Choosing a Sensor
Selecting the optimal linear position sensor comes down to purchase cost and a few key specification parameters.
Linear 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 performed 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.
This 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 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).
Of the four common sensor technologies, the first three are “contactless,” that is, they don’t 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.
To assist in choosing the optimum sensor for an application, here are a number of questions that should be answered first:
Can you define explicitly what you are attempting to accomplish with the position measurement?
While this may seem to be an obvious step, it is often overlooked during a sensor selection process and is critical to developing an overall perspective. Once you answer this question, other questions pertaining to the details of the measurement can be posed.
What is the nature of your sensing application?
Typical answers include automation or control systems, industrial process monitoring without automatic control, product quality assurance and R&D lab testing. Your response will help prioritize the specification parameters of the sensors under consideration. For example, an automatic control system would likely require better dynamic response than a typical quality assurance test, but quality assurance measurements usually require better measurement precision and especially calibration traceability.
In what environment is the sensor expected to operate and how harsh is it?
There are many issues involved in answering this question. One is the range of ambient temperatures in which the sensor will be expected to operate. Another is the level of vibration and/or shock the sensor will experience. A key issue is the degree the sensor will be exposed to moisture, coolant mists, solvent or chemical vapors, dust or grit. For example, in a typical factory floor environment, there can be serious shock impulses transmitted from the structures of punch presses and a large amount of grit that can accumulate from sanding or finishing operations. Is there possible electromagnetic interference (EMI) from arc welders, variable frequency motor drives or 2-way radios? Finally, is the sensor going to be operating in a hazardous location that would require a UL, FM or CSA hazloc-approved component?
What reliability and quality issues apply to the position sensor?
Several important factors pertain to this topic, the most important of which is whether the sensor is being used in a safety-critical application where its failure could result in serious injury to personnel. In such an application, reliability becomes the highest priority in selecting the right position sensor technology.
For less critical uses, the key quality questions relate to the measurement system’s duty cycle and expected service life. Reliability comes into sharp focus when the sensor must operate on a 24/7/365 duty cycle that can cause sensor system operating cycles to accumulate very quickly and wearout to develop in a relatively short period of time.
Another factor pertaining to reliability is the frequency of recalibration required for the position sensor, as well as its preventative maintenance program. Downtime of the position measuring system, whether it is scheduled or unscheduled, typically has a significant cost beyond the direct expenses of recalibration or maintenance. Replacing a worn out or failed sensor also can introduce downtime.
Are there any additional items needed to make the position sensor function?
Typical examples are ancillary electronics like power supplies, signal conditioners and various cables. An often overlooked extra is the need for specific mounting hardware and the labor for installing the sensor. A further requirement could be special software that must be acquired to effectively utilize the sensor.
Is there any special training or a sensor-dependent learning curve for the measuring system operator?
This is usually the most common unanticipated issue in applying a sensor in a measuring system. If this is a potential expense, then it is necessary to consider it in developing a budget for the real installed cost of the position sensor in the measuring system.
What position measurement accuracy and resolution are actually required for your application?
It’s possible for the position sensor in a measuring system to be over-specified in these regards, which can add unnecessary difficulty in making the choice of the optimum sensor from among the many technologies available. The key to this issue is a good understanding of what these parameters mean.
Then you should evaluate the effect of lower accuracy and/or resolution for the position measurement application by doing the necessary calculations. In performing these calculations, pay particular attention to the exact manner in which the specification is presented. Unfortunately, it is possible for a sensor manufacturer to utilize specmanship that favors their product and can lead to misunderstanding unless the parameter is closely scrutinized.
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.
Harold Schaevitz is President of Alliance Sensors Group.