by Richard Anslow and Michael Jackson
The last blog in this series reviewed some emerging trends in motor encoder applications. This blog looks at the benefits of using magnetic encoders based on anisotropic magnetoresistive (AMR) sensors and shows how to use components from ADI to realize a complete asset monitoring encoder solution.
Among magnetic-based position sensors, AMR sensors offer the best combination of robustness and accuracy. This sensor type is typically positioned opposite a Dipole magnet attached to the motor shaft as shown in Figure 1.
Figure 1 Using an AMR sensor to detect motor position
AMR sensors are sensitive to magnetic field direction changes, unlike Hall effect sensors which are sensitive to field intensity. This means AMR sensors are very tolerant to airgap and mechanical tolerance variation in the system. Furthermore, since there is no upper limit on the operating magnetic field, AMR sensors are extremely robust to stray magnetic fields. The ADA4571 is an AMR sensor with low latency integrated signal conditioning and has a single-ended analog output. The ADA4571 single chip provides guaranteed angular accuracy (only 0.10 typical angular error) and can operate at speeds up to 50K rpm. The ADA4571-2 is a dual version that provides full redundancy without compromising performance in applications that are safety critical. The ADA4570 is a derivative of the AAD4571 with the same performance but with a differential output for use in harsher environments. The high angular accuracy and repeatability provided by the ADA457x family improve closed-loop control, reducing motor torque ripple and noise. This single-chip architecture improves reliability, reduces size, and weight, and is easier to integrate compared to competing technologies.
Signal Conditioning and Power
The AD7380 is a dual simultaneous sampling, 16-bit SAR ADC offering many system-level benefits, including a space-saving 3 mm × 3mm package, an important feature for space-constrained encoder PCB boards. Its 4 MSPS throughput rate ensures that detailed sine and cosine cycles are captured and encoder positions are up to date. The high throughput rate enables oversampling on-chip, which reduces the time penalty of digital ASICs or microcontrollers feeding the precise encoder position to the motor. Another benefit of on-chip oversampling is that it allows for an additional 2 bits of resolution, using the AD7380’s resolution boost feature. The VCC and VDRIVE of the ADC and the supply rails of the amplifier driver can be powered by an LDO regulator, such as the LT3023. Multiple output low noise LDOs like the ADP320, LT3023, and LT3029 can be used to power all components in the signal chain.
The ADM3066E RS-485 transceiver exhibits ultra-low transmitter and receiver skew performance, which makes it ideal for transmission of a precision clock, which often features in motor encoding standards, such as EnDat 2.2.4 The ADM3065E demonstrates less than 5% deterministic jitter across typical cable lengths encountered in motor control applications. The wide supply range of the ADM3065E means this it is suitable for applications that require either a 3.3 V or 5 V transceiver power supply.
For lower resolution applications (12 bits or less), an alternative approach is to use a microcontroller with integrated ADC. The tiny MAX32672 ultra-low power Arm® Cortex® -M4F microcontroller includes a 12-bit 1 MSPS ADC with enhanced security, peripherals, and power management interfaces.
Asset Health Monitoring Encoder Solution
The complete signal chain for asset health monitoring AMR sensor-based application is shown in Figure 2. This solution features the ADXL371, an ultralow power, 3-axis, digital output, ±200 g microelectromechanical system (MEMS) accelerometer designed for machine monitoring. The cost-effective IC is available in a small 3 mm × 3 mm package and operates at temperatures up to +105°C. In instant-on mode, the ADXL371 consumes only 1.7 μA when continuously monitoring the environment for vibrations. If it detects an impact event that exceeds the internally set threshold, the device switches to normal operating mode quickly enough to record the event. Another notable component is the ADT7320, a high accuracy digital temperature sensor, which does not requires user calibration or correction and has excellent long-term stability and reliability. This IC is rated for operation over an extended range of −40°C to +150°C and is available in a small 4 mm × 4mm LFCSP package.
Figure 2 Signal chain for an AMR sensor-based asset monitoring solution
A list of ADI-recommended components for this signal chain is shown in the table below.
The next blog in this series will look at signal chains for encoders based on Hall-effect sensors as well as optical and resolver (coupled) encoder solutions.