CPI has developed a robust sensor for sensing hydraulic piston position. Stationary Applications include Aircraft production and Flight simulators. Mobile Applications include Heavy Trucks, Fire Trucks and Skid-steer loaders.
A bullet list of the basic design requirements are listed below:
The Sensor...
- must fit completely inside the hydraulic cylinder, bores as small as 3"
- must fit ordinary, high aspect-ratio, unitized cylinders as found on today's mobile equipment
- must not require core drilling of piston rod
- must be an absolute transducer
- must utilize reliable, non-contacting technologies
- must be robust to shock, vibration, EMI, temperature, pressure, and submersion
- must be less costly than current (magnetostrictive) technologies
CPI's SL Series Linear Position Transducers meet all of these requirements.
Principle of Operation
The sensors use a linear variable differential transducer (LVDT) as their core transduction means. The LVDT is ideal for hydraulic cylinder applications for a number of reasons: It is non-contacting, submersible, and immune to pressure and temperature. LVDTs, as a class, are a mature technology, well known in the industry to be highly accurate, reliable, and robust.
The SL Series utilizes a precision mechanism that couples the relatively long translation of a hydraulic cylinder to the relatively short translation of the LVDT. This highly integrated mechanism features an LVDT that is coupled to a micrometer-like threaded element. This LVDT/micrometer assembly actually forms the axis of a recoil spool mechanism (fig. A). The spool utilizes a flexible polymer coated stainless steel lead to form a reliable, repeatable coupling between piston and sensor. The lead is drawn off the recoil reel as the piston moves away from the sensor, and vice-versa. The entire unit is about the size of a large proximity sensor. In hydraulic cylinders, it typically mounts on the inside face of the cylinder end-plug, in place of the hydraulic cushion, for example (fig. B).
| Fig. A | Fig. B | |
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Features
Ease of installation and ruggedness are just two of the main attributes of the SL Series, achieved by several key design features.
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Snap-In-Place
Mounting Flange Made of a durable elastomer, the mounting flange snaps onto the prepared piston for ease of installation. By snapping the SL onto the piston, the sensor and piston can be installed into the cylinder as a single unit. For small ID cylinders, the SL cannot be installed this way. Please see our Installation Details section for further information on using the SL in these cylinders. |
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Flexible
Connection A nylon insulated stainless steel cable translates the movement of the piston to the LVDT mechanism. Because the cable is flexible, it can be wound around a spool to save space. A non-binding bolt is attached to the cable which in turn threads into the piston rod. |
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Self-Guiding
Bolt Mounts Two bolts located 180 degrees from each other hold the sensor in place. Generous lead-ins allow for an approximate lineup of the bolt holes during installation. Once the sensor is roughly in place within the cylinder, the lead-ins guide the sensor into proper position firmly against the back of the end-cap and in alignment with the high-pressure connector. |
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3/4
Round Shape The unique shape of the SL Transducer allows for passage of fluid if a fluid port happens to be located even with the transducer. |
Applications / Markets
| Hydraulic cylinders: | SL Series sensors are easily installed into virtually any hydraulic cylinder. Applications for automating production equipment continue to grow, and now mobile equipment manufactures can deploy advanced motion control. | |
| Accumulators: | With sensing lengths to 80", the SL Series sensors can provide the status (potential) of piston accummulators. | |
| Pneumatic cylinders: | The SL Series is a cost-effective approach to creating "smart" pneumatic cylinders. Stroke lengths to 80" are possible, and one sensor can be used for any stroke length - no standard stroke constraints. With no restriction on bore size, it can be easily mounted within a compact housing to a wide variety of actuators, including rodless-type, as well as standard round cylinders. |
