Controlling Two-Axis Motion of a Transducer Integrated with Instrument Control Using LabVIEW

Author(s):
John M. Harvard - Stratos

Industry:
ATE/Instrumentation

Products:
Motion Control, LabVIEW,

The Challenge:
Precisely controlling two-axis positioning of a transducer, synchronized with other tasks, such as changing transducer signal parameters, with the flexibility to accommodate control and configuration changes as the project progressed.

The Solution:
Building a PC-based system using National Instruments ValueMotion Servo, E Series DAQ, and GPIB boards controlled by LabVIEW.

Background
As part of the product development cycle for one of our clients, Stratos needed to develop a flexible control system to provide motion control and GPIB instrument control in a laboratory environment. Because the project was in the investigative stage, the control system had to be very flexible to test several prototype concepts and mechanical configurations. The success of the project depended on the use of LabVIEW to make software changes quickly to keep pace with changing user interface and motion control requirements.

System Requirements
The fast-paced prototype development critical to this project dictated a number of system choices. We needed to provide accurate two-axis motion control of a transducer with the possible addition of a third axis of motion or a second independent transducer at a later time. We needed to control the waveform, frequency, and amplitude of the electrical signal driving the transducer during the motion profile.

We also required data acquisition to collect input data from several sensors with analog outputs and control I/O to interface some user-controllable switch inputs and indicator outputs. The control system had to be flexible enough to accommodate changes, such as the testing of various motion sequences and profiles or the addition of new input switches or sensors. These changes had to keep pace with the rapid prototyping process. The entire system needed to be physically robust enough for us to move and reassemble it regularly at several sites specified by the client. In addition, we had only about eight weeks to develop this system and start initial testing.

System Architecture
A PC-based control solution was the best choice for our client because it provided the necessary flexibility on a familiar platform. We decided to use hardware products from National Instruments because of the tightly integrated performance achievable under LabVIEW control. We used a 200 MHz Pentium PC running Windows 95 and LabVIEW as our control platform.

For motion control, we used the ValueMotion PCI-Servo-4A, a 4-axis servo controller, to control our two small Portescap DC brush motors with built-in quadrature encoders. Because these motors require low power and have very low inductance windings, a higher-power PWM-type servo motor amplifier, such as a National Instruments nuDrive product, did not meet our needs. We instead breadboarded a custom dual linear servo amplifier with a small linear power supply to interface between the motors/encoders and the ValueMotion board. We also purchased the ValueMotion VI Library, which provides a powerful and timesaving interface to servo motor control from LabVIEW.

We used an AT-GPIB/TNT board to control the frequency, waveform, base amplitude, and modulation mode of the transducer drive signal generated by a Hewlett-Packard HP 33120A Function/Waveform Generator. We used an analog output from the AT-MIO-16E-10 DAQ board to control the external amplitude modulation input to the HP 33120A. We also connected a user control switch and several sensor inputs to the AT-MIO-16E-10 using the SCB-68 connector block, a shielded I/O interface with secure connections that we could easily change.

Design Challenges and Solutions
One design challenge was to coordinate control events while motion is underway. For example, in one configuration the transducer accelerates to a user-adjustable velocity and maintains that precise velocity while moving between two preset coordinates. During this motion, the system monitors motor-axis position. When the transducer reaches specified intermediate positions, we change the transducer drive signal without interrupting the motion.

This ability to monitor motion in progress and perform other control functions without interrupting the motion is a task that could not be done easily, if at all, using other motion control hardware/software solutions.
Using LabVIEW and the ValueMotion VI Library has made it easy to coordinate motor movement with other control tasks. The simpler, high-level motion VIs can be used for stand-alone moves. However, to provide the control necessary when coordinating motion with other control actions, the intermediate component motion VIs, such as Set Position Mode, Load RPM, Load Target Position, Start Motion, and Read Target Position, are more valuable.

We empirically tuned the servo PID parameters for different mechanical configurations and motor loads. The PC Runner software included with the ValueMotion board proved a useful starting tool. The Servo Init @ VI available in the ValueMotion VI Library provides a graphical interface for modifying PID parameters for each axis. Using this VI, we can save a file uniquely named for each parameter we have created; we load the appropriate file automatically by calling this VI through the application program (or top level VI) and passing it the parameter file name.

LabVIEW provided a very flexible programming platform. We could quickly make changes to the on-screen virtual user interface. It was a challenge to provide a basic diagram structure that could accommodate many system configuration changes without requiring major restructuring. However, with this architecture in place and with a number of subVIs written to handle individual custom tasks, we could make changes in a short time. With LabVIEW, we tested several prototype concepts and met the project schedule.

Summary
Using hardware products exclusively from National Instruments, coupled with the flexibility and capability of LabVIEW, provided built-in integration that proved extremely cost-effective for us. Especially in this fast prototyping program, we had no time to solve configuration and interfacing issues, build custom hardware, or write custom code.

The integrated motion control capability was critical in helping us reach our goals. Stratos Product Development Group, located in Seattle WA, provides product design and development services to solve problems for a diverse set of clients. Stratos has expertise in the areas of mechanical, electrical, software, and industrial design.

For more information, contact:

 John M. Havard

Stratos Product Development Group

2025 First Avenue, PH-B

Seattle, WA 98121

Tel: (206) 448-1388

Fax: (206) 448-7830

E-mail: johnh@stratos.com web www.stratos.com

Browse All Case Studies »