FEATURE Sensors & Sensing Systems
Figure 3: Sampling rate
type of optical encoder where diff erential sine and cosine signals from the encoder can be easily captured by the circuit. The ADA4940-2 front-end amplifi er is a dual-channel, low-noise, fully-diff erential amplifi er that drives the AD7380, a dual- channel, 16-bit, fully-diff erential, 4MSPS, simultaneous-sampling SAR ADC, housed in a 3mm × 3mm LFCSP package. The on- chip 2.5V reference would allow minimum component requirements for this circuit. The VCC
and VDRIVE of the ADC and
The ADC must feature simultaneous sampling on its channels such that the sine and cosine data points are taken at the exact same point in time, as that combination provides the shaft position information. The ADC conversion results are passed to an ASIC or a microcontroller. The motor controller queries the encoder position every PWM cycle and uses this data to drive the motor based on the instructions it receives. In the past, system designers would have to trade ADC speed or channel count to fi t restrictive board footprints.
Optimising position feedback The optical encoder resolution can be based on the number of slots inscribed from fi ne lithography on a disc, usually hundreds or thousands. Interpolating these sine and cosine signals to a high- speed, high-performance ADC will enable us to create higher-resolution encoders without requiring system changes to the encoder disc. For example, when an encoder sine and cosine signal are sampled at a slower rate, fewer values
Figure 4: Optimised feedback system design
of the signal are captured (Figure 3); this also limits the accuracy of the position cap. In the fi gure, when the ADC samples at a faster rate, more detailed values of the signal are captured and a higher accuracy position is determined. A high- speed sampling rate of the ADC allows oversampling, further improving the noise performance, removing some digital post-processing needs. At the same time, it reduces the output data rate from the ADC; that is, allowing for slower serial frequency signals, hence simplifying the digital interface. The motor position- feedback system is mounted in the motor assembly, which can be pretty small in certain applications. So size is vital to fi t in the limited PCB area of the encoder module. The emerging of multiple channel components in a single, tiny package are best suited for space saving.
Design example
An example of an optimised solution for an optical encoder position feedback system is shown in Figure 4. The circuit can be easily interfaced to an absolute
supply rails of the amplifi er driver can be powered by an LDO regulator, such as the LT3023 and LT3032. When these reference designs are interfaced – for example, with a 1024-slot optical encoder that produces 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 4MSPS 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. An extra benefi t of the AD7380’s on-chip oversampling is that it allows for an additional two bits of resolution, which can easily be used with an on-chip resolution boost feature. The resolution boost can further improve the accuracy, up to 28 bits. Application note AN-2003 details this oversampling and resolution boost feature of the AD7380.
Increasing demand Motor control system demands for higher accuracy, higher speed and miniaturisation are increasing. Optical encoders are used as motor-position sensing devices. To do this, the optical encoder signal chain must have a high level of accuracy when measuring the motor position. A high-speed, high-throughput ADC accurately captures information and feeds motor position data to the controller. The AD7380’s speed, density and performance answers the industry’s demand whilst enabling higher levels of accuracy and optimisation in the position-feedback system.
CONTACT:
Analog Devices
www.analog.com
automationmagazine.co.uk
Automation | November 2021
17
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