In the examples to follow, we will describe how to use the Teledyne LeCroy MDA810 Motor Drive Analyzer to acquire QEI A, B, and (optionally Z index) signals and convert them to static and dynamic motor shaft speed and angle values. We will show examples that use both analog and digital channel inputs.
QEI Analog Signal Acquisition
Figure 1 shows a long (2-second) capture of a QEI’s A, B, and Z signals (left side) and a 4000:1 horizontal zoom and vertical zoom (right side). We acquired the data at a sample rate of 5 MS/s using 10 MS of memory.
Note that some significant transient interference is seen on the zoomed analog signals. Acquiring the QEI signals with analog channels is an important first step to assessing signal quality and can assist in describing how to correctly set up for a later digital (MSO) acquisition.
Speed and Angle Calculation Setup
Figure 2 shows the setup dialog in the MDA for Mechanical speed, angle and torque setup. We have used the Speed & Angle Setup 1, chosen the method as Quadrature Encoder, and defined the A, B, and Z inputs to the acquired channels. We have also entered a Pulse/Rotation value of 2048, and enabled the Angle Tracking Observer (Observer Status). The MDA also is able to apply an offset angle to correct the measured QEI mechanical angle to a rotor magnetic field angle for use in debugging behaviors of control systems that rely on a rotor field angle, such as Vector field-oriented controls (FOC).
Displaying Calculated Speed and Angle Values and Waveforms
Figure 3 shows the QEI acquisition data with the Speed (orange, top right) and Angle (magenta, bottom right) Waveforms along with Numeric (mean value) and Statistical data.
The mean value of the speed is 1038 RPM; the mean value of the Angle is (an expected) 180°. We see in the Speed waveform the startup time for the digital filter used in the Angle Tracking Observer (this is a normal behavior for this type of digital filter). The Angle Tracking Observer filter reduces the variability of the measured speed value just as the motor drive control system would.
Figure 4 depicts another example, this time using a QEI that has only A and B signals. Without the Z index pulse, it is not possible to calculate the angle, so only the Speed Numeric, Statistics, and Waveform data appears.
Using Digital (MSO) Inputs for QEI Acquisition
Lastly, we show how the MDA uses its (optional) digital logic (MSO) inputs to acquire QEI A, B, and Z signals for purposes of speed and angle calculations. Using the digital inputs conserves analog inputs for other uses, such as AC line or Drive Output (PWM) waveforms, DC bus voltage and current measurement, gate-drive, sensor, or control/feedback signals.
Figure 5 shows a capture of QEI A, B, and Z index signals using DATA0, DATA1, and DATA2 inputs (DATA3 through DATA7 are also shown, but are not used). The assignment of digital input signals to A, B, and Z in the Speed & Angle setup dialog is as before, and Speed and Angle Waveforms, Numerics table mean values, and Statistical data are displayed. The Angle Tracking Observer is also used similarly to the previous analog input examples.
The Teledyne LeCroy Motor Drive Analyzer contains a powerful set of algorithms that permit calculation of Speed and Angle information from complex encoded QEI data, and display of the information as an intuitive Waveform view, as if were directly probed. Additional Angle Tracking Observer filters are also included to permit direct comparison to motor drive control system behaviors. The calculated data could be used with torque information (also calculated by the MDA) to calculate mechanical power. All of the Speed and Angle calculated information and Waveforms are time-correlated to all other signals acquired or calculated by the MDA to permit complete understanding of all inverter subsection, control system, and system-level dynamic behaviors.