Position Measurement & Control - Winter 1996 (WI96)

Contents

• Multi-Dimensional Cable Travel
  
• Price: Add-ons and Life-Cycle Costs
  
• Nemesis Air Racing Team Takes "Lie Detector" Test
  
• Engineering Note: Thread and Helix
  
• Application Note
  

A benefit of Firstmark Controls position transducers is they measure displacement when the application is not necessarily moving in a straight line or when the position transducer cable exit axis is not in line with the application movement axis. Two accessories are available for the Series 160, 161 & 162 product lines that make it easier for you to align the cable movement with your application: the Cable Guide (P/N 160045) and the Idler (P/N 160022 or 161022).

The Idler was designed originally for aircraft applications. When monitoring movement of rudders or elevators, there is a large variation in cable travel in two dimensions (one plane) that the pulley-like function of the Idler addresses quite well.

The Cable Guide was developed in response to automotive applications where there was limited motion in three dimensions. This is the case in vehicle crash testing where it is difficult to predict in which direction the monitored item might deflect. A Nylatron bushing integrated into the Cable Guide allows off-line motion within a 20° cone.

While you cannot use both accessories at the same time, both accessories can be used on the same position transducer. If you would like to use the same position transducer for multiple applications, you may want to order both accessories at the same time and swap parts as required. This operation requires changing the cable assembly.

Without either of these accessories, the cable motion at the cable exit point must be in a straight line. The following anecdote gives some details on what happens when the cable motion is not in a straight line.

Over our 28-year history, we have had calls from customers that have had problems with cable breakage. The most popular reason given for these breakages is the sharp edge of the cover where the cable exits the position transducer cut the cable.

The sharp edge of the cover in question is made of 2024 aluminum. The cable that reportedly is cut by that sharp edge is made of 304 stainless steel. Therefore, in this situation, we have an aluminum knife cutting a stainless steel cable. As you are probably aware, this is not too likely. If this were true you would have a drawer full of aluminum knives in your kitchen! In fact, we have seen cases where, due to misalignment, the stainless steel cable has sawed precise grooves into the aluminum cover.

Like most other purchases you make, the choice of a position transducer involves assessing the cost of the product in relation to other products and options available. If you find yourself evaluating Firstmark Controls products against other options, you may want to keep the following Firstmark Controls position transducer option/accessory prices in mind:

• Minimal mount fabrication and installation costs due to built-in mounting flexibility
  
• Quick maintenance design allows easy repairs and minimal downtime
  

Finally, our quick-response capability usually lets us ship to you within 7 days of your order and, in emergencies, often the same day, depending on our order backlog. This results in more project planning flexibility, quicker implementation, and lower downtime for you.

Nemesis Air Racing Team, racing the Formula 1-class Sharp DR90 Nemesis, recently added Firstmark Controls Series 150 and 173 position transducers to their aircraft to monitor throttle and stick position. Formula 1 aircraft are rule-limited to what modifications can be made to them: fixed gear, 500-pound empty weight, 66-square feet minimum wing area, fixed-pitch propeller, and a 100-hp Continental 0-200 engine. As such, the teams behind these aircraft scour the world for technologies to give them an edge within the limits of the rules.

Crew Chief Steve Ericson, a designer at Lockheed Martin's Skunk Works, mounts the products using industrial double-backed tape. Ericson notes that, "the high precision, small size, and light weight make these products ideal for our environment." Jon Sharp , pilot of the Nemesis, refers to the position transducers and the Beta Motor Sports real-time flexible data acquisition system, the FDAS 900A, as "our lie detector." Jon explained, "The position transducers and the FDAS 900A give the ground team immediate feedback on the aircraft. It sometimes catches me in a lie or two."

The highly-successful Nemesis set a new world record recently at the Reno National Championship Air Races with Firstmark Controls products aboard. With Jon Sharp at the controls, the aircraft averaged 249.904 mph over the 8-lap race. To give you a feel for the impressiveness of this record, the second place finisher was more that 17 mph slower than the Nemesis. In addition, the previous record was set in 1980 under a different set of rules that allowed higher-performance metal propellers and carburetors.

For more information on the Nemesis Air Racing Team, contact them at:

Our Series 160, 161 & 162 products use a threaded drum to maintain repeatability. You may have an application where you need to know the precise travel measured during one complete revolution of the position transducer's cable drum. This information may be useful, say, when an encoder is used as the sensing element instead of a precision potentiometer.

Because a thread is a helix, the formula for determining the distance travelled during n revolutions of the cable drum is:

      s = n*square root((2*pi*r)2 + h2)

      s = distance travelled
      n = number of drum revolutions
      r = radius of drum
      h = pitch in linear units

Note that Series 16X products do not use gear reduction and, hence, there is no backlash or creep. Call us if you need cable drum dimensions to perform this calculation for your application.

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, or mail.

Q. I ordered a Model 160-0241 position transducer with 3 inches of travel but the cable can be extended to over 6 inches. In addition, the voltage output from the potentiometer shows an open midway through the 6-inch stroke. Is there a problem?

A. No, you are using a position transducer with a single-turn potentiometer that can revolve continuously. The position transducer will rotate 1 revolution and measure 3 inches and then pass over the open on the potentiometer. It will continue to rotate until the end of the cable is reached. Position transducers with this potentiometer type allow you to easily set the zero point by rotating the potentiometer (see the User's Guide). In addition, cable breakage or cable over-extension do not break this type of potentiometer due to its continuous rotation. Multi-turn potentiometers are rendered useless when internal stops are damaged.

Series 160, 161 & 162 position transducer models with a "1" at the end (for example 161-0361) use single-turn potentiometers that continuously revolve. Series 150, 173, and 174 products use single-turn potentiometers that do not continuously revolve.

Q. What connectors can you provide to connect the cable to my application?

A. An uncrimped swivel is provided standard on all position transducers. Upon request, we can also provide a broad range of alternative connectors including rings, line connectors, screws, crimped balls, and user-specified components. If you have a specific requirement, please let us know.

Q. I need to measure 18 cm of motion but the position transducer is located over 125 cm from the application. Can I order a position transducer with a longer cable?

A. Yes, simply state on your order the extra cable length you would like. We can provide extra cable at no charge for reasonable lengths. Twelve inches (305 mm) of cable is provided standard (measured from the cable exit point with the cable in its fully retracted position) with all products.

ISSN 1527-5108