Providing a means of positional feedback for hydraulic cylinders has been a long-standing problem for engineers, especially if the cylinder is long stroke or if the operating conditions are hazardous or if the cylinder is multi stage.
Common practice uses either external or internal devices.
External devices such as LVDTs, cable extensometers and laser distance sensors are prone to damage, contamination and spurious readings caused by foreign bodies or other external influences.
Internal devices are pressure capable LVDTs mounted inside the cylinder. As such, they are protected from external hazards.
They have excellent technical performance (linearity +/- 0.2% of stroke with infinite resolution) and typically output as 0.5 / 4.5Vdc or 4 / 20mA.
However, in order to fit them into a cylinder, the rod must be hollow, which adds cost and sometimes involves deep hole boring, usually for most of the rods length. They are only generally available up to 2.0m stroke and at this length they tend to be quite costly and the deep hole boring operation becomes more difficult.
Having been faced with these issues many times we decided to come up with a system which satisfies the requirements of most applications at an acceptable cost.
The solution uses two main components, a short stroke, inexpensive internal LVDT and a device we call a draw wire converter (DWC). The DWC is an internally fitted high precision draw wire device, which accurately converts the stroke of the cylinder into a much shorter stroke, this short stroke is then monitored with the much shorter internal LVDT.
For example, the application shown here is a 250mm bore cylinder with a 3.0 metre stroke which has a DWC fitted internally, it is monitored with a 100mm stroke LVDT. This size device adds 180mm to the length of the cylinder.
The DWC comes in two versions, side entry LVDT (shown here) and end entry LVDT through the end cap. The current range covers cylinder strokes up to 5.0 metres and cylinder bores from 50mm.
The range has been developed using Penny & Giles ICT range of LVDTs.
We’ve chosen Penny & Giles as they are a market leader in this technology and their equipment is highly regarded as being robust and reliable. However, adapting for a different manufacturer is straightforward.
- All components are internal
- Protected from external hazards
- Range goes beyond 2.0 metres
- Cost effective
- Can be used in multi stage cylinders
- No need for hollow rod
- Uses established LVDT manufacturer
- Increases overall length of cylinder
- Piston has to be modified to include cable anchor
- More complex assembly procedure
- Minimal reduction in accuracy compared to direct reading LVDT