Position Measurement & Control - June 2002 (S050E)

CUSTOMER FOCUS

Flight Controls, Displacement Transducers, and Safety

Editor's Note: Thank you to Dr. Ulf Ringertz of the Royal Institute of Technology for performing the following testing and allowing this summary to be published.

Firstmark Controls position transducers have been safely used on a broad range of aircraft without incident for over 30 years. Recently, as a part of the KTH ASK-21 flight test program, the Department of Aeronautics at the Royal Institute of Technology in Stockholm, Sweden performed a breaking strength test of a Firstmark Controls cable assembly consisting of 7x7 0.018-inch (0.4572-mm) stainless steel cable (C40.0), copper sleeve (300292), and swivel (160026). Excerpts from the Institute's draft report are below. Later this year, the Institute will perform an additional test using a modified displacement cable assembly.

Summary of "Testing the Breaking Strength of a Position Transducer Cable"

All the control surfaces of the KTH ASK-21 flight test aircraft are measured using the position transducers manufactured by Firstmark Controls. The transducer consists of a potentiometer which measures the movement of a spring loaded cable. The cable is attached to the moving object and the potentiometer provides an analog voltage that is proportional to the movement of the object. In the KTH ASK-21 case, the cable is attached to the push rods in the mechanical control system as shown in the photo below.

Since the transducers are attached to the vital parts of the control system, it is essential to analyze the consequences of a possible failure of a transducer. Assuming that corrosion, wear and tear of the transducer would start to affect the performance of the transducer, the most likely event is that the transducer would not retract the cable properly. This type of failure would not affect flight safety since a slack cable has no effect on the movement of the push rod. A more serious failure mode of the transducer would be if the cable would be locked inside the transducer in a retracted position. Although this situation appears unlikely, if at all possible, the purpose of this investigation is to analyze the forces needed to brake either the cable or the connector that attaches the cable to each push rod.

The breaking strength of the cable and the connector were tested independently.

The first test involved finding the breaking strength of the displacement cable itself. The cable was run around the load cell fitting in a loop as shown in the photo below.

The loop was closed using the copper sleeve supplied with the transducers. Applying the force through displacement control of the testing machine produced the blue curve shown below.

At roughly 120 N of force, the cable started to slip in the copper sleeve giving the varying displacement force relation shown below.

Based on a conservative estimate of the breaking strength of the cable connector, the required pilot forces are well below what the average pilot can produce. For the elevator, the most critical control, the margin of safety is more than eight (8). However, for the aileron and air brake control the margin of safety is only about two (2).

If you would like a complete copy of the Insitute's test report, please contact us.

APPLICATION CORNER

The Application Corner answers your questions about using position transducers in specific applications. If you have an application question you would like answered, please contact us by phone, fax, e-mail, or Web form.

Q. Your standard products are generally provided with the displacement cable unterminated. Can you terminate the cable for us to our specification?

A. Yes. If you have a specific length where you want the cable terminated, please specify this information on your purchase order as well as what type of displacement cable termination you would like. There is no charge for this service for most termination requirements. Note that our standard termination tolerance is ± 0.01 inch (± 0.254 mm).

To terminate the displacement cable yourself, you may want to use the 160001-01 installation kit.

Q. How do I zero the output from unit to unit?

A. Below are a number of techniques you can use to zero the output on analog-output position transducer. If you have developed a technique not shown below, we would be interested in hearing about it.

• adjustable displacement cable termination - a turnbuckle, threaded plug, or similar devices allow you to adjust the cable length
• precision displacement cable leader - leaders specific to the application can be fabricated
• shims - thin layers ("shims") can be added as required on the mounting base or displacement cable termination
• rotating the potentiometer - the cam locks can be loosened and the potentiometer rotated to achieve the desired output
• trimming potentiometer - trim potentiometers can be added to the voltage divider circuit to control the zero point

Q. We need to maintain backwards compatibility with our RVDT sensors. Do you offer a position transducer with an RVDT- or LVDT-type electrical output?

A. We can provide our position transducers with an RVDT as the rotary sensor to meet the electrical output requirements of your application. Alternately, we can work with you to simulate RVDT/LVDT output using an appropriate electrical circuit.

Q. Do you have a source of magnets for mounting your product to our application?

A. For our Series 160, 161, 162, D60, D61, D62, 161H, and 162H style products, we now offer the 160005 magnet base that interfaces to both the standard base and the universal base. For more information on this product, please view our Mounting Bases and Accessories datasheet.

A supplier of magnets you may want to consider is http://www.bunting-magnetics.com.

IN ACTION

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

SSSSSHHHHHHH! Business aircraft will soon be quieter with the use of engine quieting technology that use a custom-engineered Firstmark Controls displacement transducer that acts as a limit switch. These "hush kits" shroud aircraft jet engines during takeoffs and landings to lower the sound levels ("hush") emanating from them. The Firstmark Controls displacement transducer provides input to cockpit displays indicating if the hush kit is in a retracted or extended position.

Automotive Components Manufacturer Finds "Less Is More" An automotive company was using 4 different types of non-Firstmark Controls displacement sensors for factory automation and testing purposes. These displacement sensors did not offer mounting flexibility and required the company to purchase a different sensor model for each orientation that needed to be measured. The manufacturer's manufacturing engineering group were referred to Firstmark Controls position transducers from a sister division and found that a single Firstmark Controls position transducer offered the installation flexibility to allow 1 model to replace 4 models of the previous device. The use of Firstmark Controls position transducers reduced the manufacturer's sensor inventory, fixturing requirements, and installation time.

Firstmark Controls Gets the Checkered Flag On May 26, 2002, Firstmark Controls miniature position transducers were in the winning vehicle of the Indianapolis 500. The transducers monitored driver controls for performance improvement analysis.