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the turbine blade. As with the gear-type flow meter, a transducer is mounted above the turbine and generates a pulse each time a blade passes under it.

The frequency from the transducer is proportional to the flow over a limited range.

For example, a 25 mm (1 in) turbine flow meter might typically have an accuracy of

±1% of full scale without linearisation. The same 25 mm (1 in) turbine flow meter when used with a look-up linearisation table will operate over a wider range, with an accuracy of 1% of the indicated reading. A turndown ratio of 30:1 is common.

The use of a turbine instead of gears means that the meter requires less energy to operate and has a very low pressure drop, 3 bar (44 psi) at 400 L/min (106 gsm) or lower, depending on the bore size.

The disadvantage of this type of meter is that it is quite susceptible to changes in viscosity. Thus, it is usually used for fluids with a viscosity under 100 cSt.

Other meter types

Oval gear meters are similar to conventional gear meters but use two elliptical gears that rotate together at 90º to one another inside a housing. The fluid is swept around the chamber by the gears, and the frequency of rotation is directly related to the volume of fluid passing through the meter. This type of meter generally works best with higher-viscosity fluids, but has a lower pressure drop than an equivalent conventional gear-type meter. The teeth on the gears tend to be very fine, resulting in the flow meter being more susceptible to fluid contamination than other meter types.

In theory, non-intrusive meters, such as ultrasonic meters, are very attractive in that they do not require ‘breaking into the system’ and have virtually no pressure drop. However, they are not yet widely used in hydraulics for two reasons. First, hydraulic pipe sizes are quite small, often less than 25 mm (1 in), making transit times very short and difficult to measure. Second, hydraulic connections often consist of flexible hoses manufactured from two or more materials, making it difficult to transmit signals through them. As a result, non-intrusive meters tend to be used more for permanent installations on industrial hydraulic systems using rigid pipes with larger diameters.

change in the resistance can, therefore, provide an output proportional to the force applied. Temperature will also affect the resistance, but this is normally compensated for in the construction of the load cell.

Piezo-electric devices use a material that produces a small electric charge when a force is applied to it, which again can be amplified and used to provide an electrical signal proportional to the applied force. Compared with strain gauges, piezo-electric devices often have a wider useful working range, which can be useful when a load cell has to operate under high shock loads but also needs to be accurate when measuring light loads.

The correct selection and installation of load cells is important to ensure that they cannot be overloaded and that the force being measured is transmitted correctly through the load cell. Overloading, even momentarily, can cause mechanical or electrical damage to the load cell, as can high induced voltages caused, for example, by nearby welding.

Torque transducers

Torque transducers are basically load cells designed to sense rotary force (torque) rather than linear force. Again, the most common design used with hydraulically powered machinery is the strain-gauge type, although other constructions are used in specialist applications. An added complication with torque transducers, however, is the fact that the component at which torque needs to be sensed (such as a drive shaft or drill bit) will be rotating, sometimes at high speed. This then requires some form of contact or non-contact device to transmit the signal from the rotating sensor to the non-rotating signal conditioning equipment.

Position sensors

Position sensors are also available to measure either linear or rotary position. Many different types are in use, depending on the accuracy required, the distance over which position is to be measured, the working environment, etc. The simplest type is probably the electrical potentiometer, in which a wiper is moved along a coiled wire or carbon track. Applying a voltage difference to the ends of the potentiometer means that the voltage sensed at the wiper is proportional to the wiper position. As with all mechanical contact devices, however, wear will eventually cause deterioration of the sensor performance, so for most applications it is preferable to use non-contact devices, which include the following:

• Linear variable differential transformers (LVDTs) move an iron core through electrical coils fed with an AC voltage in order to vary the inductance of one coil relative to another (Fig. 9.10). The change in inductance can then be sensed and converted to a positional signal. They have no contacting components and are suitable for measuring relatively small movements, such as the few millimetres of movement of a proportional valve spool up to several centimetres of movement of a cylinder.

• Magnetostrictive sensors use a magnet attached to the moving component (Fig. 9.11). The magnet moves along the outside of a waveguide tube and

an electrical pulse is transmitted down the waveguide. The magnetic field of the electrical pulse interacts with the magnetic field of the magnet, causing a torsion strain pulse in the waveguide. By sensing the time taken for the pulse to travel out and back, the position of the magnet, and hence the position of the moving component, can be determined. Such devices can be built into hydraulic cylinders, where they are protected from damage and form a very compact arrangement.

Speed sensors

Speed can often be derived electronically from a position signal by relating the change in the position signal to time. Tachogenerators have been used for many years to measure rotary speed (rpm). These devices produce an output voltage proportional to their driven speed and a polarity that is dependent on their drive direction.

A more compact arrangement for use with hydraulic pumps and motors is a pulse- type sensor (Fig.  9.12), which senses grooves or notches on the rotating shaft and generates a series of electronic pulses (a similar arrangement to that used in

INTERROGATION PULSE

RETURN TORSION PULSE

PERMANENT MAGNET

WAVEGUIDE PROTECTIVE TUBE SENSOR

Fig. 9.11 Magnetostrictive position sensor in a hydraulic cylinder

MOVEMENT INPUT

VOLTAGE

OUTPUT VOLTAGE IRON CORE

PRIMARY COIL SECONDARY

COIL SECONDARY

COIL

Fig. 9.10 Principle of working of LVDTs

gear-type flow meters). The frequency of the pulses indicates the shaft speed, and dual-output sensors also determine the rotational direction.

Where speed sensors are not permanently installed and where a rotating shaft is safely accessible, either contact or non-contact hand-held tachometers (Fig. 9.13) can be used to provide an output-speed reading.

Fig. 9.13 Hand-held tachometers (Photo courtesy of Checkline Europe Ltd)

FAST SPEED SLOW SPEED SENSOR

Fig. 9.12 Pulse-type speed sensor (Image courtesy of Eaton Corp.)

Vibration sensors

Measuring machine or component vibration can be a very good means of detecting early symptoms of a breakdown, in particular for components such as shaft bearings.

Vibration sensors are normally permanently installed (Fig. 9.14) so that vibration can be measured over a period of time and any abnormal trend can be acted upon before complete failure occurs.

The software that accompanies vibration sensors can now identify individual bearings by their distinctive ‘signature’. A vibration sensor mounted on a hydraulic pump, for example, is able to identify whether a front or rear bearing is approaching failure or whether some other problem is causing an abnormal operation of the pump.

CONTACT NON-CONTACT

Hand-held vibration sensors are also available for routine checking of components, but care is needed to ensure that measurements are taken in exactly the same way on each occasion.

Particle counters

Off-line particle counters have been used for many years to determine the contamination level of a fluid sample taken from a system. They typically use either an infra-red or laser light beam to cast ‘shadows’ of dirt particles onto a light- sensitive sensor to both count and size the particles passing through in a known volume of fluid.

More recently, particle counters have been developed that can be permanently installed in a system (online) to provide a readout of the contamination level in real time (Fig.  9.15). Where a system has a serious problem and contamination

Fig. 9.15 Online particle counter (Image courtesy of MP Filtri Ltd) Fig. 9.14 Portable and permanently installed vibration sensors

PORTABLE PERMANENTLY INSTALLED

is increasing rapidly, this type of sensor can warn of the issue immediately, thus providing a significant advantage over off-line sensors.

Again, being able to trend the data (recording how it changes over a period of time) will give advance warning of when wear problems are increasing and thus action needs to be taken.

Contamination analysers

More sophisticated analysers can be installed in systems to measure such things as water content and to distinguish between certain types of wear particles (e.g.

ferrous or non-ferrous).