Draw Wire Sensors Revisited

string pot or draw wire sensor operation

In ancient times when cave men were trying to measure the linear displacement of things they cleverly came up with something called a “String Pot”.  Using only stone knives and bearskins, they were able to cobble together a spool with some “string” and attach it to the object whose position was to be measured. Then with a simple potentiometer (resister) attached to a wiper, they could measure variable resistance (voltage) as an indication of position.

But as man began to walk upright, the drawbacks of string pots became apparent:  the strings broke; the spools were unreliable, they had limited accuracy, and of course the potentiometers wore out because of the contacting nature of the technology. But they were cheap to make and worked well-enough to be a tried and true technology for decades, especially in labs where environmental conditions were not a factor. It was straightforward mechanical technology. Easy to understand, easy to deploy. It required less complex electronics and signal conditioning.

What is A Draw Wire Sensor?

It turns out that String pots are really a type of draw wire sensor. This class of sensor is marked by its use of spooled wire or cable of some kind that is attached to a moving object whose linear position is to be measured.

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….

  1. No string
    • Uses 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.
  2. 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.
  1. 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.
  1. 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.
  1. 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

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 of any technology in the world.

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