The Miniature, Rugged Alternative to LVDTs and Linear Potentiometers
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Application Note for Ground Vehicles/Transportation
This Application Note was developed to assist engineers and technicians with the process of selecting, installing, and using SpaceAge Control miniature and subminiature position transducers for ground vehicle and transportation-related applications. It details why, where, and how SpaceAge Control position transducers are used in automotive and other vehicle environments.
Product Technology Summary
SpaceAge Control position transducers are cable-actuated displacement-sensing devices. The majority of ground vehicle and transportation applications use the analog electrical output provided by precision potentiometers, but encoders, synchros, and resolvers are also available.
Figure 1.0 shows how a position transducer works. Operationally, the products work by mounting in a fixed position and attaching the displacement cable to a moving object such as a suspension component or vehicle control component. As movement occurs, the cable extracts and retracts.
An internal power spring maintains cable tension. The threaded cable drum rotates a precision potentiometer, encoder, servo, or synchro that produces an electrical output proportional to the cable travel. Depending on the mounting arrangement and signal conditioning, the electrical output can indicate any number of lengths, motions, angles, or rates.
Figure 1.0: How a Position Transducer Works
Known also as string pots, yo yo pots, cable-displacement transducers, and draw wire transducers, SpaceAge Control position transducers have been used in a broad range of vehicle-related programs since they were first developed in 1968 to support NASA flight test programs. Since that time, these products have been used in projects ranging from truck and car development to crash impact research and from solar-powered vehicles to Formula 1 racing. Please contact us for a representative list of our ground vehicle and transportation customers and programs.
Before discussing where and how to use SpaceAge Control position transducers, let's look at what benefits these products bring to position measurement challenges and why you might want to consider the products for your application.
Small Size and Light Weight
The realities of physics mandate that size and weight be considered in nearly every engineering design. This is especially true in the ground vehicle and transportation industries. Because the significant aspect of the cable-actuated position transducer is a stainless steel cable, SpaceAge Control products have an inherent size and weight advantage compared to other choices.
Rod-and-cylinder products such as linear potentiometers and LVDTs require room to store the rod and cylinder when the product is in its fully retracted state. For example, a typical rod-and-cylinder solution requires 42 inches (1.07 m) of operating space to measure 20 inches (0.51 m). A SpaceAge Control product can measure the same distance using less than 22 inches (0.56 m). When fully retracted, the rod-and-cylinder product takes up over 20 inches (0.51 m) of linear space. The SpaceAge Control product takes up less than 2 inches (0.05 m). See Figure 2.0.
Figure 2.0: Comparison - Rod-and-Cylinder vs Cable Position Transducer
Correspondingly, a rod-and-cylinder design will weigh significantly more than the SpaceAge Control product. A representative comparison for products with a 20-inch (0.51-m) range show rod-and-cylinder designs weighing 22 to 32 oz. (0.62 to 0.91 kg) while the SpaceAge Control product weighs around 3 oz (0.09 kg).
While a non-contact sensor may seem to provide slight size and weight advantages over a cable position transducer, often that advantage is outweighed once the non-contact sensor electronics and signal conditioning are taken into account.
The flexible cable combined with innovative mounting bases and mechanisms enable these position transducers to be mounted in accordance with your application requirements. There is no need to re-design your project or product. This flexibility is particularly useful when measuring suspension components that simultaneously rotate and extend or when monitoring crash test objects that are moving unpredictably in three dimensions. It is also useful when tight spaces provide no room for the transducer. Cable pulleys can be used to re-route the displacement cable through conduit and other channels, letting the transducer be placed remotely from the measurement location.
Ease of Installation
Quick-connect displacement cable connectors and varied mounting arrangements make the products easy to install and remove. This saves time in setup as well as reduces time spent at the design station and in the machine shop creating specialized installation fixtures.
SpaceAge Control products use threaded cable drums and precision potentiometers (providing infinite resolution), high-resolution encoders, servos, or synchros. SpaceAge Control's threaded cable drum design ensures the cable is laid in the same place each cycle. Non-threaded drums allow the cable to wind over itself in a haphazard fashion. This reduces repeatability.
These SpaceAge Control design features combined with precision-machined aluminum parts and assembly by skilled technicians result in unparalleled repeatability and resolution. Accuracy for a 30-inch (0.76 m) Series 161 unit has been shown to be better than ±0.025% of full-scale output.
Safety and Damage
A final benefit is the safety and reduced damage factor that exists with these products relative to other choices available. The predominant alternatives to these types of transducers are:
The safety benefit of a cable-actuated position transducer over a rod-and-cylinder device is that a cable-actuated position transducer has no inflexible components that have the potential to bind, bend, or otherwise disrupt the free movement of the object being monitored. Rod-and-cylinder products need to be precisely aligned and mounted to ensure long-life and disruption-free operation. Mounting errors, non-linear application movement, and simple accidents can cause the rod-and-cylinder device to malfunction and restrict the movement of the application. If the application is a throttle, brake, or other critical part, this restriction of movement can have serious consequences.
The inherent flexibility of a cable-actuated position transducer eliminates any binding or bending problems. Should a mounting or product malfunction occur, the displacement cable connection assembly can be designed to have a minimum breaking strength of less than 10 lbs. (44.48 N).
While non-contact transducers do not present a restriction of movement hazard, they do present difficulties in ensuring data integrity due to the dirty environments ground vehicles and transportation systems are exposed to.
Leaking hydraulic and lubricating fluids, smoke, mist, and other visual and atmospheric elements reduce or eliminate the effectiveness of non-contact technologies. The result is data that cannot be relied upon and possible safety consequences if a control or feedback loop is involved.
Using high-quality precision sensing elements and anodized machined aluminum with ball-bearing-mounted shafts, SpaceAge Control position transducers will often outlast the vehicle itself. When used properly, some models are rated to exceed 50 million cycles of operation.
Due to the sensitive nature of many ground vehicle and transportation development projects, we frequently never know how our products are used. Nevertheless, for over 40 years, we have found the most frequent uses of our products to be those shown in Figure 3.0.
Figure 3.0: Position Transducer Usage Areas
Next are a few recent ground vehicle and transportation applications of SpaceAge Control position transducers with notes on product usage. Contact us if you would like more details on these applications.
NASCAR Racing: Rear Spoiler Angle
SpaceAge Control Series 160 position transducers are used to measure the rear spoiler deflection during highspeed NASCAR race vehicle test runs. Any spoiler deflection results in less downforce and a higher riding car with a slower top speed. By quantifying the amount of spoiler deflection occurring, race engineers are able to stiffen the spoiler to reduce deflection to an acceptable level. Other SpaceAge Control products are used to measure a variety of Formula 1, rally, Indy, and NASCAR race vehicle components including suspension, weight jacker, brake, throttle, and steering wheel parts.
Anthropomorphic Dummy Chest Ribcage Displacement
Eighteen subminiature SpaceAge Control products were used in an 18-month vehicle lifecycle test. Vehicle doors, hatches, and compartment access devices were instrumented to determine usage patterns. The test was completed successfully without a single position transducer malfunction.
Truck Suspension Monitoring
Series 160 and 161units are used to obtain performance data at several vehicle proving ground locations. Tests incorporate high shock and vibration inputs as well as rapid sinusoidal cycling. Position transducers are mounted both from axle to body and from wheel to body.
Hydraulic Actuation Verification
Product Usage Notes
Which Series To Use
In general, the product size, displacement, and resolution will determine which SpaceAge Control position transducer is best suited to your application. An exception to this guideline is when increased cable tension is required for very-high-frequency response applications. Due to product sizes, Series 150, 17X, and 180 products are not available with ultra-high cable tension and therefore ultra-high cable tension requirements must be met with Series 16XH products.
Position Transducer Mounts
Mounting of SpaceAge Control products is simple using a choice of mounting bases and designed-in mounting flexibility. Rotation ability of most bases allows precise alignment of the product after it has been mounted.
Series 6, 160, 161, 162, 161H, and 161H
Series 150 through 180
Other Mounting Methods
Magnetic Base - In some environments, quick and clean mounting and dismounting can be accomplished by using magnetic bases.
Double-Sided Tape - Double-sided tape provides a relatively secure and fast method of mounting the position transducers.
Industrial Adhesives - Adhesives can be directly applied to the rugged anodized aluminum cases and the application. This is particularly useful in a fast-paced test environment.
Custom Mounts - Custom mounts and configurations can be developed if your application has unique requirements.
Displacement Cable Attachment
Displacement Cable Construction - The standard displacement cable used is of 7 x 7 stainless steel construction with a minimum breaking strength of 40 lbs. (178 N). It has a diameter of 0.018-inch (0.457 mm). A larger 0.027-inch (0.686-mm) diameter cable with 90-lb. (400 N) minimum breaking strength is available for applications using ultra-high cable tension (greater than 56 oz. (15.57 N)). This type of cable is used frequently in crash test applications.
Synchros, Servos, and Encoders - Historically, most automotive applications specify precision potentiometers as the sensing element. Note that standard and custom encoders, synchros, and servos may also be specified.
Custom Designs - If your application has unique requirements not met by our standard products, please contact us. We have extensive experience modifying existing products and developing new products to meet demanding requirements. Some examples include high-frequency-response displacement transducers for vehicle vibration measurement, low-profile units for racing suspensions, and custom signal conditioning for crash test instrumentation.
If you have any questions not answered by this Application Note or would like to discuss your application, please contact us by phone, fax, or e-mail.
A representative list of ground vehicle and transportation applications where SpaceAge Control position transducers have been used: