NO20151375A1 - Method for accurately measuring rapid changes in vertical position based on change in measured pressure when the environment has fluctuating background pressure - Google Patents

Description

Method for accurate measurement of rapid changes in vertical position based on change

in measured pressure when the environment has fluctuating background pressure.

Background

The use of barometric pressure sensors, absolute pressure sensors with a measurement range above normal atmospheric pressure is a well-established method for determining vertical position. The method has long been used in various forms of navigation.

Modern microtechnology today gives us sensors that measure absolute, atmospheric pressure with a resolution of around 2 Pa. At normal pressure, this corresponds to an air column of 15 cm. In addition to resolution, long-term stability and temperature stability are important properties in a pressure sensor. For the invention in question, however, it is the resolution that is the most important property.

Pressure sensors have developed a lot in recent years. Improved micromachining technology and the development of analog and digital microelectronics have led to increasingly better and less expensive sensors. Development will continue, but in terms of resolution and sensitivity, it is the fundamental physical processes responsible for much of the noise that limit what is achievable. It does not seem realistic to expect large improvements in these characteristics.

Atmospheric pressure is determined by a number of meteorological factors in addition to the vertical position, and an accurate pressure measurement will not alone give an accurate value of the sensor's vertical position. However, since meteorological conditions vary relatively slowly with time, we can use accurate measurement of atmospheric pressure to determine rapid changes in vertical position. These will be changes with a significantly shorter time scale than the meteorological ones.

When there are changes in pressure being measured, instead of an absolute pressure sensor, a sensitive differential pressure sensor can be used where one port is connected to a chamber that is vented to the surroundings through an element with high flow resistance. A capillary tube is well suited. Together with the volume of the reference chamber, a 1st order dynamic system with time constant r will be formed.

Here vol is the reference volume, Pre-ambient pressure and Rfiuia flow resistance. The use of a ventilated reference chamber in equipment for measuring height change has been described before, among others in [1] and [2]. In these descriptions, it appears that the measurement signals are directly related to height change or rate of change. In [3], a measurement system with the possibility of combining signals from several sensors is described. Beyond this, this description bears little resemblance to our invention. In equipment for measuring the rate of climb for aircraft, the technique of a differential pressure transmitter with a ventilated reference cell has been known and used since long ago. [4], [5], [6], [7]

Our application of a differential pressure transmitter with a ventilated reference cell will need a sensitivity that is significantly higher than that stated in the aforementioned references. The measurement area will only be a few meters with a resolution or accuracy of around 1 cm. There are MEMS differential sensors on the market with a measurement range of +/- 250 Pa, which corresponds to a height change of +/- 19 m at normal atmospheric pressure. The noise voltage from such sensors will typically correspond to a height change of around 1 cm. Many applications will only use a smaller part of the available measurement range, and these measurements will be more accurate if a differential pressure sensor with an even higher sensitivity than the one mentioned can be used.

Invention

When pressure change is measured with an accuracy that corresponds to around 1 cm height change at normal atmospheric pressure, we will quickly discover that the ambient pressure also fluctuates significantly. This applies both indoors and outdoors and it can be due to a number of different conditions. Doors and windows that are opened and closed, pressure from fans and ventilation systems are obvious sources of this type of "pressure noise". Such noise can be more powerful than pressure changes from significant changes in altitude and they will often have a course with the same time scale as the changes in altitude we want to record.

We can remove pressure noise from measurement signals that represent height change, if we use two pressure sensors. If we take the difference between the measured pressures, the pressure noise from the surroundings will be cancelled, and the measurement signal will be a measure of the height change between the two pressure sensors. If we leave one sensor fixed in the room, we will measure the height change of the other, but there may also be situations where both sensors are movable. This is illustrated in figure 1 and figure 2.1 both figures represent the strong plot, 1, the difference signal and the two weaker plots, 2 and 3, are each of the two pressure signals. In Figure 1, sensor 2 is held still and sensor 3 is raised and lowered. We see that significant background fluctuations are completely gone in plot 1, the difference signal. In Figure 2, both sensors are moved on. When starting from the same height, the difference signal will in this case express the height difference between the two pressure sensors.

There can be many situations where you want to detect rapid changes in height in the meter range with centimeter accuracy. We are particularly concerned with the application of the measuring principle in a fall detector. This is a device for detection and notification that the user has fallen and remained lying down without being able to get up again. The user wears the device with a pressure sensor on it. This is connected wirelessly with a stationary part which also contains a pressure sensor. In addition, the stationary part performs the signal processing required to detect a fall based on the measured pressure changes. It also has a connection to a notification system to notify if the user has fallen and remains lying down without getting up again. A rapid increase in pressure that represents known height changes that correspond to falls from different positions will be detected as a fall if the pressure does not decrease as a sign that the user stands up again. Over the past 15 years, a lot of work has been done to develop a simple and well-functioning fall detector. So far, all are based on using the motion sensor as the primary sensor for fall detection. We are not aware of any descriptions where a pressure change sensor is used as the primary sensor for fall detection. The principle of basing fall detection on the difference in pressure change between two sensors is also not described.

A fall detector can also be realized by the user wearing both sensors. Then they must be placed on parts of the body where the height changes that accompany a fall are significantly different. This can be on the neck and on the calf, but other locations can also be appropriate. It is also not obvious that only two pressure change sensors should be used. By using more, we can count on increased sensitivity and fewer errors. Here there will be a trade-off between function and complexity.

When using two or more sensors attached to the user, ambiguity will arise in the difference signals, since these express changes in height between the two sensors. If one falls from a standing position and lies on the floor or if one lies down in a bed, the difference will be the same. In order to distinguish between different events that produce the same change in the difference signal, we must analyze the correspondence or correlation between the difference and each of the pressure change signals. As the noise component is the same in both, the difference in correlation between the difference signal and each of the measured signals will express how much of the change is written from each of the measured signals.

In a practical realization of the invention, there must be communication between the sensors and between the sensors and a central control module. Wireless communication will normally be practical, but electrical connection will also be able to be used between sensors worn in different parts of the body.

A fall detector that uses two pressure change sensors carried by the user will not be linked to a reference that is fixed in a room. It will be able to be used freely outdoors as well as indoors. It may therefore seem like a better principle than where the reference is fixed. The last mentioned system, on the other hand, has the possibility of indoor position tracking in that the system must be connected to the nearest fixed reference at all times. In addition, the ambiguity that was described above will be eliminated, and the device carried by the user will be simpler and lighter. Both systems thus have strengths and weaknesses.

Figure 3 shows a fall detector based on a movable and a fixed sensor. The user wears a sensor, s, and this communicates with a fixed unit that contains the sensor, s, processing, p, and communication part, c. Figure 4 shows a fall detector based on several moving sensors. Here, the user wears two sensors. In the figure, these communicate with a fixed part for processing, p, and communication, c. This unit can be permanently mounted as shown in the figure, but it can also be carried by the user. The system then becomes completely mobile and can be used both indoors and outdoors.

Exploitation of the invention

Use of the invention for measuring height change by means of pressure change in environments with fluctuating background pressure can be used in different measurement systems. We have seen how it can be used in a fall detector. The same principle can be used for positioning in connection with storage and transport. It can also record height changes related to games, training and other physical movement. This coincides with applications discussed in [1] and [2]. Our invention will have significant advantages if accuracy in the centimeter range is required. This will not be possible to achieve without the use of a reference to cancel fluctuations in the background pressure.

Claims (5)

1. Procedure for using pressure change measurement to accurately determine the height change of an object that is moved in a room where the background pressure fluctuates, by instead of basing the measurement on the pressure change from a simple sensor attached to the object, the measurement is based on the difference between the signals from two pressure change sensors where one is attached to the object and the other is permanently mounted in the room where the measurements are carried out. 2. Method for using pressure change measurement to accurately determine the difference in height change between two objects that are both moved in a room where the background pressure fluctuates, which follows claim 1 in that the fixed sensor is also made movable and is attached to one of the objects. 3. Procedure for registering that a person, who is staying in a room, falls, which complies with claim 1, in that a change in the body's vertical position, which characterizes a fall, is produced by the difference between the pressure change measured with a sensor attached to the person's body and the pressure change measured with a fixed sensor. 4. Procedure for recording that a person, who is indoors or outdoors, falls, which complies with claim 2, in that a change in the vertical position of two or more body parts, which characterizes a fall, is produced by the difference between the pressure change measured with a sensor attached to each of the relevant body parts. 5. Method for removing ambiguity in fall detection, which follows claim 4, in that the correlation between each of the sensor signals and the difference signal is calculated, in this way to determine to what extent each of the sensor signals contributes to the difference signal. References [1] WO 2007062377 A2, Enhanced hang timer for console simulation. [2] US 20120083705 Al, Activity Monitoring Systems and Methods of Operating Same. [3] US 20090150029 Al, Capacitive integrated mems multi-sensor [4] US 2142338 A, Rate of climb indicator. [5] US 3451265 A, Fast response apparatus for measuring rate of change of pressure. [6] US 4890103 A, Rate-of-climb indicator. [7] US 2275719 A, Rate of climb indicator.