Precision Tape Magnetization Leads to Precision Position Measurement
The key to ultimate accuracy for any magnetic linear encoder system is the precision of the magnetic encoding on the tape (sometimes called a scale). Sensors inside the encoder read head respond to the strength and position of the magnetic flux coming from the magnetic poles encoded onto the tape. Precise placement of these poles – and just as importantly, the precise shape of these poles – is critical to the ultimate level of accuracy that can be delivered by the encoder system. Any inaccuracy in the position, strength, or shape of these fields will directly influence the accuracy of the encoder’s indicated position. This effect is amplified with increasing gap distance between the tape and the encoder read head. The further away from the tape, the weaker and more indistinct the shape and position of the magnetic poles becomes.
Not All Magnetic Tapes Are Created Equal
Many magnetic encoder tapes on the market are surface magnetized utilizing the conventional parallel magnetization process. This is a straightforward technique that results in an encoder tape that meets performance specifications at close gap settings between the read head and the tape.
A more recent tape magnetization process called Permagnet® produces magnetic poles with improved control over their strength, shape, and location on the tape. The best way to appreciate the advantages of this technology is to compare magnetic scans of some conventionally magnetized tapes to some examples of tapes encoded with the Permagnet® process. Note the visible difference in sharp definition of the magnetic poles that is produced by the newer technology.
Conventionally Magnetized Tape – Sample #1
Tape Magnetization with Permagnet® Technology – Sample #2
The stronger, more sharply-defined magnetic poles produced by Permagnet® technology enables encoders to be more tolerant of variation in the working distance between the encoder read head and the tape. Reduced dispersion and distortion of the magnetic fields at any distance within the specified working range reduces the influence of distance variation on the accuracy of the position measurement in real-world applications.
Summary of Application Benefits
Improved linearity at close working distances for ultimate system accuracy
Improved linearity at longer working distances
Higher tolerance to deviations in the working distance, with reduced non-linearity
Less need to closely control the working distance in the application, saving cost by reducing painstaking setup and alignment effort
Full system accuracy, even if gap distance varies during operation
Better linearity for any given pole spacing on the tape
In a previous installment here on SENSORTECH, we explored the three M’s of linear position feedback application (Linear Feedback Sensors – The Three M’s). One of those three M’s stands for Motion Control. When we talk about motion control applications for industrial linear position sensors, we’re often referring to closed-loop servo-hydraulics. In these applications, the linear position sensor, which is usually installed into a hydraulic cylinder, plays a key role in the ability to accurately and reliably control the motion of very large, heavy loads.
Nowhere is closed-loop servo hydraulics more prominently utilized than in primary wood processing – where raw logs are transformed into all manner of finished board lumber. Applications such as saws, edgers, planers, along with many more, rely heavily on closed-loop servo-hydraulics. In many cases, hydraulic actuators get the job done when other types -electric, pneumatic – simply can’t.
If you’d like to get a look at some of these application, or to learn more about how linear positions sensors are used in the applications, a good place to start would be at an event where many of the machinery builders and suppliers gather in one place for a few days. Does such an event exist? (I hear you asking).
Well of course it does! It just so happens this very thing will be taking place in Portland, OR in the middle of October 2014. If you would like to learn more about these interesting applications in general, and how linear position sensors are used in particular, you might want visit Balluff at the Timber Processing and Energy Expo. Click the link below for more information.
Much has been written here on SensorTech about the value of industrial networking in the machine automation realm. As the trend towards industrial networking continues to expand, we see more and more network-capable sensors coming to the fore. Linear position sensors are no exception.
Network-connected linear position sensors take the concept of continuous, absolute linear position feedback a step or two forward by allowing the position sensor to be directly connected to the network, and also providing additional information in the form of sensor-level diagnostics.
Available in two varieties, one for basic position monitoring, and one capable of closed-loop positioning tasks, the Micropulse EtherCAT transducer is a good example of the continuing evolution of basic sensors towards more “intelligent” network-capable sensors.
For more information on industrial networking products, start here.
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.
A question came in recently concerning the maximum recommended cable length for analog sensors. Even as digital interfaces gain popularity, sensors with analog interfaces (0-10V, 4-20 mA, etc.) still represent the overwhelming majority of continuous position sensors used in industrial applications.
The question about maximum cable length for analog sensors comes up pretty frequently. Generally speaking, the issue is that electrical conductors, even good ones, have some resistance to the flow of current (signals). If the resistance of the conductor (the cable) gets high enough, the sensor’s signal can be degraded to the point where accuracy suffers, or even to the point where it becomes unusable. Unfortunately, there is no hard and fast answer to the question. Variables such as wire gauge, whether or not the cable is shielded, where and how the cable is routed, what other types of devices are nearby, and other factors come into play, and need to be considered. A discussion about all of these variables could fill a book, but we can make some general recommendations:
I recently had a conversation with a customer that resulted in one of those forehead-slapping “duh” moments for me, and I thought it might be worth passing along. Here’s the story:
The customer had an application that required an analog linear feedback sensor that provided an output of 1 volt to 5 volts over the linear stroke range. Now, a 1-5V output is not very common, and the particular sensor he was interested in was only available with either a 0-10V or a 4-20 mA output. What to do? Perhaps the answer should have been obvious to me, but it was the customer who provided the solution this time: “couldn’t I use a 4-20 mA output and 250 ohm resistor to get my 1-5V output?” Why, yes….yes you could (smack…..duh!). And I know it will work, because we have the law on our side. Ohm’s Law, that is: E = IR, or voltage equals current x resistance.
Let’s check it:
4 (mA) x 250 (ohms) = 1 (volt)
20 (mA) x 250 (ohms) = 5 (volts)
So there you have it. Take a very common 4-20 mA output and drop it across a 250 ohm resistor and, lo and behold, you have your less common 1-5V signal. And, if you do this conversion right at the input to the controller, you get the added benefit of increased noise immunity of the 4-20 mA signal.
And, yes, I’m sure I knew of this little trick at one time. Maybe the part of my brain where this information was stored got overwritten by the names of the contestants on The Amazing Race or by the rollout plans for my million dollar consumer product idea: Dehydrated Water (just add water). But let’s keep that just between us, ok?
When we talk to people about applications for continuous linear position sensors, we often point out the advantages that can be realized by “upgrading” a machine and/or a process by incorporating continuous position feedback. In this post, I’d like to offer up a case in point. This “Application Spotlight” showcases the real and tangible advantages that can be realized by using continuous linear position sensors, such as:
• Improving machine/process efficiency
• Reducing set-up and changeover time
• Reducing planned downtime
• Error-proofing the process
So, you see, we’re not just making this stuff up! Download this case study here.
As previously discussed, the world of linear position sensors is pretty diverse. There are many types of linear sensors available in many different form factors, employing many different technologies, and coming in at many different price points. For the sake of discussion, let’s imagine you’re shopping for a linear position sensor, and you’ve decidedon a form factor. You’ve settled on a position sensor that will be externally mounted on your machine. And you don’t really care much about the “under the hood” technology; you just care that the sensor does what it’s supposed to do when it’s installed. Now, let’s further assume that you find a couple of different sensors that you think will do the job, and the only difference is the cost. It makes sense to choose the lowest cost option, right? Well, maybe not.
Hysteresis, resolution, repeatability, non-linearity, null-point, temperature coefficient, accuracy. These are but a handful of the many terms associated with linear position sensors. To the uninitiated, it can be rather daunting. And, unfortunately, there is a lot of room for ambiguity and confusion.
For example, let’s take a look at the term “accuracy”, as in “how accurate is this this linear position sensor?” It seems like a fairly straightforward
question, right? But in reality, it’s not that simple. Whenever I get asked that question, my response is “what do you mean by accuracy?” To which, I usually get a response something like “what do you mean what do I mean by accuracy?” The fact is that the term “accuracy” means different things to different people. The person asking the question may want to know the absolute straight-line, absolute positional accuracy (non-linearity) of the sensor. Or, they may be referring to how accurately the sensor can repeat the same indicated value at the same position over subsequent moves (repeatability). Or, perhaps what they’re really interested in is the smallest amount of position change that the sensor can detect (resolution). So, as you can see, it’s not a simple question after all.
In numerous types of analog position sensors, resolution is expressed in terms of bits, e.g, 8-bit, 12-bit, 16-bit, etc. But what does that really mean? In a previous entry, I discussed what I called Digitally Derived Analog Signals, which provides a basic overview of how Digital-to-Analog Converters (DAC’s) are used to generate analog sensor signals. You may recall from that entry that when someone says a sensor has “16-bit resolution”, what they really mean is that the sensor employs a 16-bit DAC, which is capable of processing 216 discrete values, and representing any one of those values as a corresponding analog signal.
To help better understand what these binary numbers actually mean, I thought it might be helpful to provide a quick-reference chart showing the equivalent decimal values of numbers from 20 to 232.
(click to enlarge)The values in bold represent some of the more commonly used DAC’s for industrial sensors.