By Richard Anslow and Michael Jackson
The previous blog in this series showed how encoders based on AMR sensors can be used to monitor the condition of motorized equipment. This blog shows how to enhance the resolution of an optical encoder by simply modifying the components in the signal chain, thereby avoiding the expense and effort involved in changing the coded disk used to track motor position.
Capturing Absolute Position with an Optical Encoder
A typical optical encoder used for capturing the absolute position of a rotating motor shaft consists of an LED light source, a marked disc attached to the motor shaft, and a photodetector (Figure 1). The disc features a masked pattern of opaque sections that obscure the light and transparent section that allows it to pass through. Photodetectors sense the resulting light pattern and convert it into a digital electric signal. As the disc turns, small amplitude sine and cosine signals (in the mV or µV) range are produced. These signals are fed to an analog signal conditioning circuitry, usually consisting of a discrete amplifier or an analog PGA, which amplifies the signal up to approximately 1 V p-p— the typical input voltage range required to maximize the dynamic range of an ADC. A simultaneous sampling ADC ensures that the sine and cosine signals are sampled and converted at the exact instant in time before being passed to an ASIC or microcontroller to calculate the shaft position as it rotates. The motor controller queries the encoder position every cycle and uses this data to drive the motor based on the data it receives. In the past, system designers were required to trade ADC speed against channel count because of board size limitations.
Figure 1 Typical optical position feedback system
Dual Channel Oversampling Delivers Extra Benefits
New motor control applications require even higher accuracy position detection. Typically, increasing the resolution of an optical encoder would mean changing or swapping the disc for one with a higher number of slots – a time-consuming and costly redesign to the motor system. However, an alternative approach, which doesn’t require changing the disc, is to interpolate the sine and cosine signals using a high-speed, high-performance ADC. By sampling at a faster rate, more detailed signal values are captured, and the shaft position is determined with a higher degree of accuracy. High-speed oversampling also improves the noise performance, reducing the amount of overhead on the digital postprocessor. At the same time, it reduces the output data rate from the ADC, allowing for slower serial frequency signals, hence simplifying the digital interface. A dual-channel solution of this type has a smaller footprint, meaning it can be easily mounted on the motor assembly.
Optimized Signal Chain Solution
An optimized signal-chain solution for an optical encoder position-feedback system is shown in Figure 2. This circuit can be easily interfaced to an absolute optical encoder where differential sine and cosine signals from the encoder can be easily captured. The ADA4940-2 front-end amplifier is a dual-channel, low noise, fully differential amplifier that drives the AD7380, a dual-channel, 16-bit, fully differential, 4 MSPS, simultaneous sampling SAR ADC, housed in a small 3 mm × 3 mm LFCSP package. Using the integrated on-chip 2.5 V reference helps to reduce the overall component count. The VCC and VDRIVE of the ADC, as well as the supply rails of the amplifier driver, can be powered by an LDO regulator, like the LT3023 or LT3032. When these reference designs are interfaced—for example, with a 1024-slot optical encoder producing 1024 cycles of sine and cosine in one revolution of the encoder disc—the 16-bit AD7380 samples each encoder slot at 216 codes, overall increasing the encoder resolution up to 26 bits. The 4 MSPS throughput rate ensures that detailed sine and cosine cycles are captured and accurate encoder positions are maintained. The high throughput rate enables on-chip oversampling, which reduces any delay in the ASICs or microcontroller feeding the precise encoder position to the motor. An extra benefit of the AD7380’s on-chip oversampling is that it provides an additional 2 bits of resolution, which can be easily used with an on-chip resolution boost feature. This resolution further improves accuracy up to 28 bits.
Figure 2 Optimized signal chain for an optical encoder
Click here to learn more about Analog Devices' position encoder solutions for next-generation sustainable motor control. The final blog in this series will show how to overcome challenges encountered when designing signal chains for resolvers.