How to accurately detect motor position in faster, smaller applications?
Latest update time：2024-03-26
Reads：
This article describes common issues designers in the industrial automation space face when designing position sensing interfaces for motor control, namely sensing position in faster, smaller applications. Utilizing the information captured from the encoder to accurately measure motor position is important for the successful operation of automation and machine equipment. Fast, high-resolution, dual-channel simultaneous sampling analog-to-digital converters (ADCs) are an important component of this system.
Motor rotation information such as position, speed and direction must be accurate to produce precise drives and controllers for a variety of emerging applications, such as assembly machines that assemble micro-components into space-constrained PCB areas. Recently, motor control has begun to move toward miniaturization, resulting in new surgical robot applications in the medical and health industries and new drone applications in the aerospace and defense fields. Smaller motor controllers are also leading to new applications in industrial and commercial assembly. The challenge for designers is to meet the high accuracy requirements of position feedback sensors in high-speed applications while integrating all components into a limited PCB area to fit inside a tiny package, such as a robotic arm.
Figure 1. Closed-loop motor control feedback system.
motor control
The motor control loop (shown in Figure 1) mainly consists of the motor, controller and position feedback interface. The motor rotates the rotating shaft and drives the robotic arm to move accordingly. The motor controller controls when the motor applies force, stops, or continues to rotate. The position interface in the loop provides speed and position information to the controller. For assembly machines that assemble micro surface-mount PCBs, this data is key to the normal operation. These applications all require accurate position measurement information about rotating objects.
The resolution of the position sensor must be high enough to accurately detect the position of the motor shaft, pick up the corresponding micro-component, and place the component into the corresponding position on the board. Additionally, the higher the motor speed, the higher the loop bandwidth required and the lower the latency.
position feedback system
In low-end applications, position detection may be possible using incremental sensors and comparators, but in high-end applications, a more complex signal chain is required. These feedback systems include position sensors, followed by analog front-end signal conditioning, ADCs, and ADC drivers through which the data passes before entering the digital domain. One of the most accurate position sensors is the optical encoder. Optical encoders consist of an LED light source, a marking disc attached to the motor shaft, and a photodetector. The disc contains opaque and transparent mask areas that block light or allow it to pass through. Photodetectors detect these lights, and the on/off light signal is converted into an electronic signal.
As the disk rotates, photodetectors (coordinated with the disk's patterns) generate small sine and cosine signals (mV or µV levels). This system is typical of absolute position optical encoders. These signals enter analog signal conditioning circuitry (typically consisting of discrete amplifiers or analog PGAs used to obtain signals up to 1 V peak-to-peak range), usually to match the ADC input voltage range to the maximum dynamic range. Each amplified sine and cosine signal is then captured by the driver amplifier of the synchronous sampling ADC.
Each channel of the ADC must support simultaneous sampling so that sine and cosine data points are acquired simultaneously, and the combination of these data points provides axis position information. The ADC conversion results are sent to the ASIC or microcontroller. The motor controller queries the encoder position every PWM cycle and then uses that data to drive the motor based on the instructions received. In the past, system designers had to sacrifice ADC speed or channel count in order to integrate into limited board space.
Figure 2. Position feedback system.
Optimize position feedback
As technology continues to develop, innovations in motor control applications require high-precision position detection. The resolution of an optical encoder may be determined by the number of finely photoetched grooves on the disk, usually hundreds or thousands. By inserting these sine and cosine signals into a high-speed, high-performance ADC, a higher-resolution encoder can be created without system changes to the encoder disc. For example, when sampling the encoder's sine and cosine signals at a slower rate, only a few signal values are captured, as shown in Figure 3; this limits the accuracy of the position capacitance. In Figure 3, when the ADC samples at a faster rate, more detailed signal values can be obtained, resulting in a more accurate position determination. The ADC's high-speed sampling rate enables oversampling, further improving noise performance and eliminating some digital post-processing requirements. At the same time, the ADC's output data rate can be reduced; that is, slower serial frequency signals are supported, thus simplifying the digital interface. The motor position feedback system is mounted on the motor assembly, which in some applications can be very small. Therefore, the size is the key to whether it can be installed in the PCB area with limited area of the encoder module. Integrating multiple channel components in a single tiny package is a great space saver.
Figure
3. Sampling
rate
.
Optical Encoder Position Feedback Design Example
Figure 4 shows an example of an optimized solution for an optical encoder position feedback system. This circuit is easily interfaced with an absolute type optical encoder and the circuit then easily captures the 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, a dual-channel, 16-bit fully differential 4 MSPS simultaneous sampling SAR ADC packaged in a small 3 mm × 3 mm LFCSP package. The on-chip 2.5 V reference allows this circuit to use a minimum number of components. The ADC's V
CC
and V
DRIVE
, as well as the amplifier driver's power rails, can be powered by LDO regulators such as the LT3023
and LT3032. When these reference designs are interconnected (for example, using a 1024-slot optical encoder that generates 1024 sine and cosine cycles in one encoder disk period), the 16-bit AD7380 samples 2
16
codes in each encoder slot , increasing the overall resolution of the encoder to 26 bits. The 4 MSPS throughput rate ensures detailed information on sine and cosine cycles is captured, as well as up-to-date encoder position information. The high throughput rates enable on-chip oversampling, which reduces the time delay required for a digital ASIC or microcontroller to feedback precise encoder position to the motor. Another benefit of the AD7380's on-chip oversampling is that it can add an additional 2 bits of resolution, which can be used in conjunction with the on-chip resolution enhancement function. The resolution enhancement function can further improve the accuracy, up to 28 bits.
Figure 4. Optimized feedback system design.
in conclusion
Motor control systems require higher accuracy, higher speed, and a higher degree of miniaturization, and optical encoders are used as motor position detection devices. Therefore, the optical encoder signal chain must be highly accurate when measuring motor position. The high-speed, high-throughput ADC accurately captures the information and then sends the motor position data to the controller. The AD7380 has the speed, density and performance to meet industry requirements while enabling higher accuracy and system optimization in position feedback systems.
????Click to explore the ADI “core” world
·
·
京公网安备 11010802033920号