Why an SL Series Sensor is not a “String-Pot”

In the world of hydraulic linear position sensors, the term “string pot” is well known to many engineers.  Also known as a draw wire sensor, string encoder or my personal favorite, a “yo yo pot”, these devices have been around since the 1960’s and were originally used to test the movement of airplane parts during fatigue testing.  Later the simple technology was adopted as a general solution to the measurement of linear displacement. They were considered to be a simple and inexpensive solution for applications like automotive and aerospace test fixtures, factory automation, and industrial machinery.  They also found use in the design of hydraulic cylinders, as a simple way to denote the displacement of the piston in the cylinder.  They remain to this day probably the simplest and cheapest implementation of a linear displacement measurement system.

What does a String Pot look like?

Even modern day string pot designs have the same 4 basic parts that they had 50 years ago.

1. Measuring cable – can be string but modern pots usually use steel.
2. Spool – a constant diameter winding spool critical for calibrating displacement accurately.
3. Spring – keeps the cable responsive during retraction without creating measurement drag.
4. Rotational Sensor – as the name suggests, this is typically a “pot” or variable resister whose impedance is controlled by a wiper attached to the spool. This is a “contacting” technology and susceptible to mechanical fatigue over time. 

On the surface, CPI SL-Series sensors do indeed bear some resemblance to string-pots: both typically use a steel cable wrapped around a spring loaded spool for measurement of displacement as the spool reels and unreels…

But that is basically where the comparison ends…

CPI SL Series LVDT based Linear Position Sensors – Not Your Mother’s “String Pot”

In every area critical to durability and performance, CPI has extended the design of the class of linear transducers known as “Draw Wire Sensors” into something perfectly suited to reliable usage in extreme environments. The results are not only patented, but result in performance and durability parameters that bear little resemblance to their string pot ancestors...

No stringUses a stainless steel braided cable. The type of steel used (302, 304, 316) is selected for the particular application environment. Cable winding and diameter is also carefully selected for the size of the sensor, the cylinder stroke length, and the overall use case.

No pot In our design, the objective sensor is a Linear Variable Differential Transducer (LVDT). Immune to heat, liquid, temperature, RFI, EMI, shock, vibration….and it’s NON-CONTACTING, so it never wears out.  

No Sealed Volumes in the assembly The CPI sensor has no sealed cavities or subassemblies. The device is immune to pressure or liquid environments because it is made entirely from precision-machined metal parts: No seals. No closed air volumes. As such it can be immersed, and mounted on the oil or gas side of the cylinder.

Patented Translating Spool String pots have a fixed spool and cable feed point. This is not optimal. CPI’s Linear Sensors have a patented Translating Spool. The spool moves laterally as it winds or unwinds. This means that the spool can hold more cable, more securely. In a string pot, the cable has to move over itself as it winds up. In our product, the spool moves out of the way of the cable.

Patented Linear-to-Rotary-to-Linear technology String pots work by turning a potentiometer with a cable spool. Many of our clients have abandoned String-Pots because “they don’t sense fast enough”.  Essentially the transducer is directly tied to the rapid rotation of the spool and provides unreliable readings at higher rotational speeds. By contrast, CPI’s sensor works by reducing, via a precision micrometer thread mechanism, the long linear translation of the object to be sensed. The first reduction is to a rotary motion (the spool); then to a short, easily calibrated linear translation (the LVDT). No other linear sensing technology works like this. That’s why we have approximately 20 US and International patents granted to our linear position sensor technology.

Summary of CPI Sensor vs. String Pot

In essence, our sensor does not even compete in any market you would consider using a String Pot sensor in. For large scale powerful hydraulics, or hydraulics deployed in heavy duty vehicles, or cylinders operating underwater, or in highly vibrational and corrosive environments, our SL series position sensor technology is arguably the only high reliability hydraulic cylinder linear position measurement solution in the world.

For more information visit us at www.cpi-nj.com.

But when you talk to us, just don’t call it a string pot….(-:

-Mac Stuhler

This blog was originally published on our website at http://www.cpi-nj.com.