Whether using a piston or a bladder, hydraulic accumulators are all about precharge pressure. When sizing an accumulator, specifying the correct volume and precharge pressure can mean the difference between a system that runs smoothly and one that doesn’t.
Monitoring the precharge pressure of a bladder accumulator is fairly straightforward: install a pressure transducer to detect the pressure of the precharge gas in the bladder.
But piston accumulators pose a different scenario. Because piston seals are dynamic, they are subject to wear and eventually will leak. This can cause the piston to drift, so even though a pressure transducer may indicate the correct pressure, the piston may not be where it should based on the pressure. If the piston is too close to the end cap, then a sudden increase in system pressure could cause the piston to slam into the end cap. For this reason, designers often specify an accumulator with a longer stroke than necessary to provide extra length in case the piston has drifted.
To address these challenges, Control Products introduced a line of linear position sensors that detect the absolute position of pistons in hydraulic accumulators. The SL series sensors use a stainless steel cable that enters the accumulator though an SAE size 8 fluid port and attaches to a 7/16-20 hole machined into the piston. The cable is connected to an LVDT that provides a continuous and absolute analog or digital output signal indicative of the piston location. The use of the LVDT means it is a completely non-contact sensing technology. The LVDT can be encapsulated and sealed such that it can operate on either the gas or oil side of the piston. The complete solution can be tailored to operate at the high pressure requirements of most hydraulic accumulators.
The SL series sensor is available in three standard sizes:
• the SL0390 accommodates strokes to 1 m;
• the SL1200 measures strokes to 3 m;
• the SL1490 handles strokes to 7m.
Custom sizes for long stroke or telescoping cylinders are available. Call CPI today to discuss your application at (973) 887-9400.
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