Author: Jonathan Colao, application engineer of ADI company

This paper introduces the common problems faced by designers in the field of industrial automation when designing the position detection interface for motor control, that is, they can detect the position in the application with faster speed and smaller size. Using the information captured from the encoder to accurately measure the motor position is important for the successful operation of automation and machine equipment. Fast, high-resolution, dual channel synchronous sampling analog-to-digital converter (ADC) is an important component of this system.

brief introduction

Motor rotation information such as position, speed and direction must be accurate to produce accurate drives and controllers for various emerging applications, such as assembly machines that assemble micro components into PCB areas with limited space. Recently, motor control has begun to move towards miniaturization, resulting in new surgical robot applications in the medical and health industry and new UAV applications in aerospace and defense fields. Smaller motor controllers also lead to new applications in industrial and commercial assembly. For designers, the challenge is to meet the high-precision requirements of position feedback sensors in high-speed applications, while integrating all components into a limited PCB area to install inside micro packages, such as robotic arms.

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Figure 1 Closed loop motor control feedback system

Motor control

The motor control loop (as shown in Figure 1) is mainly composed of motor, controller and position feedback interface. The motor rotates the rotating shaft to drive the manipulator to move with it. 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 the assembly machine with micro surface mount PCB, these data are the key to its normal operation. These applications need to obtain accurate position measurement information about rotating objects.

The resolution of the position sensor must be very high enough to accurately detect the position of the motor shaft, take the corresponding micro component and place the component in the corresponding position on the board. In addition, the higher the motor speed, the higher the loop bandwidth required and the lower the delay.

Position feedback system

In low-end applications, position detection may be implemented by using incremental sensors and comparators, but in high-end applications, more complex signal chains are required. These feedback systems include position sensors, followed by analog front-end signal conditioning, ADC and ADC driver. The data passes through them before entering the digital domain. The most accurate position sensor is optical encoder. The optical encoder consists of an LED light source, a marking disc connected to the motor shaft and a photodetector. The disc contains opaque and transparent mask areas that block or allow light to pass through. The photodetector detects these lights, and the on / off light signal is converted into an electronic signal.

As the disk rotates, the photodetector (matched with the mode of the disk) generates small sinusoidal and cosine signals (MV or µ V level). This system is a typical system used in absolute position optical encoder. These signals enter the analog signal conditioning circuit (generally composed of split amplifier or analog PGA to obtain signals up to 1 V peak to peak range), usually in order to make the input voltage range of ADC match the maximum dynamic range. Each amplified sine and cosine signal is then captured by the driving amplifier of the synchronous sampling ADC.

Each channel of ADC must support synchronous sampling in order to obtain sine and cosine data points at the same time, and the combination of these data points provides axis position information. ADC conversion results will be sent to ASIC or microcontroller. The motor controller queries the encoder position in each PWM cycle, and then uses the data to drive the motor according to the received instruction. In the past, in order to integrate into a limited board space, system designers had to sacrifice ADC speed or channel number.

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Figure 2 Position feedback system

Optimized position feedback

With the continuous development of technology, the motor control applications that need to implement high-precision position detection are constantly innovated. The resolution of the optical encoder may be determined by the number of fine lithography slots on the disc, usually hundreds or thousands. After these sine and cosine signals are inserted into a high-speed, high-performance ADC, a higher resolution encoder can be created without system changes to the encoder disk. For example, when sampling the sine and cosine signals of the encoder at a lower rate, only a few signal values will be captured, as shown in Fig. 3; This limits the accuracy of the position capacitance. In Fig. 3, when the ADC samples at a faster rate, more detailed signal values can be obtained to determine the position more accurately. The high-speed sampling rate of ADC supports oversampling, which further improves the noise performance and eliminates some digital post-processing requirements. At the same time, the output data rate of ADC 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 may be very small in some applications. Therefore, the size is the key to whether the encoder module can be installed in the PCB area with limited area. Integrating multiple channel components in a single micro package is very helpful to save space.

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Figure 3 Sampling rate

Design example of optical encoder position feedback

Fig. 4 shows an example of an optimized solution suitable for an optical encoder position feedback system. This circuit is easy to connect with the absolute type optical encoder, and then the circuit can easily capture the differential sine and cosine signals from the encoder. Ada4940-2 front-end amplifier is a dual channel, low-noise fully differential amplifier, which is used to drive ad7380. The latter is a dual channel, 16 bit fully differential 4 MSPs synchronous sampling SAR ADC, using 3 mm × 3 mm small lfcsp package. The on-chip 2.5 V reference voltage source allows the circuit to use a minimum number of components. The VCC and vdrive of the ADC and the power rail of the amplifier driver can be powered by LDO regulators, such as lt3023 and lt3032. When these reference designs are interconnected (for example, using a 1024 slot optical encoder to generate 1024 sine and cosine cycles in one encoder disk cycle), the 16 bit ad7380 samples in each encoder slot of 216 codes, improving the overall resolution of the encoder to 26 bits. 4 MSPs throughput rate ensures that the details of sine and cosine cycles and the latest encoder position information are captured. The high throughput rate supports on-chip oversampling, which reduces the time delay when the digital ASIC or microcontroller feeds back the accurate encoder position to the motor. Another advantage of on-chip oversampling of ad7380 is that it can add an additional 2-bit resolution, which can be used in combination with on-chip resolution enhancement. The resolution enhancement function can further improve the accuracy, up to 28 bits. The application note an-2003 introduces this oversampling and resolution enhancement function of ad7380 in detail.

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Figure 4 Optimized feedback system design

conclusion

Motor control system needs higher precision, higher speed and higher degree of miniaturization. The optical encoder is used as a motor position detection device. Therefore, when measuring the motor position, the optical encoder signal chain must have high precision. The high speed and high throughput ADC accurately captures the information, and then sends the motor position data to the controller. The speed, density and performance of ad7380 can meet the industry requirements. At the same time, it can achieve higher accuracy in the position feedback system and optimize the system.

author

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Jonathan Colao

Jonathan Colao is a product application engineer in the precision Converter Technology Department of ADI company. He joined ADI company as a product engineer in 2006. His work location is located in jiamidi, Philippines. He graduated from cagayande oroxavier University in the Philippines with a bachelor’s degree in electronic engineering.

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