The Pitfalls of Draw Wire Sensors
Overcome by CPI SL Series Sensor Systems

Draw wire sensors, as a product class, have been around for a long time.  Core sensing technologies have ranged from rotary potentiometers to optical encoders, and product configurations span the miniature range (max range of 100mm) through large industrial sensors (10M and beyond).  Given the complexities of integrating rod-type sensors into long stroke cylinders, and worse yet, piston accumulators, there has been strong motivation to incorporate a draw wire sensor into these hydraulic systems – not to mention, the rod-type sensors will not work with a telescoping cylinder. A number of manufacturers have created draw wire sensor systems for use in hydraulic cylinders and accumulators with varying degrees of success. CPI has been developing draw wire sensors specifically for use in the high pressure hydraulic environment for over 20 years.   Following are problems/concerns encountered with draw wire sensors in hydraulic systems, and the corresponding design solutions offered by CPI SL Series sensors:

1. The draw wire spool shaft must pass through a dynamic seal which separates the sensor electronics from the high-pressure environment. The dynamic seal is cumbersome, and ultimately fails.  >>> There are no dynamic seals in the CPI’s sensor system. The only seal is a standard (static) SAE port through which the magnetostrictive probe passes.
2. Sensor dither under very high cycle life will ultimately create dead zones in the sensor. >>> Since CPI’s sensor system relies on non-contacting technology, the life is virtually infinite.
3. Draw wire sensors are not robust enough to endure the high pressure environment. >>> CPI’s SL Series sensors are completely vented.  The pressure rating of the magnetostrictive probe is typically 350 bar static, 690 bar peak: there is no practical pressure limitation with respect to the draw wire mechanism.
4. If there is a sensor failure, it is not accessible for servicing. >>> The sensor probe and signal conditioner are completely accessible from outside the cylinder. The draw wire core is designed to operate well beyond the lifespan of hydraulic systems.
5. ATEX/IECeX ratings are required. >>> Since CPI SL Series sensors leverage the highly developed capabilities of magnetostrictive technology, virtually all intrinsic safety and other ratings, including SIL2 and sensor redundancy are available.
6. The cable will break due to high acceleration. >>> This is a valid concern with all draw wire sensors, particularly in piston accumulators.  Proper care must be taken when moving the piston with shop air, for example.  CPI SL Series sensors incorporate the most robust stainless steel cables in the industry.   In addition, highly developed shock absorbing cable connectors are available to meet application requirements.
7. The draw wire core (recoil mechanism) will not stand up to long-life requirements. >>> The high-endurance failure mode for draw wire sensors is often the recoil or power spring. Power springs for all SL Series sensors are over specified such that no stress thresholds are encountered, even during full-stroke cycles.
8. Draw wire sensors are susceptible to airborne contaminates, i.e. salt air, oil vapor. >>> This is a valid concern, particularly with potentiometer and encoder-based units.  However, SL Series sensors are completely immune to contaminates, as all electronics are completely isolated from the atmosphere.  In fact, SL Series sensors operate submerged in water (even sea water), and can be configured for subsea use.