Position Measurement & Control - August 2001 (S050B)

Contents

• DO-160D / ED-14D Environmental Testing Completed
• Application Corner
  • Conversion Among Acceleration, Velocity, Displacement, and Frequency
  • Thermal Effect
  • Displacement Cable Sag (Catenary Curve)
• In Action
  • Timing Is Everything
  • R2D2, Meet the University of Utah
  • Windshield Wiper Testing
  • Riding the Trains
• News You Can Use
  • After Hours Assistance
  • What Is Accuracy?
  • Read All About It: Position Transducer Installation Guide

DO-160D / ED-14D Environmental Testing Completed!

Firstmark Controls products have been used in aircraft flight data recorder ("black box") applications for over 5 years. A comprehensive suite of environmental tests has been conducted on a range of Firstmark Controls products to verify the suitability for extended aircraft use, both inside and outside the aircraft pressure vessel. This test suite, performed according to DO-160D / ED-14D (Environmental Conditions and Test Procedures for Airborne Equipment), is now complete and a test report is available.

Battle tested: series of 5 position transducers after going through the DO-160D / ED-14D environmental test suite.

The environmental conditions of this test were established by RTCA's DO-160D document. Firstmark Controls was responsible for collecting data and performing functional tests to verify compliance with equipment performance standards. Five position transducer designs were tested: three baseline products along with two OEM designs. The position transducers were tested in the same environmental conditions, even though those conditions exceeded design specifications of some of the products.

A test system that could accommodate the various test configurations was used to test the position transducers (except for the fungus test which was a qualitative test). The test system consisted of a mounting plate, rotating cam, brushless motor, and data acquisition system. The test system operated the position transducers both electrically and mechanically. Data was recorded on the hard drive of a laptop computer.

The data acquisition system consisted of a wiring harness, laptop computer, data acquisition card and software, and a multichannel virtual instrument. The virtual instrument had the capability of recording and viewing data six channels at a sampling rate of 10,000 Hz. The resolution of the 12-bit analog to digital converter was ±10 mV. System noise was observed as high as ±30 mV. Efforts to reduce noise were successful at some times and unsuccessful at others due to setup variability. The majority of system noise had a characteristic 60 Hz backdrop implying it stemmed from conventional AC line noise from the surrounding laboratory equipment.

727 flight control surface monitored by position transducer design tested to DO-160D. Photo courtesy of Avionics Support Group

The computer was programmed to send a 5 VDC signal to the position transducers via the wiring harness. The wiring harness consisted of a distribution bus and shielded cable bundle. The cable bundle was comprised of input/output wires connecting to each individual position transducer. The excitation voltage to each transducer could be isolated if required. Thus, if one position transducer failed, the remaining four position transducers could be monitored independently.

Both the configurations of the test system and the method used to collect data varied depending on the test conducted. There were a total of 20 test configurations used in combination with 3 test methods.

Three primary test methods were used to determine compliance with equipment standards. Each primary test method had sub-categories that were used depending on the test set-up.

• Method 1 - Equipment was energized with 5 VDC and actuated during the test. Data was recorded using the data acquisition (DAQ) system during the test. Observations were recorded for pre-run, run, and post-run time.
• Method 2 - Equipment was energized with 5 VDC and held in a fixed position during the test. Data was recorded using the DAQ system during the test. Observations were recorded for pre-run, run, and post-run time. A photograph was taken upon conclusion of the test.
• Method 3 - The functional test was performed at the conclusion of the test. Data was recorded using the DAQ system or by qualitative analysis. Observations were recorded for pre-test and post-test runs.

As the manufacturer of the position transducers, Firstmark Controls’s responsibility was to determine compliance with applicable equipment performance standards. Early in the development of the test system, it became evident not all product specifications could be monitored during or after the test. Therefore, the test system was designed to monitor the position transducers primary function. The primary function of a position transducer is to measure displacement. A properly operating position transducer behaves like a voltage divider such that voltage output is proportional to linear displacement of the stainless steel cable. Accuracy and repeatability are primary functions of a position transducer as is the output smoothness. Temperature was predicted to have a small effect on internal resistance so resistance was measured at ambient conditions and -55° C to determine if theoretical thermal resistance coefficients were valid.

What was the result of all the testing? Unfortunately, 90 days of testing cannot be summarized in a short article. However, one unit passed all tests. Two units passed all tests except one test each (shock and waterproofness, respectively) due to the potentiometer manufacturer's specification being exceeded and due to a non-conforming manufacturing procedure. As expected, the final two units passed all tests except waterproofness and contamination due to their unsealed sensor cavity.

To obtain a copy of the final test report, please request one by completing our Comments and Questions form.

Series 160 position transducer shown installed on flight control cable. Photo courtesy of ASM

Application Corner

The Application Corner is dedicated to answering your questions about using position transducers in specific applications. If you have an application question you would like answered, please let us know by phone, fax, e-mail, or mail.

Q. Your data sheets give a specification for maximum acceleration. However, I need to know the fastest displacement cable velocity that can be attained with your products. How do I convert your acceleration specification to velocity?

A. For sinusoidal motion, you can use our Sinusoidal Motion Calculator that will allow you to convert between acceleration, velocity, displacement, and frequency.

Q. We will be installing your position transducers on aircraft. What effect on accuracy will the temperature change have?

A. The most significant temperature effect occurs on the stranded, Type 304 stainless steel displacement cable. Our Thermal Effect Calculator provides means to calculate this effect which is usually insignificant or can be compensated for.

Incidentally, because our analog potentiometer-based products are designed to be used as voltage dividers (voltage variance) and not rheostats (resistance variance), there is no temperature effect related to the potentiometer itself. This is because there is no voltage change due to the temperature change: the potentiometer's resistance change occurs uniformly throughout the resistive material.

Q. If the displacement cable is acted upon by gravity or another force, what is the effect on the position transducer's accuracy?

A. Because the mass of the cable per unit length is so small and the cable tension is relatively high, cable sag does not generally produce significant error unless the cable length is exceptionally long (over 60 feet). Visit our Displacement Cable Sag (Catenary) Calculator for more details on this effect.

In Action

Every month we see numerous fascinating applications of our position transducers. Here are a few recent ones:

Timing Is Everything Troubleshooting engine performance issues can be daunting and time consuming. Recently, Series 173 units were used to monitor the timing belt of automotive engines to give an indication when undue vibration was occurring.

R2D2, Meet the University of Utah Science fiction and television have embedded images of what a robot should look like and do. The University of Utah recently used Series 160 position transducers to monitor the legs on its robot used for robot mobility testing.

Windshield Wiper Testing Windshield wiper testing machines get feedback on wiper position from Series 173 position transducers.

Riding the Trains Trains must travel thousands of miles reliably and without maintenance. Series 160 position transducers are used to monitor displacement and vibration. The information gathered is used to design better suspension components and to troubleshoot reliability issues.

News You Can Use

Here are some updates and reminders regarding Firstmark Controls.

After Hours Assistance Need immediate help outside of our normal business hours of Monday - Thursday, 6:30 am to 5:00 pm (Pacific time)? Then, use our After Hours Assistance: 1-866-912-6232. We will assist you immediately with your question or issue.

What Is Accuracy? We are often asked, "What is the accuracy of your products?" Because of the lack of a common definition of "accuracy," we cannot quote a figure without knowing a little bit more about what your accuracy requirements are. With that in mind, here is a quick review of accuracy and terms related to accuracy.

Read All About It: Position Transducer Installation Guide Firstmark Controls position transducers are easy to use and rugged. They are so easy to use that you may be tempted to use them before fully understanding how they work resulting in damage to the product.

Before using the product, please study the Position Transducer Installation Guide (Acrobat document). Doing so will likely save you time and ensure all operating guidelines are followed.

ISSN 1527-5108