アプリケーションノート 7601

Applications of MAX11192/MAX11195/MAX11198 ADCs for Motor Monitoring and Control

筆者: Tom Au-Yeung

要約:

This application note discusses applications of the MAX11192/5/8 ADCs for motor monitoring and control.


Introduction

The magnetic encoders are used to sense mechanical motion and generate digital signals corresponding to that motion to measure the speed and position of a typical motor system. To accomplish these two tasks, an ADC with two simultaneous-sampling, balanced differential inputs and a separate data output for each channel is needed. The MAX11192/MAX11195/MAX11198 are specifically designed for this motor monitoring and control applications. These ADCs feature best-in-class sample rates and resolution in a tiny 2mm x 3mm package. An integrated voltage reference and reference buffers help designers to minimize board space, component count, and system cost. To fully capture the benefits and optimal performance of this device, the proper choice of input driver amplifiers for different types of applications is essential. This application note provides an overview of how a typical motor system is used to measure the speed and position and discusses the needed performance of the ADCs as well as the suitable driver amplifiers for optimum motor monitoring and control.

Overview

Figure 1 shows a typical block diagram of a motor monitoring and control system. The motor drive controller monitors the status of the motor from the data provided by the ADCs in the magnetic encoder. Based on this data, the motor drive controller generates the needed commands to regulate the motor. The hall sensor in the magnetic encoder is used to sense the position information based on changes in the magnetic field produced by the rotational shaft of the motor. The analog signals from the output of the hall sensors are then converted into speed and position information by the ADCs. This feedback data enables the motor drive controller to process the information and provide the commands to control the system accurately.

Motor Monitoring and Control Typical block diagramFigure 1. Motor Monitoring and Control Typical block diagram.

Figure 2 shows the SEW MOVITRAC® with a built-in magnetic encoder. For more information, refer to the SEW EURODRIVE.

SEW Motor MOVITRAC® with a built-in magnetic encoderFigure 2. SEW Motor MOVITRAC® with a built-in magnetic encoder.

Speed and Position Measurement

Figure 3 shows a typical magnetic encoder block diagram. The magnetic encoder, an integrated part of the motor, incorporates a hall sensor to sense the speed and rotational position and generates analog signals in response to that motion. A hall sensor can only detect the magnetic field strength in one direction. Therefore, to capture the position changes in the rotational motor, two hall sensors are needed to detect both the X and Y axes. These two hall sensors detect the angular position of the magnet located on the encoder and provide two analog output voltage values that are orthogonal. These two orthogonal analog outputs (sine and cosine) are fed to the inputs of the driver amplifiers followed by the ADCs to generate the corresponding digital signals for the microcontroller. These two obtained X (cosine) and Y (sine) signals are then converted to angles using the trigonometric functions as follows.

tanθ = Y/X
Or θ = arctan(Y/X)

This magnetic encoder requires fast and precise ADCs such as the MAX11192/MAX11195/MAX11198 family of 2Msps sampling rates to be able to catch the rapid movement of the motor. In addition, the ADCs require appropriate driver amplifiers such as the MAX44242, MAX44263, or MAX4432 to yield accurate data.

Typical Magnetic Encoder block diagramFigure 3. Typical Magnetic Encoder block diagram.

Precision Encoder Speed and Position Measurement Using the MAX11192/MAX11195/MAX11198

The magnetic encoders in motors are exposed to very noisy environments. In addition, they are required to capture the high speeds and the fast-changing rotational positions of motors accurately. Hence, ADCs that can sample data at a high rate in the order of mega samples per second (Msps) are needed. These ADCs require the following characteristics for optimal performance:

  1. Precision Measurement: To measure the high speed of the motor in the kHz range and position precisely, ADCs with at least 12-bit resolution or higher and a sampling rate of at least 10x that of the maximum signal frequency, such as the MAX11192/MAX11195/MAX11198 12-bit/14-bit/16-bit, 2Msps ADC are needed. In addition, to provide optimum input signal conditioning, a high-speed and low-noise amplifier such as the MAX44263 or MAX44242/MAX4432 is used to drive the MAX11192 12-bit ADCs and the MAX11195/8 14-bit/16-bit ADCs, respectively.
  2. Differential Measurement: To minimize the common-mode noise in the noisy motor environment, ADCs with balanced and differential input signals are needed.
  3. Dual and Simultaneous Sampling: To accurately measure the position of the encoder, ADCs with dual-channel outputs and simultaneous sampling are required to capture the sine and cosine signals from the hall sensors.

The MAX11192/MAX11195/MAX11198 SAR ADCs are designed specifically to provide these three essential features, making them desirable devices for the magnetic encoder in motor monitoring and control applications.

Analog Inputs

The analog inputs of the MAX11192/MAX11195/MAX11198, AIN+ and AIN-, are designed for balanced differential signals. The input signals can range from 0V to VREF. Therefore, the differential input interval [VDIFF = (AIN+) - (AIN-)] ranges from –VREF to +VREF, and the full-scale range is FSR = 2 × VREF. Figure 4 shows the MAX11192/MAX11195/MAX11198 analog input signal voltage ranges.

The MAX11192/MAX11195/MAX11198 analog input signal voltage ranges where VREF + 250mV ≤ VAVDD ≤ 5.25VFigure 4. The MAX11192/MAX11195/MAX11198 analog input signal voltage ranges where VREF + 250mV ≤ VAVDD ≤ 5.25V.

The differential analog inputs must be centered with respect to a common-mode signal of VREF/2, with a tolerance of ±100mV. The reference voltage can range from 250mV to 2.5V below the reference supply AVDD. This reference voltage range guarantees adequate headroom for the internal reference buffers. For input signals other than this required unipolar differential configuration, level-shifting and phase conversion can be implemented to achieve the specified input voltage ranges. For more information, see the Different Configurations of Driver Amplifiers section.

Driver Amplifiers

The ADC driver amplifier should have a sufficiently low noise density of 6nV/√Hz or less in the bandwidth of interest because the driver amplifier noise affects the signal-to-noise (SNR) significantly.

In addition, to take full advantage of the ADC's excellent dynamic performance, the driver amplifiers must have equal or better total harmonic distortion (THD) performance than those of the MAX11192/MAX11195/MAX11198. This prevents the driver amplifier distortion in the signal path from limiting the overall system dynamic performance. Table 1 lists some potential ADC driver amplifiers for the MAX11192/MAX11195/MAX11198 ADC.

Table 1. Potential Driver Amplifiers for the MAX11192/MAX11158

Amplifier Input-Noise Ddensity (nV/√Hz) THD (dB) Signal Bandwidth (MHz) VDC (V)

MAX4432 2.8 N/A 180 ±5V
MAX44242 5 -124 10 2.7V to 20V
MAX44263 12.7 -110 15 1.8V to 5.5V

MAX11198 ADC Dynamic Performance with Various Driver Amplifiers

The MAX11198 dynamic performance is evaluated with various driver amplifiers. The conditions are AVDD = 5V, OVDD = 3.3V, and VREF = 2.5V unless otherwise noted.

Table 2 shows the results of the effective number of bits (ENOB).

Table 2. MAX11198 Dynamic Performance ENOB with Various Driver Amplifiers

fIN (kHz) Sample Rate (ksps) ENOB
MAX11198 and MAX44242 MAX11198 and MAX44263 MAX11198 and MAX4432 (VREF = 2.5V) MAX11198 and MAX443 (VREF = 4.096V)
1 100 14.2 14 14.5 15
10 50 14.2 14 14.5 15
10 100 14.2 14 14.5 15
10 500 14.2 14 14.5 15
10 1000 14 13.8 14.5 15
10 2000 13.1 13.7 14.4 14.9
100 1000 12.2 11.3 14.1 14.8
100 2000 12.5 11.5 14.1 14.8

Table 3 summarizes the results of SNR with different driver amplifiers.

Table 3. MAX11198 Dynamic Performance SNR with Various Driver Amplifiers

fIN (kHz) Sample Rate (ksps) SNR
MAX11198 and MAX44242 MAX11198 and MAX44263 MAX11198 and MAX4432 (VREF=2.5V) MAX11198 and MAX4432 (VREF=4.096V)
1 100 87.5 86.1 89.2 92.4
10 50 87.5 86 89.1 92.3
10 100 87.4 86.1 89.1 92.3
10 500 87.3 86 89.2 92.4
10 1000 86.2 85.1 89.1 92.3
10 2000 81 84.6 88.7 91.9
100 1000 83.5 79.6 87.2 91
100 2000 80.4 78.3 84.3 91

Table 4 shows the THD performance with different driver amplifiers.

Table 4. MAX11198 Dynamic Performance THD with Various Driver Amplifiers

fIN (kHz) Sample Rate (ksps) THD
MAX11198 and MAX44242 MAX11198 and MAX44263 MAX11198 and MAX4432
(VREF=2.5V)
MAX11198 and MAX4432
(VREF=4.096V)
1 100 107.1 109.1 111.2 112.2
10 50 104.7 111 111.6 112.6
10 106.9 116.2 108.9 110.9 112.4
10 500 107.6 108.1 111.8 114.4
10 1000 106.6 102.7 110.1 114.1
10 2000 94.2 99 108.8 107.2
100 1000 75.7 70.2 97.6 102.5
100 2000 79.5 71.8 95.7 104.8

Figure 5 summarizes the MAX11198 ENOB with different driver amplifiers at 10kHz.

MAX11198 ENOB vs. sample rate with different driver amplifiersFigure 5. MAX11198 ENOB vs. sample rate with different driver amplifiers.

Figure 6 shows the dynamic performance of the MAX11198 with the MAX4432 driver amplifier captured by the MAX11198 evaluation kit (EV kit) software graphic user interface (GUI) at 1kHz/100ksps.

The MAX4432 yields the best dynamic performance as it has the lowest noise density of 2.8nV/√Hz and the widest bandwidth of 180MHz.

The MAX11198 with the MAX4432 driver amplifier dynamic performance at fIN = 1kHz, sample rate = 100ksps, and VREF = 4.096VFigure 6. The MAX11198 with the MAX4432 driver amplifier dynamic performance at fIN = 1kHz, sample rate = 100ksps, and VREF = 4.096V.

Figure 7 shows the 1kHz Sine and Cosine waveforms at the analog inputs captured by the MAX11198 ADC.

1kHz Sine and Cosine Waveforms captured by the MAX11198EVKITFigure 7. 1kHz Sine and Cosine Waveforms captured by the MAX11198EVKIT.

Conclusion

With the implementation of dual differential and simultaneous ADCs with a high sampling rate of 2Msps and an accurate internal reference voltage, the MAX11192/MAX11195/MAX11198 ADCs are optimized for precision magnetic encoders in motor monitoring and control applications. The differential inputs reduce the noise from the motor, and the integrated voltage reference of these ADCs reduces cost and board size. The high sampling rate of 2Msps is a perfect feature for the high-speed application demanded by the motor. From the experimental data in the lab, the MAX4432 is the best driver amplifier for the MAX11198 thanks to its low voltage noise density and wide frequency bandwidth. The MAX44242 is an excellent choice for the MAX11195 with a 14-bit resolution ADC, and the MAX44263 is a suitable selection to pair with the 12-bit MAX11192 ADC.

References