Linear position sensing is a specific and common industrial automation need — with many options for solving application challenges.
By Andrew Waugh, AutomationDirect
Industrial equipment and machinery usually involve a good amount of rotating and actuating parts. Effective automation of these elements calls for sensors to reliably detect the position of actuators and associated mechanisms. This can be quite simple if it is only necessary to detect the presence or absence of hardware at a fixed end-of-travel. Even rotary position, while more difficult, still permits a rotary encoder sensor to be localized at a shaft.
More challenging, however, is the need for detecting linear motion and position over a much longer operating distance or with greater accuracy. For instance, a gantry crane can span great distances and may be installed and operating in harsh outdoor or factory environments but require ±1-in. accuracy. On the other hand, a material handling machine may be more compact and protected but require much more accurate feedback for the location of multi-axis X-Y-Z carriers.
There are several linear position sensing options available, ranging from simple classic technologies all the way up to advanced devices. This article reviews some linear position sensing technologies and discusses where each method is best used for accurate and reliable detection.
Let’s get it straight
Linear positioning is used for an extremely specific, but highly deployed, subset of motion geometries, with multiple design considerations for sensing, Figure 1.
Rotational motion is readily sensed with rotary encoders. Sometimes rotary motion is translated to linear motion using gears or other mechanisms. In either case, it is possible to infer the linear position based on how much rotation has occurred. However, if there is any failure in the mechanism, then the rotary sensor and the linear position lose their relationship and the sensing will not be correct.
Limit or position switches are simple and reliable but can only detect a linear position at the point where they are installed. These sensors may be based on mechanical, optical, proximity, or ultrasonic methods, but only provide a binary position, or perhaps relatively crude resolution. Position switches of this type are usually installed at the end of travel for the mechanism, but it is possible to put a target on the equipment and arrange many position switches along the path of travel. Still, the sensing only occurs at those points, so if the equipment moves away from a switch, its position is indeterminate.
The best true linear position sensing technologies will provide a continuous analog output signal, or a digital signal via Ethernet connectivity, to accurately report the target position in real-time with sufficient resolution.
Following are the leading linear position sensing technologies, with a brief look at the advantages and potential problems of each.
A linear potentiometer is a tried-and-true configuration consisting of a wiper that moves along a resistor as the equipment moves through the full length of stroke, providing a variable resistance related directly to position, Figure 1. Signal conditioners can convert this reading into other electrical output levels. Linear potentiometers are simple, inexpensive, and easy to work with, Figure 2.
However, as a physical device subject to constant mechanical swiping, they wear out over time and need to be replaced. They may not be resistant to liquids and contaminants, and the form factor must be large enough to accommodate the fully extended and retracted rod stroke, which sometimes limits use due to installation space constraints.
Magnetostrictive linear position sensing technologies may look similar to potentiometers, but they employ a magnet travelling along a rigid linear housing. The magnet can be arranged on the surface of the housing or as a ring around it, providing a great deal of mounting flexibility and more importantly avoiding mechanical wear issues. These sensors usually have on-board circuitry to condition the output to various desirable signal levels. Magnetostrictive technology is more costly than potentiometers or linear inductive solutions, but the added benefits often offset the extra cost, especially in challenging applications.
Linear inductive position transmitters, Figure 3, reside in a middle ground between potentiometers and magnetostrictive devices, both technically and commercially. They look like a potentiometer but use inductive technology to monitor the rod stroke of an armature. These non-contact devices with on-board electronics avoid the wear and tear of physical sensors and provide a range of output signals.
Like magnetostrictive, linear variable differential transformer (LVDT) technology also uses an armature to identify variable positions. The armature is free floating, and may be captively connected to the target, or may be spring loaded so they actively push the armature into contact with the moving target at all times. These sensors use a magnet in the armature whose position is electrically referenced to primary and secondary windings in the main housing which it passes through. LVDT’s are similar to linear inductive sensors in many ways, but a key distinction is they can also be designed for AC voltage circuits allowing greater installation flexibility.
Draw wire sensors are actually a rotary device, operated as a cable is mechanically pulled into or out of the device. The cable is attached to the moving equipment, with the draw wire sensor housing in a fixed position. This is somewhat like a tape measure, but with a rotary pot or encoder on the winding device to make the measurement. These are often used for long-distance positioning where a stroke-length device cannot be feasibly installed.
For certain physical configurations, linear position sensing can be accomplished by installing a scale along the length of travel. The linear encoder, which may be optical, magnetic, or even capacitive, is arranged to “see” this tape as it travels by it during motion. Optical units are historically more common and provide a good linear signal by using an enclosed through-beam sensor to read coded gaps using light pulses. Magnetic versions use a coded-magnet scale, which is more durable and can be installed for longer lengths. They also provide better resolution and have greater immunity to negative environmental impacts.
When accuracy is important and the installation environment can be suitably protected, a laser-based linear position sensor often provides the best solution. Laser devices provide excellent resolution and responsive feedback, but only if the line-of-sight remains unobstructed and the target is properly prepared. These devices are completely non-contact, but this technology may not work if the environmental conditions, surface reflections, or even physical interferences obscure the signal.
Many of the preceding position sensor devices come in similar housing styles and form factors, making them easy to work with, replace, and upgrade — with specific solutions selected based on the needs of the application. Cylindrical housings in ½-in. and ¾-in. are common, as are rectangular units.
Practical installation practices for many linear position sensing devices are often quite similar, with a sensor housing mounted to a fixed surface and the shaft portion connected to the moving or travelling object to be measured. Some mounting attachments pivot to provide a degree of freedom of motion, while still tracking linear movement along a single axis. Sensors may be positively attached to their targets or may be spring loaded to track movement while accepting a degree of sliding.
Linear position solutions
Linear position measurement is commonly needed to monitor and control the operation of machinery and equipment. The correct configurations will reliably detect the position of target hardware with sufficient precision for a given environment.
Users can choose from many technologies, ranging from simple to advanced, to obtain the best solution for their applications. When questions arise, suppliers offering a wide range of solutions can be consulted for recommendations and best practices.