Using NI-Motion, NI-DAQ, and SCXI to Build an Automated Test Bed for Characterizing Nonlinear and Dynamic Properties of Electric Motors and Actuators

Author(s):
Chetan Kapoor - Robotics Research Group, The University of Texas at Austin
Jae Gu Yoo - Robotics Research Group, The University of Texas at Austin

Industry:
University/Education, Machines/Mechanics

Products:
LabVIEW, PXI/CompactPCI, Motion Control

The Challenge:
Designing and developing an electric motor and actuator test bed for measuring and analyzing an array of physical properties under static and dynamic loading.

The Solution:
Building a mechanically and electrically reconfigurable test bed that uses a PXI-based motion controller and DAQ, combined with SCXI signal conditioning and LabVIEW for control, data collection, analysis, and user interface.

Increased Productivity with Many Functions
A modern electric actuator is an integration of prime mover, gear train, clutch, brake, bearings, and motion controller. These actuators find use in highly programmable applications that normally have a nonlinear dynamic load. However, most electric motors, which are a critical part of an actuator, are statically analyzed with little available data related to their performance under varying temperature and dynamic loading. We can use this data, if available, to significantly increase control performance, assure quality, reliability, conformance to design, and confirm suitability of an actuator for a particular application. This customer solution describes the design and implementation of a test bed that provides extensive automated test capability of up to 1 HP actuators that span from low speed high torque to high speed low torque. For increased productivity, data collection performance, and synchronization, we used a high-quality motion controller and other software and hardware components from National Instruments in the test bed.

A Test Bed to Accommodate a High-Speed Motor
We designed this test bed to accommodate actuators/electric motors up to 1 HP in capacity. As different 1 HP systems can have a large variation in torque-speed relationships, we designed and selected the mechanical components of the test bed for both high-speed and high-torque operations. We chose a 4,000 RPM, 10 HP (peak 5,000 RPM and 35.6 ft-lb torque) motor as the load motor. Its primary purpose was to provide a programmable dynamic load on the test motor. We connected the test motor and load motor through a series of mechanical couplings selected for minimal inertia, maximum stiffness, high torque, and high-speed capacity. In between the mechanical couplings exist other components such as a brake, clutch, torque sensor, and four-bar linkage for a physical nonlinear load. We designed the attachment points on the test bed, so we can mechanically reconfigure it to accommodate different test motors and other measurement or loading hardware such as a gear train. We needed several sensors, including current sensors, position encoders, thermistors, and a torque sensor to build the performance data sets and to monitor the test motor state variables in real time. The current sensors measured the three phase input currents to the test motor. We used the encoder to determine torque ripple, correlating output torque with the precise rotor position. Temperature sensors evaluated motor capacity under various duty cycles. We used a torque sensor to fully measure the performance of an actuator and to enable torque control of the load motor.

Smooth Instrumentation, Control, and Software Development
National Instruments products proved critical to the instrumentation, control, and software development of this test bed. For motion control, we used the NI PXI-7350 8-axis motion control card. We used two axes of this board for the load motor and one for the test motor. Because of 50 percent lower cost, we selected a drive without sinusoidal commutation for the load motor. As such, we used the capabilities of PXI-7350 to provide the sinusoidal commutation for the load motor. The load motor drive does not need position and hall feedback, as the motion controller performs the load motor commutation of the current signal. Because of this, the hall and position feedback signals were directly connected to the PXI-7350. For the test motor, encoder and hall signals are needed for feedback to the amplifier and complete the sinusoidal commutation loop to run the test motor. To control the test motor in torque mode, we connected the torque sensor analog signal to one of the analog input channel on the NI motion controller. We then used this channel as a feedback signal for the test motor. With this arrangement, we controlled the speed of the load motor and the torque of the test motor. Default setting for the brake installed in the system was on, whereas default for the clutch was off. We did this to prevent the unintentional transfer of a large force from the load motor to the motor under test.

Quality Collection of Sensor Data
The state values of interest from sensors in the test bed were position, velocity, acceleration, torque, temperature, voltage, current, and magnetic flux density. However, the parameters estimated from the embedded sensors in the test bed included signal noise, so we used mathematical tools to reduce this noise. Use of PXI with the SCXI module for signal preconditioning cleaned up the noisy raw data from the test bed. For this, we chose SCXI 8th order programmable Bessel filter. The SCXI-1142 Bessel lowpass filter provides 80 dB attenuation and the passband magnitude response begins to drop off immediately with 7 kHz cutoff frequency. Also, the Bessel filter does not have any phase shift and it is not sensitive to overshoot or ringing in the step response. All eight conditioned signals were multiplexed in the rear connector, to which we could connect a single input channel of the DAQ device. We used a PXI-6040E multifunctional DAQ board to read the 11 analog input signals at 250 kS/s, 12-bit resolution. We used the Real-Time System Integration (RTSI) bus connected internally across the rear connectors of different boards for high-speed data synchronization. For this, LabVIEW generated a trigger output on the motion controller for the DAQ board at desired positions. We used -periodic breakpoint in the LabVIEW program to clock in an equidistant rate on the position or velocity of the load motor, which operated in velocity control mode. Because of this, we could collect sensor data synchronized with load motor speed. We used NI-Motion and Measurement & Automation Explorer (MAX) configuration utility to provide complete control over the characteristic of the actuator control system. The MAX used a Graphical User Interface (GUI) to initialize the motion board, configure each axis, and set control loop gains and motion thresholds.

Successful Solution for Various Automated Tests
Using the flexible and powerful LabVIEW software to integrate the NI-Motion Controller with PXI modular instrumentation, we built a successful solution for the control of the actuator test system. For accurate experimental data measurement, we implemented a signal conditioner and efficiently collected our data using a data acquisition board. This actuator test bed is suitable for automated testing of various motors and actuators using detailed test regimes developed at the Robotics Research Group, The University of Texas at Austin, USA.

For more information, contact:
Jae Gu Yoo
The University of Texas at Austin
10100 Burnet Rd. Bldg.160
Austin, TX 78758
Phone: 512-471-6825
Fax: 512-471-3987
E-mail: jaegu@mail.utexas.edu
Web: www.robotics.utexas.edu

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