True Cost of Ownership for Commonly Used Linear Position Sensors Part 1, 2, and 3Part 1Linear position measurement is a frequent requirement for many industrial automation or process control systems, QA functions, and R&D testing. The actual position measurement function is normally done by some type of position sensor, the selection of which quite often is based on just a few key specification parameters and its purchase cost. However, using the sensor’s purchase cost for economic guidance can be very misleading. The real cost of utilizing that sensor must be considered based on installation costs, operational costs over the period of time it is in service, maintenance and recalibration costs, operator training, and possible replacement cost. When taken together, these costs, along with several other expenses which will be identified later in this article, add up to the True Cost of Ownership or TCO. This series of articles will help you to evaluate the TCO of commonly used linear position sensors. The first article identifies the most commonly utilized industrial position sensing technologies and poses a set of questions to ask yourself about various position sensor application issues to assist you to make an optimum choice of sensor from both a technical and an economic perspective. The second article starts by examining the strengths and weaknesses of these technologies, presents a comparison chart of their specification parameters with a brief explanation of what those specifications mean, and helps to focus on the technical aspects involved in TCO evaluation. The third article pulls together the answers about costs derived from the answers to the queries in the initial article, combines that information with the choices made as a result of the examination of sensor technologies and the weighting applied to the specifications parameters in the second article, and shows how these factors combine into the TCO. The most commonly utilized industrial position sensing technologies incorporate a moving element which is attached to the object or workpiece whose position is being measured, while the body of the sensor is fixed in position. Typical of these are Linear Variable Differential Transformers (LVDTs), Linear Variable Inductance Transducers (LVITs), Magnetostrictive Linear Displacement Transducers), (MLDTs, also known as LDTs), and Linear Resistance Potentiometers (POTs). And while these sensors all operate on different physical principles, they are all analog output sensors. There is another style of linear position sensor that has a digital output, glass scale encoders, but their use is largely limited to the machine tool and CMM markets primarily because they are not considered robust enough for the majority of industrial automation or mobile hydraulics applications. Some position sensing technologies function without requiring any physical contact with the object whose position is being measured, and are often called “non-contacting” position sensors. Typical examples are proximity, ultrasonic, laser, radar, optical, and similar sensors. However, these types of sensors all have drawbacks that can limit their use, such as specific target materials required for proximity, environmental effects on ultrasonic, high initial cost as well as the eye protection requirement for lasers, etc. For these reasons, non-contacting position sensing technologies will be excluded from the focus of this article. Of the four common sensor technologies, the first three are “contactless”, that is, they do not require any physical or electrical contact between their moving element and their fixed part. Pots, however, do have a wiper that contacts their internal resistance element that is fundamental to their operation, so they are not contactless. The foregoing explanation should illustrate the difference in the meaning of “contactless” and “non-contacting” as applied to position sensors. All four sensor technologies will be described in detail in the second article of this series. But before comparing and contrasting the details of these common industrial position sensing technologies, there are a number of questions you should answer to assist you in choosing the optimum sensor for your application from among them. Can you define explicitly what are you attempting to accomplish with the position measurement? What is the nature of your sensing application? In what environment is the sensor expected to operate and how harsh is it? What reliability and quality issues apply to the position sensor? Are there any additional items needed to make the position sensor function? Is there any special training or a sensor- dependent learning curve for the measuring system operator? What position measurement accuracy and resolution are actually required for your application? The foregoing questions assist in the selection of the correct sensor so that a prospective position sensor user or specifier will gain insight into various factors. These factors go into the choice of a position sensor for a measurement application and can establish a weighted priority list of specification parameters for the optimum sensor choice that uses the information derived from the answers or responses to these questions. The next article in this series continues with a more detailed technical explanation of the strengths and weaknesses of the four technologies cited above, accompanied by a comparison chart of the significant specifications and parameters, along with a brief explanation of the meaning of these specifications. Then the third article delves into all the cost factors that make up the true installed cost of a position sensor and how you can assimilate the results into a TCO of the position measuring system.
Part 2To help you evaluate the True Cost of Ownership (TCO) of commonly used industrial linear position sensors, our first article identified the four most commonly utilized industrial position-sensing technologies. It also posed questions to ask yourself about various position-sensor application issues to assist you in making an optimum choice of sensor from a technical and an economic perspective. Our second article examines the strengths and weaknesses of these technologies, presents a comparison chart of their specification parameters together with a brief explanation of what those specifications mean, and helps you to focus on the technical aspects of a TCO evaluation. The most commonly utilized industrial position sensing technologies are linear variable differential transformers (LVDTs), linear variable inductance transducers (LVITs), magnetostrictive linear displacement transducers (MLDTs, also known as LDTs), and linear resistance potentiometers (POTS).
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