input module, replacing the original input signals of the photoelectric signals. The program flowchart is illustrated in Figure 6. 

In summary, the photoelectric detection device itself is composed of electronic components and has certain technical requirements for its installation environment. Especially when used in harsh environments, appropriate protective measures must be taken to ensure that the photoelectric detection device operates under the technical conditions specified by the product, thereby allowing it to fully exhibit its technical performance.

Applying PC programs in the control system for real-time process control monitoring or intervention can overcome various deficiencies associated with the use of photoelectric devices in the system, which is an effective way to enhance system reliability.