Products Hydraulics & pneumatics
PC pump prevents Tower Bridge flooding A NOV Mono Compact C Range
progressive cavity (PC) pump is used in each of London’s Tower Bridge’s legs to transfer ground water to the surface 100ft above, from where it is transferred to the main drainage system. Without the pump, the towers would fill up with water and flood up to river level. The pump uses positive displacement for process control, variable speed for accurate dosing and a suction lift capability of over 8m. NOV Mono
www.mono-pumps.com Enter 212
Measuring the displacement of hydraulic cylinders A new approach to measuring
the displacement of hydraulic cylinders has been jointly devel- oped by Micro-Epsilon and Sensor-Technik Wiedemann. The solution is based on a draw-wire sensor, which is inte- grated inside the cylinder at the bottom, where it is protected from external influences. The measuring wire is attached to the bottom of the piston. One chal- lenge here was the design of the sensor and the signal routing to the outside, as a bore hole in the cylinder housing is always a weak point. Pressures are up to 600bar in the cylinder and leak- age must be prevented.
Each movement of the piston causes a rotation of the wire drum, which is divided using a gearbox onto two shafts with different rotational speeds. A magnet is positioned on each shaft at the bottom of the cylin- der, whose positions can be measured by external, magnetic angle sensors. Using a suitable gearbox, each combination of the magnet position only occurs once across the complete
measuring range. The sensor therefore shows the characteris- tics of an absolute encoder. Due to the high pressures, the cylinder walls must be constructed from relatively thick metal, but a magnetic signal transmission through these is not suitable for measurement require- ments. However, a solution was found for this without having to weaken the cylinder design. At the points where the two gear shafts reach the bottom of the cylinder, the steel is tapered and a special membrane is welded on. Using this, a magnetic signal can be transmitted with sufficient quality. Using FEM calculations, the minimum possible wall thick- ness was determined.
The electronics on the outside were designed to be extremely flat and can easily be attached to the bottom of the cylinder. Due to the draw-wire sensor, the system can easily be adapted to different cylinder lengths and diameters and to a range of operating conditions. According to Micro-Epsilon, its wireSENSOR WDS-TZ10 is
suitable for cylinders that have a lifting height between 0.5m and 15m. Cylinders with this type of displacement measurement are ideally suited to harsh environ- ments and in mobile construction machinery and off highway vehicles, explains the company. For short cylinder strokes, the TZ10 can be used with the magnetostrictive method. Micro-Epsilon T: 0151 355 6070
Enter 213
www.micro-epsilon.co.uk
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