I recently ran across an interesting article that explored some of the factors involved in selecting hydraulic cylinders. The article, entitled “3 Steps to Choosing the Right Hydraulic Cylinder” was very informative and helpful. But what if you need a “smart cylinder”, i.e. a cylinder that can provide absolute position feedback? Just as it’s important to select the proper cylinder to match the mechanical requirements of your application, it’s also important to select the right sensor to meet the electrical requirements.
So, to that end, I’d like to piggyback on the cylinder selection article with this one, which will look at 3 steps to choosing the right in-cylinder position sensor. In particular, I’ll be talking about rod-type magnetostrictive linear position sensors that are designed to be installed into industrial hydraulic cylinders to provide absolute position feedback.
Before we get to step 1, let’s talk about the cylinder itself. So-called smart cylinders are typically prepped by the cylinder manufacturer to accept a magnetostrictive position transducer. Prepping consists of gun drilling the cylinder rod, machining a port on the endcap, and installing a magnet on the face of the piston. For more information about smart cylinders, consult with your cylinder supplier.
Step 1 – Choose the Required Stroke Length
The stroke length of the position sensor usually matches the stroke length of the cylinder. When specifying a position sensor, you usually call out the working electrical stroke. Although the overall physical length of the sensor is going to be longer than the working electrical stroke, this is usually not a concern because the cylinder manufacturer accounts for this added length when prepping the cylinder.
Step 2 – Choose the Sensor’s Electrical Output
There are quite a few possible choices when it comes to the electrical signal that the sensor provides as position feedback. But the good news is that simple analog interfaces are, overwhelmingly, the most commonly used. Analog 0-10 Vdc or 4-20 mA interfaces probably make up 70-80% of all in-cylinder feedback in use. Of course, other interfaces are available (Synchronous Serial Interface, digital pulse interfaces, fieldbus interfaces). But these are less commonly used. And, really, the choice of the electrical interface is probably going to be dictated by an already-in-place control architecture. One of big advantages of magnetostrictive sensors is that they are available with electrical outputs to meet the needs of nearly any control system.
Step 3 – Choose the Physical Sensor Design
In-cylinder magnetostrictive position sensors are available in a variety of housing styles. Which one you choose really depends on the nature of the application. Standard sensors, where the electronics are housing in an anodized aluminum enclosure, are the most common. These sensors are suitable for most typical industrial applications. For more demanding applications, such as those where the sensor may be at risk of physical damage, sensors with compact, rugged, all stainless steel housings are available. There are even sensors capable of withstanding high-pressure washdown applications.
Taking these three basic steps into consideration should go a long way towards making the process easy and straightforward.