Testing Automated Medical Imaging Tables with the NI Motion Control Card

Author(s):
V. Arunachalam - Soliton Technologies Private Limited
Ganesh Devaraj, Ph.D. - Soliton Technologies Private Limited
Gopal Swaminathan - Soliton Technologies Private Limited

Industry:
Medical/ Medical Instrumentation

Products:
Motion Control, LabVIEW

The Challenge:
Developing, in eight weeks, a cost-effective solution for production testing of sophisticated tables that are used to position patients in nuclear-medicine-based medical imaging machines.

The Solution:
Using the National Instruments PCI-7344 motion control card and LabVIEW to meet the requirements of this system.

Building an Automated Test System

A major manufacturer of medical imaging machines requested that we, Soliton Technologies Private Limited, build an automated test system to test tables that are used to support and position patients during imaging. The tabletop, on which the patient would lie, had to be moved up and down as well as horizontally along its length, so that the part of the patient being imaged could be precisely positioned with respect to the source and detector in the machine. These movements had to be very smooth with no perceptible jerks, so as to minimize any discomfort to the patient.

The table has two DC servo motors for the vertical and horizontal motions, numerous limit switches to keep track of the tabletop position for safety, and encoders to provide feedback about the tabletop movement. The test system had to test the motors, limit switches, and encoders, while ensuring that there was no abnormal friction during the motion that could overload the motors or result in jerks. The test system also had to actuate a clutch used for power transfer from one of the motors and measure the slip produced at the clutch.

Since the requirements called for a multitude of tests, some of which were quite nonstandard like jerky motion detection, we determined that a computer-based system was required. It was clear right at the start that a motion control card was necessary to control the motors, but after a detailed study, we realized that the NI PCI-7344 had all of the necessary capabilities to address each and every test requirement.

System Description and Benefits

The table is normally interfaced to the imaging machine’s gantry, which houses the motor power supplies and drives and the table control system. In order to exercise the table through all its motions, we had to simulate the gantry using our automated testing equipment. The power supplies and drives were housed in a panel along with the PC.

As anticipated, we found that the friction and "sticky" positions during motion could be determined by monitoring the motor current using the analog input channels on the

PCI-7344. We performed the test by moving the table horizontally or vertically at a constant speed, while monitoring the current drawn by the corresponding motor from the drive using Hall Effect Current Transducers.

We used the NI UMI-7644 to bring out the terminals of the PCI-7344 for connection with the table system. We connected the horizontal motion system (motor, encoder, and home, plus two end-limit switches) to the corresponding terminals of Axis 1, and the vertical motion system to Axis 2. We connected an additional encoder on the shaft at the output of the horizontal motor clutch to the encoder of Axis 3. The table also had additional limit switches, which we connected to the DIO lines of the card. We polled their status in software. Finally, we optoisolated all the digital lines using a custom board, which also included a buffer to drive the relay in the table that activated the clutch.

The primary tests exercised the motors and checked the motor current, distance between all the limit switches using the encoder output, and repeatability of these readings. Another class of tests was the measurement of the stopping distance. It was important that the table stop smoothly and within the specified distance in a highly repeatable manner, when the stop command was given by the technologist while positioning the patient. This parameter was measured by getting the table to move at the specified speed and giving a motion command to stop with a specified deceleration profile and counting the number of pulses from the encoder before the table came to a standstill. In all of these tests, we assumed that the motion controller was highly repeatable and any variation in measurements was a result of variations between the tables. The repeatability tests on the same table verified that this assumption was correct. Finally, the clutch slip was measured by counting the output of the two encoders, one on the motor and one on the shaft after the clutch, and calculating the percentage difference for a full length of travel. We performed all of these tests with different speed and load conditions, and for each test, we predefined the pass and fail criteria. The test results were stored and were available for post-test review.

The customer also considered testing the table using the imaging machine’s gantry to exercise the table and using additional data acquisition equipment for making the measurements. The advantages of the computer-based system over the manual testing with a gantry are as follows:

Comparison Parameter	Manual Testing with Gantry	PC-Based Automated Test System
Cost and Flexibility	The gantry, with its control system, was a very expensive option. Also, it would only be able to test one type of table.	Since only one motion control card was required, our cost savings was more than 50 percent. The system is fully programmable to test any kind of table, making this solution substantially more beneficial.
Size	The gantry would consume a lot of production floor space.	All of the components are housed in a standard panel, making it 60 percent more compact. Since the system can test other table types, the space savings is very significant.
Quantitative Measurements	The smoothness of the table movements are only judged qualitatively.	The friction along the entire length of travel is determined quantitatively and compared with limits. All parameters are measured and recorded for six-sigma analysis.

 

A Simple, Cost-Effective System

The striking feature of this solution is its simplicity. Using a single motion control card, we automated all of the tests. We were able to match the capabilities of the motion controllers in the imaging machines with the specs on the NI high-performance controller, PCI-7344, making this solution a very cost-effective universal tester of imaging tables.

For more information, contact:

Ganesh Devaraj, Ph.D.

Soliton Technologies Private Limited

#420, 3rd Floor Bluemoon Icon,

20th Main, 6th B Cross,

6th Block Koramangala,

Bangalore – 560095

India

Tel: +91 (80) 25504677

Fax: +91 (80) 25504692

E-mail: ganesh@solitontech.com

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