We’ve posted numerous articles here on the Sensortech blog about linear position sensors used for applications such as hydraulic cylinder position feedback, plastic injection molding machinery, tire manufacturing machinery, etc. What all of the applications have in common is that we’re generally talking about fairly long linear travels, usually longer than 12″, sometimes up to 300″.
But in applications such as spindle clamp positioning on machine tools or positioning of
linear movements on automated assembly machinery, travels are sometimes only a couple of inches, and the available space to mount a position sensor is extremely limited. Fortunately, there are highly capable linear position sensors that are perfectly suited for such applications.
For example, there are sensors that use an array of inductive coils to detect the
precise linear position of a simple metal target. These sensors, with working strokes ranging from < 1″ up to around 5″ have are extremely compact, with very little dead zone. That means they fit into very tight spaces, where other type of linear position sensors simply couldn’t.
Typically, these types of sensors provide a position signal in the form of an analog voltage (0-10V) or current (4-20 mA). Increasingly though, IO-Link interfaces are gaining in popularity, offering simplified wiring, better noise immunity, built-in diagnostics, and the ability to easily get the position data into virtually any industrial field-bus architecture.
When maintenance technicians replace linear position sensors (also known as probes or wands) from hydraulic cylinders, it can leave a terrible mess, waste hydraulic oils, and expose the individual to harmful hot fluids. Also, the change out process can expose the hydraulic system to unwanted contaminants. After the sensor replacement has been completed, there can also be more work yet to do during the outage such as replacing fluids and air-bleeding cylinders.
Hydraulic linear position sensors with field-replaceable electronics/sensing elements eliminate these concerns. Such sensors, so-called Rapid Replacement Module (RRM) sensors, allow the “guts” of the sensor to be replaced, while the stainless steel pressure tube remains in the cylinder. The hydraulic seal is never compromised. That means that during the replacement process there is no danger of oil spillage and no need for environmental containment procedures. There is also no need to bleed air from the hydraulic system and no danger of dirt or wood debris entering the open hydraulic port. Finally, there is no danger of repair personnel getting burned by hot oil.
The RRM is an option for Balluff’s BTL7 Z/B Rod Series used in applications for the lumber industry, plastic injection and blow molding, tire and rubber manufacturing, stamping presses, die casting, and all types of automated machinery where a continuous, absolute position signal is required. Applications in industries such as Oil & Gas and Process Control are especially critical when it comes to downtime. For these applications, this Rapid Replacement Module capability is especially advantageous.
Peter Nachtwey of Delta Computer Systems has written an excellent primer on electro-hydraulic motion control. In addition to many design and component selection tips, he highlights the benefits of magnetostrictive linear displacement transducers (MLDT) for position feedback to the controller. Check out the article in the July edition of Design World online, called “A Second Look at Electro-Hydraulic Motion Control Systems.”