Georgia Tech Uses NI LabVIEW Control Design Toolkit and LabVIEW Simulation Module for a Cylindrical Coordinate Measurement Machine

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"Using the LabVIEW Control Design and Simulation Module, we found it easy to design and implement control systems."

- Thomas Kurfess, Ph.D., Georgia Institute of Technology

The Challenge:
Reducing setup time by actively centering a bearing ring on a rotating spindle in a cylindrical coordinate measurement machine.

The Solution:
Using the NI LabVIEW Control Design and Simulation Module and NI hardware to design, implement, and test the control system for automated part centering in a cylindrical measurement machine.

Author(s):
Laine Mears - Georgia Institute of Technology
Thomas Kurfess, Ph.D. - Georgia Institute of Technology

The Need for a More Efficient System

Currently, the mechanical dimensions of finished bearing rings are measured in the factory using a cylindrical coordinate measurement machine with a manual centering process. The operator places the finished bearing ring on a table mounted to a precision spindle and begins the manual centering process. The operator monitors a digital readout from an LVDT displacement sensor and manually taps the bearing until it is centered and the LVDT reading falls within a preset tolerance. Typically, the manual centering step takes one minute to center a part within the 2.5 microns tolerance window. At this point, the machine measures the mechanical dimensions of the ring by scanning over the surface of the part with a contact probe. The operator spends 15 percent of the total measurement cycle time centering the part. Therefore, by reducing the time required for the centering process, we can significantly reduce the cycle time and labor costs of the part measurement process.

By designing an active control system to automate the manual centering process, we can reduce the centering cycle time.  Manual centering is not only used for bearing metrology but in production processes as well. Therefore, manufacturing engineers could also use an automated centering approach to reduce the cycle time of various production processes for cylindrical parts.

Automating the Centering Process

We designed an automated system that consists of a linear slide and a precision spindle.  Both motion devices include air bearings to improve precision and smooth motion. The linear slide contains a brushless linear motor and a linear encoder. The spindle uses a brushless motor and a rotary encoder. We used an LVDT displacement sensor as a measurement probe and mounted it to the linear slide. We also mounted a fixed pusher contact to the slide and used it to actuate the bearing ring.

The operation of the system is divided into three separate stages:

1. Servo following stage

2. Pushing stage

3. Modification stage

In the servo following stage, we use the LVDT measurement probe deviation from null position to command the linear slide velocity. The system captures the probe absolute tip position with respect to rotational spindle position. In addition, the system filters the raw data using a Kalman Filter.

In the pushing stage, we use experimental data and modeling to identify the spindle position where the bearing surface is the greatest distance away from the spindle center.  We use this as the target for the bearing position. The system commands the linear slide to the desired target position using a trapezoidal velocity profile.

Finally, in the modification stage, we compare the push stage results to expected values. We then make adjustments before the next servo following and push stages. Our overall measures of success include centering time reduction from the current manual centering process and the ability to achieve repeatable centering tolerance.

We currently use an NI PXI-7350 motion controller for velocity control of both the linear and rotary motor. We implemented higher-level control loops on the PXI controller using NI LabVIEW Real-Time with parallel LabVIEW timed loops.

We used the NI LabVIEW Control Design and Simulation Module to design and analyze the higher-level control loops in the system. We also used it to design a Kalman Filter for the noisy measurement probe output. The system then models the filtered data with a single-lobe sinewave using least-squares curve fitting.

We used the NI LabVIEW Control Design and Simulation Module to develop simulations of the various control loops in the system.  For example, we modeled a motion control loop using a subsystem for the PID control law used in the motion controller, a transfer function for the motor drive along with a saturation block, and the transfer function representing the motor dynamics. We included both position and velocity feedback in the model.

Future Work Using NI Products

In the future, we plan to move the motion control loops from the PXI-7350 to the LabVIEW NI SoftMotion Module and the NI PXI-R Series module to increase the control update rate and implement more advanced control schemes at this level.  This becomes especially important as we try to push heavier bearing rings.  The increased mass can increase nonlinear effects such as friction between the ring and the rotating spindle. Also, we plan to use the NI LabVIEW System Identification Toolkit to identify the plant model from measured data.  By having a more accurate model, we can better develop advanced controllers.

Benefits of Using LabVIEW For Control Design and Simulation

Using the LabVIEW control design and simulation tools, we found it easy to design and implement control systems. We also experience this ease-of-use when changes or updates to the programs are needed.  With LabVIEW, we can quickly integrate different code components together – including code written in other programming languages. Finally, by having tight integration of the NI software with the hardware, we can reduce the time required to implement the system.

Author Information:
Thomas Kurfess, Ph.D.
Georgia Institute of Technology
813 Ferst. St.
Altanta, GA 30332-0405
United States
Tel: (404) 894-0301
Fax: (404) 894-9342
tom.kurfess@me.gatech.edu

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