Rotation Measurement Using Optical Mouse Sensor

This design example uses the sensor in the optical computer mouse to measure the rotation of the disc, in which the disc can be mechanically connected to any rotating device. By changing the position of the sensor along the radius of the disc, the pulse per rotation can be adjusted.

The CMOS optical sensor of the mouse chip can provide a non mechanical tracking engine. In the chip, image capture, digitization and digital processing can be completed. Take the simple and low-cost om02 for example. The sensor measures the position by collecting surface image frames, and determines the motion direction and distance through mathematical operation. The sensor is installed in a polystyrene optical package and is designed to be used with high brightness LEDs. It has a complete and compact tracking engine; There are no moving parts and precise optical alignment is not required. Om02 can generate quadrature output signals for X and Y direction motion. The resolution is about 0.0025 inches and the movement speed is up to 16 inches per second.

The orthogonal X-direction output signal generated by the chip simulates the output of the ordinary encoder. Both X and Y signals can be used in 2D systems. Om02 generates X1 and X2 quadrature signals at a frequency of up to about 25 kHz. Figure 1 shows a timing diagram of forward x movement (right direction). This quadrature output can also be used for DC stepper motor control when needed.Figure 1: example of quadrature output waveform ( X motion).According to the instructions in the IC data book, an internal oscillator can be used, and the capacitor COSC can be used at this time (Fig. 2). The resistance ROSC defines the frame rate: the smaller its value, the higher the corresponding rate.

Figure 2: disc rotation measurement with om02 sensor.Connecting the X1 and X2 outputs to the XOR gate can double the data rate, but the direction information will be lost.Physical implementation

In order to obtain good optical recognition effect of surface pattern, the detection disc or other surfaces must have certain texture, pattern, scratch or brush surface treatment (Fig. 3).Figure 3: physical principle of rotation detection.The structure shown in Figure 4 has been successfully applied to assembly lines, conveyor belts, label posting devices, printing on moving objects and other applications to provide synchronous motion. More than 100 products produced still work well after years of operation.

Figure 4: structure that has been successfully applied to production.There are other sensor ICs on the market, which can handle different light sources and have different output, speed and other indicators. For example, pan3101 CMOS optical mouse sensor uses SPI, and pan101bcmos optical navigation sensor has SPI and quadrature output at the same time.Schematic example of SPI interface

Sensors with SPI interface (or USB interface using more than one IC) are not allowed to track each pulse separately because they send packets (Fig. 5). For hard real-time applications, sensors that provide quadrature output are preferred.It will be very interesting to build an encoder with a wireless computer mouse, and it may be more interesting to use the sensors in the digital caliper, because most of them have I2C interfaces.Figure 5: schematic example of SPI interface.