This article outlines common challenges faced by designers in the industrial automation field when implementing position-detection interfaces for motor control, specifically detecting position in faster and smaller applications. Capturing encoder information for precise motor position measurement is critical for reliable operation of automation and machinery. Fast, high-resolution, dual-channel synchronous-sample analog-to-digital converters (ADCs) are important components in such systems.
Motor control
The motor control loop, shown in Figure 1, consists mainly of a motor, a controller, and a position feedback interface. The motor rotates the shaft, which moves the mechanical arm. The motor controller determines when to apply torque, when to stop, or when to continue rotating. The position interface provides the controller with speed and position information. For pick-and-place machines that assemble tiny surface-mount components into constrained PCB areas, this data is critical for normal operation. These applications require accurate measurements of the rotating element's position.
Position sensors must offer very high resolution to detect the motor shaft position precisely, pick the correct tiny component, and place it on the intended location on the board. Higher motor speeds demand greater control-loop bandwidth and lower latency.
Position feedback system
Low-end applications may use incremental sensors and comparators for position detection, but high-end applications require more sophisticated signal chains. These feedback systems include position sensors, analog front-end signal conditioning, ADCs, and ADC drivers, with data passing through these stages before entering the digital domain. Optical encoders provide the most precise position sensing. An optical encoder typically consists of an LED light source, a patterned disc attached to the motor shaft, and a photodetector. The disc has opaque and transparent mask regions that block or transmit light. The photodetector converts the resulting light on/off patterns into electrical signals.
As the disc rotates, the photodetector produces small sine and cosine signals (in the millivolt or microvolt range) that correspond to absolute position. These signals go into analog signal conditioning, typically implemented with discrete amplifiers or an analog programmable gain amplifier to achieve up to 1 V peak-to-peak signals so they match the ADC input range. Each amplified sine and cosine signal is then captured by the ADC's driver amplifier and synchronously sampled by the ADC.
Each ADC channel must support synchronous sampling so that sine and cosine samples are acquired simultaneously. The ADC results are sent to an ASIC or microcontroller. The motor controller queries encoder position each PWM cycle and uses that data to drive the motor. Historically, designers often had to sacrifice ADC speed or channel count to fit systems into limited PCB space.
Figure 1. Closed-loop motor control feedback system.
Optimizing position feedback
Advances in technology have driven novel motor control applications that require high-precision position measurement. The optical encoder resolution is determined by the number of finely patterned slots on the disc, often in the hundreds or thousands. Sampling the encoder's sine and cosine signals with a high-performance ADC can increase effective resolution without changing the encoder disc. For example, sampling at a low rate captures only a few signal values per cycle, which limits position accuracy. Sampling at a higher rate yields more detailed signal points and improves position determination. A high ADC sampling rate also supports oversampling to improve noise performance and reduce digital post-processing requirements. At the same time, the ADC output data rate can be reduced, allowing a slower serial interface and simplifying the digital interface.
Encoder modules are often mounted on compact motor assemblies that can be very small in some applications. The ability to fit the encoder electronics into a limited PCB area is therefore crucial. Integrating multiple channels into a single small package is advantageous for saving space.
Figure 2. Position feedback system.
Optical encoder position feedback design example
Figure 4 shows an optimized solution example for an optical encoder position feedback system. This circuit easily connects to an absolute optical encoder and captures differential sine and cosine signals from the encoder. The ADA4940-2 front-end amplifier is a dual-channel, low-noise, fully differential amplifier used to drive the AD7380, which is a dual-channel, 16-bit, 4 MSPS fully differential SAR ADC in a 3 mm × 3 mm LFCSP package. The on-chip 2.5 V reference allows the design to use a minimum number of components.
The ADC VCC and VDRIVE, and the amplifier driver power rails, can be supplied by low-dropout regulators such as the LT3023 and LT3032. When these reference designs are combined—for example, with a 1024-slot optical encoder that generates 1024 sine and cosine cycles per disc revolution—the 16-bit AD7380 can sample 2^16 codes across each encoder slot, increasing the overall encoder resolution by many effective bits. A 4 MSPS throughput ensures detailed capture of sine and cosine cycles and up-to-date encoder position information. The high throughput supports on-chip oversampling, which reduces latency for the digital ASIC or microcontroller to provide precise encoder feedback to the motor. On-chip oversampling can add effective resolution; combined with on-chip resolution enhancement features, overall accuracy can be further improved.
Figure 3. Sampling rate.
Figure 4. Optimized feedback system design.
Conclusion
Motor control systems require higher precision, higher speed, and greater miniaturization, and optical encoders are commonly used for motor position detection. To measure motor position accurately, the optical encoder signal chain must maintain high precision. High-speed, high-throughput ADCs capture encoder information accurately and deliver position data to the controller. ADCs that combine speed, channel density, and performance enable improved precision in position feedback systems and support system optimization.
