How to Choose Among Draw Wire, LVDT, RVDT, Potentiometer, Optical Encoder, Ultrasonic, Magnetostrictive, and Other Technologies
Note: Thank you to Circuit Cellar INK magazine for allowing us to print an abridged version of an article from their May 1998 issue. For more information on Circuit Cellar INK magazine, please visit their Web site (http://www.circuitcellar.com) or e-mail them at email@example.com.
As an application development manager for a position transducer supplier, I get numerous queries on how to solve a broad range of position measurement challenges. These inquiries run the gamut from the common (aircraft flight control surface movement) to the exotic (Formula One racecar suspension travel) to the seemingly impossible (three-dimensional tracking of a golf ball in flight from a fixed position).
These position-measurement challenges usually share one common element. They can be solved using a variety of solutions, but it's not always easy to determine the best one.
There are possibly more options for measuring position than any other type of sensed variable. While there may be more suppliers for pressure transducers, the variety of position-transducer types and technologies is unmatched.
The 1997 Thomas Register lists 264 suppliers of pressure transducers and 229 suppliers of displacement and position transducers. However, there are 13 categories related to displacement and position measurement, compared to just four categories for pressure measurement.
In this article, I introduce you to various position transducer selection parameters. You'll also find information on position-measurement techniques, technologies, and choices.
A brief note on semantics: for ease of communication, this guide refers to position transducers and sensors as being the same. While that is not strictly true, it is generally irrelevant whether you are using a position transducer or a position sensor. The purpose of both is the same - to find out where something is!
Transducers that are the focus of this article provide position, displacement, and proximity measurements, which are defined as :
- position - location of the object's coordinates with respect to a selected reference
- displacement - movement from one position to another for a specific distance or angle
- proximity - a critical distance signaled by an on/off output
In this article, I focus primarily on transducers for position and displacement measurement. Unless otherwise noted, I use the term "position transducer" to refer to displacement and proximity transducers as well.
On what basis should you select a position transducer? As a starting point, let's look at the laundry list of parameters shown in Figure 1. While this list is not all-inclusive, it helps you begin to decide what parameters are relevant to your application.
|What are your requirements? This table helps you rank your most important parameters and value specifications. Select relevant parameters, prioritize them, and then choose the appropriate value for the parameters.
Perhaps the first parameter to address in any application is whether the transducer can physically touch the object being monitored. If your application is sensitive to outside influences, a noncontact transducer may be the most appropriate. Otherwise, a contact sensor might offer advantages not found in a noncontact sensor.
At first thought, noncontact transducers may seem like the superior solution for all applications. However, the decision isn't that clear cut.
Noncontact products can emit potentially harmful laser- or ultrasonic-based signals. These products also rely on having a clear visual environment to operate in. Frequency response isn't always as high as with a contact sensor, but costs are often higher. Finally, operating-temperature ranges are typically not as broad.
Another parameter to consider early on is whether you need to measure linear or rotary movement. Note that using cable position transducers (like the one shown in Photo 1), cams, pulleys, levers, electronics, software, and other methods can enable a rotary transducer to measure linear motion, and vice versa. Lack of space, cost, and ease of mounting are a few reasons for doing this.
Once you decide if you require a contact or noncontact solution and are measuring rotary or linear movement, selecting a transducer technology becomes much easier.
Next, determine if you're monitoring one-dimensional or multidimensional motion. If the motion is multidimensional, see if you need to measure in multiple dimensions or if the object is moving in multiple dimensions and you only have to measure one of them. Often, multidimensional motion is measured with multiple one-dimensional transducers.
Also, think about the type of signal you need to obtain. If you need a signal that specifies a unique position, specify a transducer with absolute output.
However, if all you need is relative position from a prior position or a simple on/off indicator, then incremental or threshold technology is more appropriate.
An important difference between incremental and absolute transducers is that incremental transducers typically need to be reinitialized after powerdown by moving the monitored object to a home position at powerup. This limitation is unacceptable in some applications.
Threshold measurements are on/off in nature and usually involve limit switches or similar devices. As you might guess, absolute devices are usually more expensive than incremental or threshold devices.
Travel, also known as range, varies from microns to hundreds of feet (or more, depending on your definition of "transducer"). The range of many precision transducers is limited to 10 inches or less.
If your application needs to operate on the Space Station or some other size- and weight-sensitive platform, you need to specify the maximum values for the transducer's dimensions and weight.
The application's operating environment can have a large impact on your technology choice as well. You need to determine what operating and storage temperatures the device will be in and whether you need to meet commercial, industrial, or military environmental requirements.
Also consider whether excessive humidity, moisture, shock, vibration, or EMF will be encountered. See if your environment has other unique aspects, such as high or low pressure or the presence of hazardous or corrosive chemicals.
An often-overlooked parameter is the method and time required for transducer installation and mounting. For testing applications, this parameter may not be so important. However, OEM and large-volume applications often require simple installation and removal to reduce labor costs and enable easy maintenance.
See if the transducer can only be mounted with manufacturer-provided special mounting bases or if a variety of mounting techniques can be used. Besides the common threaded-fastener approach, some other nonpermanent mounting techniques include suction cups, magnets, industrial adhesives, grooved fittings, and clamping.
In going through the previous parameters, you might have asked yourself, "Hey, what about accuracy?" While accuracy is certainly important and sometimes critical, it's often the last degree of freedom in the selection of a transducer.
As you may know from experience, accuracy is not a well-agreed-on term. Typically, various components of accuracy - linearity, repeatability, resolution, and hysteresis - are quoted for vendor convenience or per user requirements.
With the availability of software calibration tools today, linearity isn't as important as it once was. For many applications, in fact, repeatability is the most important component.
Accuracy is typically specified in absolute units like mils or microns or in relative units such as percent of full-scale measurement. If you are comparing the accuracy of one device against another, ensure you are comparing apples to apples.
For example, see if the accuracies being quoted are at a single temperature or over a temperature range. If you need it, find out if temperature compensation is available.
If you expect to see significant numbers of cycles or if the transducer will be in service for an extended period of time, specify the lifetime and reliability requirements as well. When choosing the transducer, find out what warranties are offered as well as how maintenance and repairs are handled.
A transducer that can be repaired in-house can reduce costs significantly. You should also consider what type of periodic recalibration is recommended and whether calibration procedures are provided.
It's a good idea to ask vendors what type of use their transducers see most often. Common uses include OEM, retrofit, industrial control, commercial, and test and measurement. Hopefully, the transducer has seen previous use in your type of application.
In the early stages of transducer specification, product cost sometimes doesn't even make the list. More often than not, this parameter gains importance as the project moves forward.
When determining costs, make sure to look at the initial acquisition cost as well as the cost over the product's life. For example, are special signal-conditioning electronics, power supplies, electrical connectors, housings, installation tools, or mounting fixtures required?
Ask the vendor for typical repair, maintenance, and replacement costs. And, inquire about the cost of the transducer in volume and single-unit quantities. The cost savings (e.g., a cost of $100 in volume but $600 in single quantities) may be an important factor if small-quantity replacement units will be needed in the future.
Another parameter that's occasionally overlooked is the time it takes the product to be delivered to you after you order it. The custom nature of some transducers combined with production processes and manufacturing economics requires lead times of eight weeks or more.
This delivery schedule might be acceptable now, but what about in six months when you need extra quantities or a spare part? Evaluate whether or not you can afford to be without a part for an extended period of time.
Obviously, the transducer is going to be a part of a system. So, determine your preferred electrical input and output requirements. Common output choices include analog AC and DC voltage, resistive, current (4-20 mA), digital, and visual (meter).
Increasingly, outputs using sensor bus protocols are being offered. Most position transducers require 50 V or less, and some are self-powered.
Finally, for fast-moving applications, determine the maximum velocity or acceleration that needs to be monitored. Ensure that your data-acquisition or control system has an adequate sampling rate to record the resulting datastream.
|Cable position transducers provide extended ranges in small sizes. Flexible cable allows for easy installation.
Check Your Requirements
Now that you're aware of the key parameters, you need to determine which ones are relevant to your application and of these relevant parameters, which are most critical.
If you don't prioritize your requirements, it's going to be difficult to make a selection decision. You may come to the conclusion that there is no transducer that can meet your needs. This may be true, but it's more likely that your requirements are too stringent and that you need to make a tradeoff to arrive at the optimum selection.
For example, an engineer recently approached our company looking for a transducer with ±0.0001-inch resolution over 30 inches, and he wanted to keep the cost under $500. He was adamant that all three specifications be met. Our products didn't meet all of his specifications and we were at a loss as to where we would refer him.
After some more discussion, we found out that the resolution requirement was only necessary over a limited portion of the total range and that the cost goal, while important, did have some flexibility. Hence, in this situation, range was most important, followed by resolution, and then cost.
The moral of this story: focus on your top requirements. Make the best decision you can, given the specifications you need. And keep in mind, you can't have everything - unfortunately.
In this article, we discussed some parameters for selecting position transducers. But we didn't discuss what type of technology you should select for your position transducer.
The constant change in transducer technology and the difficulty in generalizing about a particular technology's capabilities and limitations do not allow for us to cover this area in detail here.
Additionally, choosing the technology should come after determining and prioritizing your requirements. Once your requirements are well-known, the choice of technology tends to be self-selecting.
For example, just knowing whether you require a contact or noncontact technology can cut your choices almost in half. If you need the latter, a laser position sensor may be a good choice.
To get a feel for the capabilities of some of the more prevalent linear position-measurement technologies, Figure 2 maps out how these technologies compare against each other based on cost, accuracy, and maximum range. Note that not all technologies are shown.
|It's true: you can't have it all. As with many specification decisions, tradeoffs must be made when you're selecting a position transducer. This graph shows the typical performance of some linear position transducers as compared by maximum range, best accuracy, and cost.
It may be difficult to clearly define the parameter values you require as well as which parameters are most important in your application. However, it can be even more difficult to obtain these parameters from vendors and then compare one vendor's statements against another's.
To get information on products beyond what you see in the vendor's product literature, review transducer- and instrumentation-related publications for articles on position-measurement products and technologies.
Ask your colleagues about their experiences and recommendations. Information on their failures and successes can be quite valuable.
Search Web engines and Internet newsgroups. Numerous engineering, instrumentation, and measurement-oriented newsgroups can be reached via search engines. Extensive sources of position-transducer manufacturers can be found in a number of online directories, databases, and guide.
Contact vendors and request references of similar applications. Ask these references why they selected the product they did and whether they're happy with their decision. Find out what other options they considered.
Finally, see if the vendor has product samples or evaluation units you can use for testing before purchase. If the vendor is hesitant to do this, offer to provide them with a test report summarizing your evaluation. This information may be valuable to them, and they may be more willing to assist you.
 J. Fraden, AIP Handbook of Modern Sensors, American Institute of Physics, New York, NY, p. 264, 1993, 1996.
I. Busch-Vishniac, Electromechanical Sensors and Actuators, Springer-Verlag, New York, NY, 1998.
Schaevitz Engineering, Handbook of Measurement and Control, Pennsauken, NJ, 1976.
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