Method and apparatus for determining the speed and direction of movement of a fluid relative to a body

This invention relates to a method and apparatus for determining the speed and direction of movement of a fluid relative to a body, such as a test head, placed in the fluid.

The test head can itself be at a known orientation to a body of interest thus providing a means of determining the speed and orientation of the flow of the fluid relative to the body.

An example of a body of interest is a boat hull. A wind instrument incorporating this invention would provide the wind speed and direction relative to the boat hull orientation.

Another example of a body of interest would be the earth. A wind instrument orientated to the cardinal directions of the earth and incorporating this invention would provide the wind speed and direction relative to the surface of the earth at the altitude of deployment. This may be used in weather monitoring applications.

Conventional wind instruments achieve these aims by the utilisation of rotating mechanical parts for wind speed, and weather vanes for wind direction. Furthermore, such conventional instruments require electrical components to be mounted within the test head. Such mechanical and electrical arrangements are prone to poor reliability and increased costs.

Wind instruments have a wide range of applications, from marine use to weather stations to wind turbines for electricity generation. In many of these applications, the wind instrument is not easily accessible for maintenance and is expected to operate over a wide range of climatic conditions in continuous operation over long periods of time.

In sailboat applications, the instrument is placed on the mast head and may be required to operate in everything from arctic to tropical conditions with wind speeds from still to hurricane force and in a salt water spray environment. In advanced sailboat applications, the wind instruments may be linked in the control loop to the steering mechanism of the yacht, allowing high performance tracking of the yachts course relative to the wind. Similar conditions can be found in wind powered electricity generation systems, where the instruments are used within the control loop to start or stop rotation and trim rotor pitch.

In most applications the wind instrumentation is provided by a combination of a wind vane and rotating anemometer. The wind vane provides the wind direction relative to a reference orientation, and the anemometer rotates at a rate dependent on the speed of the wind. In both cases, the mechanical systems are fitted with electrical, electromagnetic, or electro-optical transducers to provide the signals to the instrument control and display head.

The continuous operation of the instrument when deployed, places exacting requirements on the mechanical components used. Further, the nature of the environments in which these must operate, means that they must be well sealed against water, salt and dust ingress. Together these factors mean that top quality wind instruments are expensive due to the incorporation of high quality mechanical components with well controlled

manufacture. Even so, they continue to form a significant maintenance problem having lower reliability than other aspects of the systems they lie within.

This invention solves these drawbacks by providing a means for measuring wind speed and direction with no moving parts in the fluid flow. Further, the electrical transducer components of the invention can be mounted some distance from the components in the fluid flow, so a system can be built where all the active parts are easily sealed from environmental hazards and often placed in significantly more accessible places. Another advantage of the present invention is that it operates in a manner such that self diagnostics of performance degradation or faulty components is inherent. This is unlike a rotating mechanical system, where increased friction or bearing wear can give rise to undetected incorrect readings.

According to a first aspect of the present invention there is provided a device for determining the speed and direction of movement of a fluid relative to a body, said device comprising a plurality of pitot tubes or probes, each tube having a longitudinal axis arranged at fixed annular separation in a common plane and aligned with a common central axis, each tube communicating with a respective pressure sensor for determining the pressure in the tube, means being provided for determining the speed and direction of a fluid flow with respect to said body based upon the measurement of pressure from each pressure sensor and predetermined reference characteristics.

Preferably the plurality of tubes are arranged at equal angular separation, each tube extending radially from said common central axis. In a preferred embodiment five equally spaced tubes are provided.

Each tube may be mounted on and extend from a common central shaft or column, said central shaft or column being substantially aligned with said common central axis.

Preferably the pitot tubes or probes extend into the flowing fluid with no shielding to minimize disturbance to the flow of fluid around the device.

Preferably the means for determining the speed and direction of the fluid flow comprises a microelectronic controller.

According to a second aspect of the present invention there is provided a method for determining the speed and direction of movement of a fluid relative to a body, said method comprising the steps of arranging a plurality of pitot tubes or probes within the fluid at fixed locations with respect to the body, each tube having a longitudinal axis arranged in a common plane and aligned with a common central axis, each tube communicating with a respective pressure sensor for determining the pressure in the tube; determining the speed and direction of the fluid with respect to the body based upon the pressure in each tube determined by the pressure sensors and known pressure response characteristics for the tubes.

In one embodiment, the step of determining the speed and direction of the fluid with respect to the body comprises identifying which tube has the greatest pressure reading (referred to hereafter as the reference probe); comparing the pressure readings of the remaining tubes to known reference characteristics to determine the direction of the direction of fluid flow with respect to the reference probe; dividing the pressure measured for each probes by their respective, preferably normalised, pressures for

the determined flow direction to determine a scalar and comparing the determined scalar with known scalers to determine the actual fluid speed with respect to the body.

In an alternative embodiment, the step of determining the speed and direction of the fluid with respect to the body comprises comparing the pressure readings from the tubes with pre-programmed known pressure response characteristics for calibrated wind speeds and directions and determining the wind speed and direction based on the closest match between the measured pressure readings and the pre-programmed sets of pressure response characteristics.

The method may comprise the further step of comparing the relative pressure outputs from all the probes to the expected relative pressures for the known pressure response characteristics to detect component faults and/or failures.

The pitot tube is a well known device for the measurement of wind speed. The flow of air is stopped by a sealed tube pointing into the wind raising the pressure inside the tube. By a solution to Bernouli's equation, the velocity of the wind is shown to be equal to the square root of twice the pressure rise divided by the density of the fluid.

However a pitot tube only follows this equation when pointing straight into the wind. If the wind direction is shifted from straight ahead, the pressure difference in the probe no longer follows the relationship to wind speed as defined by the solution to Bernouli's equation.

In figure 1 , a graph is shown depicting the normalised output of a sealed tube with a square tip profile rotated in a flow of air. (A normalised output

is one where the output at any angle of incidence to the flow has been divided by the difference between the largest output and the smallest output found when the probe is fully rotated in the flow). Normalisation results in the total amplitude of the characteristic being 1.

The probe is depicted schematically in figure 2. The probe 202 is facing into the fluid flow 201 and is supported by a shaft 203. The pressure is measured at point 204, and the whole assembly is rotated in the flow as depicted by the arrow 205.

Referring to the graph of figure 1 , beginning at an angle of 0°, the probe is directly into the wind. This gives a positive pressure normalised to 0.4. As the probe is rotated, the pressure decreases until at approximately 60°, the pressure is 0. Further rotation causes the pressure to become negative, reaching a maximum negative peak, normalised to

-0.6, when the probe is at 90° to the flow. Rotation further towards 180°, where the probe is facing directly away from the flow, causes the pressure to increase again to a pressure slightly below 0.

The graph in figure 1 is therefore the normalised pressure characteristic of a sealed tube rotated in a flowing fluid. On its own, this pressure characteristic could only be used to determine either wind speed at a known angle to the flow, or angle of flow at a known windspeed. However, this invention uses a plurality of probes, engineered such that their position relative to each other is fixed and known.

Further, they are separated such that their characteristics overlap in a unique manner over the wind angle of interest. The resulting system is such that a unique set of simultaneous pressure measurements is only possible for a unique flow speed and flow angle.

In a probe assembly according to the present invention, when the probe pressures are measured, an automated system for resolving this to flow rate and flow angle can be provided. The automated system may use an algorithm to carry out this resolution. Some algorithms may be static, using only the pressure outputs current to determine the flow speed and direction, and some may be dynamic using also the historical flow speed and direction to calculate the current flow speed and direction.

The number of probes selected may be greater or less depending on the nature of the algorithm and the chosen sophistication of the device used for implementing the algorithm.

The algorithm used to calculate wind speed and direction from probe pressure inputs, inherently detects faulty, misconnected or misaligned probes.

An embodiment of the present invention will now be described, by way of example only, with reference to Fig 3.

A shaft 301 is fitted with five pitot tubes or probes 302 arranged in a common plane and aligned with a common central axis with an annular separation of 72°. The probes 302 are connected by hollow pipes 303 to a bank of pressure transducers 304. The pressure transducers change the pressure in the pipes 303, to an electrical signal which is read by the algorithm unit 305. This algorithm unit 305 uses the measured pressure of each probe to calculate the wind speed and direction and sends this to a display output 306.

The shaft 301 and probe assembly is mounted in the flow so the resulting displayed wind speed and direction is relative to the orientation of probes and therefore the mounting system.

The probes 302 are formed from tubes having a 10mm outside diameter with a 2mm wall thickness formed from a plastic material. The tubes are mounted in blind rebates on the shaft 301. Holes at the base of each tube are made for entry of 4mm outside diameter PVC tubes, which are sealed in place by adhesive. The PVC pipes may be of any length up to and including, but not limited to, 10m.

There are many arrangements of probe possible, for example passing the pressure connection pipes through the inside of the shaft, creating dedicated manifolds for pressure pipe interfaces and others. A keyed manifold is particularly useful in ensuring the probes are not misconnected to their pressure transducers.

Each probe is connected to its own pressure transducer. Pressure transduction is carried out by silicon diaphragm devices operating from a 5V supply. Example devices are Sensortechnic's PCLA2X5, which provide a pressure range of +/- 2.5mbar. More than one set of pressure devices may be mounted in parallel to extend the fluid flow rate measured.

Additional scaling analogue electronics are provided to match the output range of the transducers to the input range of an A/D converter, which forms the first stage of the algorithm unit, 305. The algorithm unit may be engineered around a single low cost microcontroller with integrated 12 bit A/D converters, memory and instruction set. An example device is the Microchip dsPIC30F3013.

The output device 306 may be provided by a generic or custom liquid crystal display and driver with interface to the microcontroller. This displays wind speed and wind direction. Alternatively the output may be sent using telemetry to a display or data collection system remote from the algorithm unit.

In operation the probe assembly is mounted as outside plant with one of the probes, nominally numbered 1 from 5 where the other 4 are number 2, 3, 4 and 5 going clockwise from 1 , is aligned with a reference orientation. If the outside plant is a mast top, probe number 1 will be orientated to align with the boat's keel. If the outside plant is a weather station, probe 1 will be aligned with grid North.

The connection pipes, 303 are taken through a hermetically sealed lead- through to the electrical system within. The pressure readings from the probes are presented to the algorithm unit. This has been preprogrammed with the normalised pressure response characteristic of figure 1 replicated and shifted by 72° each time to build up the full 5 probe system. The scalars required to perform the normalisation for each wind speed are also pre-programmed.

The algorithm identifies which of the 5 probes are dominant, then examines the relative position of the other 4 pressure readings relative to the dominant probe. The dominant probe is the probe with the highest pressure in this example. This probe becomes the reference probe. The pre-programmed characteristic is based on a segment of the 360° test head were the segment is centred on a single probe. The first step therefore uses the highest pressure sampled to identify the reference probe which has in the embodiment a reference angle of 0°, if probe 1 , 72°, if probe 2, 144° if probe 3, 216° if probe 4 and 288° if probe 5. From

the known normalised pressure characteristics, the other probes describe a unique pattern of pressures relative to the reference probe for each angle of displacement from the reference probe. While the actual pressure measured in each probe is different than the normalised pressure, the relative pressures of one probe to another is the same for all wind speeds at any given angle relative to the reference probe. Thus the algorithm performs tests on the arithmetic ratios of one measured probe pressure to another and compares these to the arithmetic ratios at specific wind angles from the reference normalised characteristic. This process is repeated iteratively until a match is found. This gives a value of wind angle. The ratio of the measured pressures to the normalised pressures provides a scalar, which is compared with the pre-programmed scalars to provide the wind speed.

The algorithm can also identify faulty and /or failing components by identifying pressure readings from probes which deviate from expected readings compared to the readings from other probes and the known normalised pressure characteristics of the probe assembly, such information being provided to the user via the output device 306.

This basic algorithm may be engineered in the hardware described. This invention may use any algorithm involving fundamentally derived characteristics of probes assemblies, empirically derived characteristics of probe assemblies, or tabularised characteristics of probe assemblies and operate these in dedicated hardware or non specific computing devices such as Windows, Unix or Linux based systems.

The probe depicted in figure 3 has a square tip profile. Alterations of this tip profile can alter the characteristic and this may be used in some

applications to increase the sensitivity, or reduce the number of probes needed.

The present invention may be used as a dedicated flow rate and direction system as described, or as a component of a larger system for either monitoring fluid flow, or controlling the response of systems to fluid flow.