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Optical Encoder Small Size, High Precision, Reliable Work

Nov 01, 2017

Optical Encoder is a sensor which converts the mechanical geometric displacement of an output shaft into a pulse or a digital amount by photoelectric conversion. This is currently the most used sensor, Optical Encoder is a grating disc and photoelectric detection device. The grating disc is a certain diameter of the circular plate on the opening of several rectangular holes. Because the optical encoder disk is coaxial with the motor, when the motor rotates, the grating disk rotates with the motor at the same speed, and the detector of the electronic components, such as the light-emitting diode, detects the output of several pulse signals, the schematic diagram is shown in Fig. 1, and the number of output pulses per second can reflect the current speed of the motor. In addition, in order to determine the direction of rotation, the code plate can also provide phase difference 90 of the two-phase pulse.

According to the principle of detection, the encoder can be divided into optical type, magnetic type, inductive type and capacitance type. According to its calibration method and signal output form, can be divided into incremental, absolute and mixed type three kinds. Incremental encoder is the direct use of photoelectric conversion principle output three sets of square wave pulse A, B and z phase; A, b two groups of pulse phase Difference 90 Sea Maid award your Gu unloading milling dory is more than magnificus phase for each turn a pulse, used for datum point positioning. Its advantage is the principle of simple construction, mechanical average life can be more than tens of thousands of hours, anti-interference ability, high reliability, suitable for long-distance transmission. The disadvantage is that the absolute position information of the axis rotation cannot be output. 1.2 Absolute Encoder Absolute Encoder is a direct output of digital sensor, in its circular encoder along the radial number of concentric code path, each road is composed of light and opaque fan area, the number of adjacent yards is double relations, the number of yards is its binary digit number, on the side of the code plate is the source , the other side corresponds to each channel has a photosensitive element, when the code plate is in different position, each photosensitive element transforms the corresponding level signal according to the illumination or not, forms the binary system number. This encoder is characterized by no counter, in any position of the hinge can be read a fixed and position corresponding to the digital code. Obviously, the more code path, the higher the resolution, for a N-bit binary resolution of the encoder, the code plate must have n barcode path. At present, there are 16-digit absolute encoder products in China. Absolute Encoder is the use of natural binary or circular binary (Grimes code) mode of photoelectric conversion. Absolute Encoder and Incremental encoder is different from the disc on the light, opaque line graphics, Absolute Encoder can have a number of code, according to the code on the readout code, the absolute position detection. Code design can be used binary code, loop code, binary complement and so on. It is characterized by:

The absolute value of the angle coordinate can be read directly;

No cumulative error;

The location information will not be lost after the power is removed. However, the resolution is determined by the binary digits, that is, the precision depends on the number of digits, there are currently 10, 14 bits and so on.

Mixed absolute Value Encoder hybrid absolute value encoder, which outputs two sets of information: a set of information used to detect the position of the magnetic pole, with absolute information function, and another set of fully with the incremental encoder output information. Optical Encoder is an angle (angular velocity) detection device, it will input to the axis of the angle, using the photoelectric conversion principle to convert to the corresponding electrical pulse or digital volume, with small size, high precision, reliable work, interface digitization and other advantages. It is widely used in CNC machine tools, turntable, servo drive, robot, radar, military target detection, such as the need to detect the angle of the device and equipment.

Application Circuit of Optical Encoder

EPC-755A Optical Encoder has good performance, it has strong anti-interference ability in angle measurement and displacement measurement, and has stable and reliable output pulse signal, and the pulse signal can get the digital signal after counting. So, when we're developing a car-driving simulator, the rotation angle of the steering wheel is selected as a sensor, and the output circuit of the EPC-755A Optical Encoder is selected as the open type of the collector, and the output resolution selects 360 pulses/loops, considering that the steering wheel of the car is bidirectional, which can rotate clockwise and rotate clockwise. It is necessary to phase out the output signal of the encoder before it can be counted.

When the Optical Encoder rotates clockwise, channel a output waveform ahead Channel B output waveform 90°, D trigger output Q (waveform W1) for the high level, Q (waveform W2) for the low level, above with the door open, counting pulses through (waveform W3), sent to the two-way counter 74ls193 pulse input CU, To perform an addition count; At this point, the following is closed with the gate, and its output is a high level (waveform W4). When the Optical Encoder rotates counterclockwise, the channel a output waveform is delayed 90° than the channel B output waveform, D flip-flop output Q (waveform W1) for the low level, Q (waveform W2) for the high level, the above with the door closed, the output of the high level (waveform W3); At this point, the following with the non-door open, counting pulse through (waveform W4), Send to two-way counter 74ls193 to reduce pulse input CD, the subtraction count. When the steering wheel rotates clockwise and counterclockwise, the maximum rotation angle of the car is two and a half, the encoder with a resolution of 360 pulses/loops is selected, the maximum output pulse number is 900, and the actual counting circuit used is composed of 3 pieces of 74ls193, which are reset (CLR signal) when the system is initialized by electricity. The initial value is set to 800H, that is, $number (LD signal); So, when the steering wheel rotates clockwise, the output range of the counting circuit is 2048~2948, and when the steering wheel rotates counterclockwise, the output range of the counting circuit is 2048~1148 The data output d0~d11 of the counting circuit is sent to a data processing circuit.

Actual use, the steering wheel frequently rotates clockwise and counterclockwise, because of the quantization error, after a long period of time, the steering wheel back in the counting circuit output may not be 2048, but there are a few words of deviation; To solve this problem, we added a steering wheel back to the detection circuit, system work, When the data processing circuit is in the non operational state of the simulator, the system detects the circuit in the back, if the steering wheel is in the back state, and the output of the counting circuit is not 2048, the counting circuit can be reset and the initial value reset.