DE4417827C2 - Method and sensor for presence detection of people - Google Patents

Description

The invention relates to a method and a sensor for detecting the presence of people a location, especially on vehicle seats, on steering wheels or handles Drawbar steered vehicles, with at least one to a voltage source connected measuring electrode an electric field in the vicinity of the Location is generated and changes in the capacity of the measuring electrode (s) be measured, and a corresponding presence sensor.

The detection of the presence of people in a vehicle can prevent this appropriate accident protection, the control of an airbag or a belt warning indicator and serve others more. For example, according to an EC directive, everyone has to move forklift truck equipped with a safety device that the ord presence of a driver is detected. In the absence of the driver is the Switch the forklift immediately without power. For drawbar steered vehicles presence detection can be accident-preventing if the vehicle is moving moved towards the operating personnel without the respective person being able to avoid them possesses in order to get to safety. In this case the drawbar must steered the vehicle to be stopped automatically when its handles were released.

A possibility of presence detection realized in forklift trucks is thus provided called seat switch, which with a suspension mechanism in the seat of the vehicle are installed. The switch is closed by the weight of the driver and opened by relieving the seat, thereby delaying the travel current of the Forklift is interrupted. The disadvantage of such seat switches is that that they are the weight of the driver again with changing drivers must be fitted, and that there are false triggers due to unevenness of the Lane is coming. When driving over such bumps, vibrations occur Forklift trucks that are transferred to the driver, whose effective weight this fluctuates greatly over time. Likewise, when braking hard or at night the driver lean forward to decrease the weight acting on the seat switch, which opens the switch and interrupts the forklift traction current. Such false trips can affect the driver or the load being transported impact directly.  

DE 43 01 000 A1 discloses a capacitive detector system with several electrodes beard, which are integrated for example in a vehicle seat for personal detection. Here, a sensor electrode is opposite a ground electrode to form a Capacitor CA attached. The capacitance of this capacitor changes to Ab dependence on the occupant and the covering of the vehicle seat. By a further center electrode, which is attached between the two electrodes mentioned, two further capacitors CB and CC are formed. The potential of Center electrode is set so that the capacitance of the capacitors CB and CC cancel each other out. The signal taken from the capacitor CA is then independent of the upholstery of the vehicle seat, making changes to this Dielectric not be included. The detector system according to this document thus only prevents changes in the environmental conditions, insofar as they do Dielectric concern that between sensor electrode and ground electrode is to be eliminated from the measurement signal used for personal detection.

The object of the present invention is a method for the detection of the perso presence with a corresponding presence sensor, as at the beginning mentions to develop, regardless of environmental influences, such as Road conditions, temperature fluctuations, dirt, as well as regardless of different body weights of the operating personnel and largely without Manipulation possibilities the presence of people, especially in or on Vehicles, is reliably recognized.

This object is achieved by the features of claim 1. On The presence sensor according to the invention has the features of claim 9.

Such a presence sensor for personal detection fulfills all the requirements mentioned changes and can be easily realized economically. With a forklift for example, said presence sensor housed in the seat, d. H. it be at least one under the seat cover or in the plastic foam of the seat Measuring electrode and outside of the immediate vicinity of the person at least inserted another reference electrode, each generating an electric field and then the capacities assigned to the measuring and reference electrodes continuously measured and monitored by means of an evaluation circuit. It is from the Assuming that about 75% of human tissue is water, which has a high relative permittivity of about 80. Compare for comparison the values of the dielectric constant, for example of glass only 10, of paper approx. 5,  of rubber about 2.6 and of air and other gases about 1. The fact that the relative dielectric constant of human tissue that of most others Substances exceeding one to two orders of magnitude are used to detect the Presence of people used in the present case.

Relative to a certain reference potential, for example the grounded vehicle housing or a second measuring electrode, which has in the vicinity of the stay Halteses attached measuring electrode has a capacity that is essentially by the Given the geometry of the electrode arrangement and the relative dielectric constant is. Since the capacity of an electrode arrangement is directly proportional to the relative Dielek is a tricity number, in the ideal case theoretically the capacity should be at least change at least one order of magnitude if the space between measuring and reference elec trode (completely) is filled by the person to be detected. Because the detection person usually does not cover the entire space between the measuring and reference electrodes occupies and only a certain area of the generated by the measuring electrode (s) influenced electrical field, the changes in capacity are smaller, however still clearly measurable (factor 2 to over 10).

This occurs when a person enters the area surrounding his whereabouts witnessed change in capacity of the electrode arrangement is inventively by means of an evaluation circuit and can be used, for example, in forklifts serve, in the event of the absence of a driver, the vehicle automatically without power to switch. In this case, certain serve beyond a threshold value to be set going capacity changes to determine the presence or absence of the vehicle rers.

In order to make the presence sensors independent of environmental influences According to the invention, at least one further in the vicinity of the location Reference electrode attached, which is connected to a voltage source, wherein then the measuring and reference electrode (s) to a relative change in capacitance measuring evaluation circuit are connected.

The reference electrode (s) is (are) attached so that their capacity is determined by the application absence or absence of a person at the place of residence is not affected. The The capacity of this reference electrode is then exclusively dependent on environmental influences dependent (e.g. moisture) and can be used as a reference capacity for the capacity of the actual  Measuring electrode (s) are used. Go into this relative change in capacity world influences no longer.

The presence sensor system according to the invention works independently of the driver type as of the vehicle type, without conscious action on the part of the driver, is tamper-proof and interference-proof against environmental influences such as dirt, vibrations or tempera door fluctuations and can be inexpensively integrated into any vehicle. The Ener Power supply for forklifts with electric motors can already be provided where batteries are made.

Changes in capacitance of the measuring and reference electrode (s) can be done with the Measure the presence sensor according to the invention in different ways.  

For this purpose, the measuring electrode is referred to a reference potential or considered two (or more) measuring electrodes as a capacitor. The same applies to the Refe reference electrode (s).

Advantageously, changes in capacity over changes in this Ka capacity measured voltage. For example, the measurement Charge the electrode (s) formed by means of a voltage source. Then The capacitor voltage is continuously measured by the capacitance and the Total charge of the capacitor depends. On the other hand, it is possible to use the condenser Tor to connect to an AC voltage source and its capacity tap and measure certain voltage.

It can be advantageous to make changes in capacity over changes caused by this Capacity to measure the frequency of an oscillator. For this, the measuring and the reference electrode (s) are each integrated as a capacitor in an oscillator a frequency-voltage converter or a multivibrator circuit can be closed.

The frequency of the oscillator determined by this capacitance changes on application sensibility of a person in the measuring field. The inversely proportional to the dielectric constant Frequency change is detected, for example, by means of a frequency-voltage converter converts an output signal that is proportional to the change in capacitance. Of the The comparison with an adjustable threshold value of this output signal enables the Exclusion of objects in the measuring field, whose dielectric constant between the of air and that of a human being. By calculating the capacity in the same way Changes in the reference electrode (s) can be relative to environmental influences determine dependent capacity changes.

Another variant is the measurement of changes in capacity via changes the cut-off frequency of a filter determined by this capacity (high or low pass). For this purpose, the measuring and reference electrode (s) are each as Kon integrated in a filter.

In the simplest case of the low pass, the cutoff frequency depends on which the off output voltage of the low pass below a value of about 70% of the input voltage drops, inversely from the capacity. If one person takes part in the stay stops place, increases the capacitance of the measuring capacitor, whereby the  The cutoff frequency of the low pass decreases and the amplitude of the input at the low pass applied AC voltage is transmitted damped to the output. The sub a certain threshold value can be used as an indication of the presence of the per be used.

Finally, it is advantageous to make changes in capacity over changes in bridges to determine the diagonal voltage of a measuring bridge adjusted by means of this capacitance len. For this purpose, the measuring and reference electrode (s) are used as capacitors in a Integrated measuring bridge.

For example, a known measuring bridge, which is the through the measuring electrode (s) generated capacity contains, adjusted. If the capacity changes, the bridge becomes out of tune, d. H. the bridge diagonal voltage is not equal to zero.

It is particularly advantageous to use a known Vienna-Robinson bridge, in of both the capacitance formed by the measuring electrode (s) and that by the Reference electrode (s) formed capacitance are integrated. The capacities of the measuring and reference capacitor are suitable resistors in the branches of the Vienna Robinson bridge used, the bridge diagonal voltage at a certain Operating frequency becomes zero.

To save the measuring and reference electrode (s) as space-saving as possible, for example Attaching seats or handles are suitably linear or flat Electrodes used. For example, linear electrodes can be conveniently inserted into Work handles into the seats.

If changes in capacitance are measured using a measuring bridge, it is favorable to use an AC voltage source with a frequency as the voltage source from 1 to 100 kHz, preferably from 10 to 20 kHz. Such frequencies zen can be generated easily and with the intended use fail-safe. In this regard, one with this frequency is particularly suitable operated square or sinusoidal voltage.

The measuring and reference electrode (s) are advantageously linear or flat. The Electrodes can be made of a conductive metal, such as aluminum or copper, that is electrically isolated from the environment. When using two measuring electrodes it is desirable to fill the space between the measuring electrodes from the electrical  Field is fulfilled, as completely as possible with the body of the person to be detected to fill in to keep the capacity change as large as possible.

For example, two linear measuring electrodes can be placed in the seat of a fork integrate staplers. In this case, the resulting capacity is directly proportional to relative dielectric constant of the dielectric located between the electrodes and descends with the conductor length, the wire radius and with decreasing conductor was standing. Due to the geometry dependency of the electrode arrangement, one is poor deriform arrangement of linear electrodes favorable. Are two linear measurement electrodes meandering in the seat, the two- up to four times the capacity values compared to a person sitting on the seat to the capacity with an empty seat. Since the person in this case over and not between between the electrodes, the area of the capacity change and thus the Limited dynamics. However, linear electrodes can advantageously be elongated and / or space-limited locations, such as handles of drawbar steered vehicles to be brought.

In the case of flat measuring and / or reference electrodes, the respective surfaces can angle, d. H. by the surface vectors perpendicular to these surfaces formed angles, assume different values. For example, two flä chige measuring electrodes in the seat next to each other, or a flat The measuring electrode is in the seat cushion, the other in the cushion of the backrest. in the In the first case the surface angle is 0 °, in the second case 90 °. The Ka also hangs here capacity from the geometry of the electrode arrangement and linearly from the relative Di electricity number from. If both surface vectors are perpendicular to each other, an et what to expect higher capacity than for parallel surface vectors. Mes Solutions for flat measuring electrodes gave 8 to 10 times the capacitance values with app sender compared to the capacity without a person. Be with a forklift one measuring electrode in the seat, the other built into the backrest, is at In this arrangement, it should be noted that bending the upper body reduces the capacitance Solution significantly affected because the space between the electrodes is then no longer full is constantly filled in by the person. That is why it is particularly beneficial to the two flat electrodes with a surface angle of 0 ° both side by side in the Integrate the seat cushion of the forklift seat.

To the inventive method for presence detection of people closer to explain, this is based on the schematically shown in the drawing Vari ante of a presence sensor according to the invention described in more detail.

In the single figure, a presence sensor system according to the invention for people in a vehicle, such as a forklift, is shown schematically. The figure shows the seat 4 as well as the evaluation circuit 7 electronically connected to it as the location of the person.

The measuring electrodes 2 , 3 and the reference electrodes 5 , 6 are integrated in the seat cushion of the seat 4 under the cover of the seat. These are flat electrodes that largely adapt to the body and movements of the driver. The electrodes can be made of aluminum foil, for example, and insulated using a polyethylene foil. The two measuring electrodes 2 , 3, when connected to voltage, generate an electric field at the location of the person. The reference electrodes 5 and 6 form a small capacitor which, according to the invention, is mounted outside the immediate vicinity of the location and whose field cannot be influenced by the presence of a person. The reference capacitor consists of approximately 3 × 3 cm² small metal plates, the capacity of which is influenced only by the fluctuations in humidity and temperature.

The measuring electrodes 2 , 3 are installed as a measuring capacitor in a branch of a Wien-Robin son measuring bridge 8 by connecting the measuring capacitor in parallel to form a resistor. The reference capacitor, which is composed of the two reference electrodes 5 , 6 , is inserted into the same branch of the measuring bridge by being connected in series with the second resistor located there. Corresponding to the capacitances of the capacitors and the values of the resistors, the measuring bridge 8 is matched by a suitable choice of the resistors for a specific operating frequency. This operating frequency results from the values of the two capacitances and resistances in one branch of the measuring bridge 8 . In this exemplary embodiment, an operating frequency of 16 kHz was selected, which is generated by an oscillator, ie the voltage source 1 . The four resistors are then dimensioned in accordance with the balancing conditions for the measuring bridge 8 , it being possible to use at least one balancing potentiometer. From the voltage source 1 , the measuring bridge is now operated with an alternating voltage of 16 kHz, with a square-wave or sine voltage supplying good results here.

According to the invention, changes in the capacitance of the measuring electrodes 2 , 3 relative to that of the reference electrodes 5 , 6 are measured by measuring and monitoring the voltage between the centers of the two branches of the measuring bridge 8 (bridge diagonal voltage). This bridge diagonal voltage is increased by means of an Instrumentenver amplifier 9 , without stressing the measuring bridge 8 . The signal is then rectified in a rectifier 10 , with a filter eliminating harmonics of the square wave voltage. Subsequently, the signal corresponding to the vibration amplitude of the bridge diagonal voltage of the detuned measuring bridge 8 is fed to a comparator 11 , which compares the input signal with an adjustable DC voltage threshold value and delivers a measurable output signal when the threshold value is exceeded. A slight detuning of the measuring bridge 8 in the absence of a person can be compensated for in this type of evaluation circuit 7 by appropriately setting the threshold value on the comparator 11 .

The presence sensor system according to the invention was tested with various people and objects. The changes in capacity for presence and absence result in a factor between 8 to 10 in the case of different people, but only a factor of 1 to 2 in the case of different items that could be placed on the seat 4 in everyday operation. By adjusting the threshold value on the comparator, such changes in capacity caused by various objects can be eliminated. The influence of clothing, size and weight of the people is less than 20% of the measured capacitance values, so that reliable person detection remains ensured. If the field strength is sufficient, it is even possible for the driver to get up slightly from the seat 4 without the measurement signal being significantly influenced. False triggers, as were customary in previous circuits when vibrations of the vehicle occurred, are excluded in the presence sensor system according to the invention, since the electrodes follow the driver's body, so that the measurement signal remains unchanged. The costs of the presence sensors implemented in this exemplary embodiment are still below those of previous seat switches for presence detection. The omission of the reference electrodes, for example in closed vehicles (airbag monitoring, belt warning display) and the replacement of the evaluation circuit 7 by other evaluation circuits described for measuring changes in capacity can further reduce the technical outlay and thus the costs.

The presence sensor system according to the invention works independently of the driver and Vehicle type, can be installed in the vehicle in such a way that deliberate manipulation is almost impossible, is immune to dirt, moisture, vibration conditions and temperature fluctuations can be determined using the existing energy supply operate supply in the vehicle and can also be retrofitted in before existing vehicles are installed.

Claims (17)

1.Method for detecting the presence of people at a location, in particular on vehicle seats, on steering wheels or handles of drawbar-steered vehicles, an electric field being generated in the vicinity of the location by at least one measuring electrode connected to a voltage source and changes in the capacity of the measuring electrode ( n) are measured, characterized in that at least one further reference electrode ( 5 , 6 ) connected to a voltage source ( 1 ) generates an electric field outside the immediate vicinity of the location ( 4 ) and relative changes in the capacitance of reference ( 5 , 6 ) and measuring electrode (s) ( 2 , 3 ) are measured. 2. The method according to claim 1, characterized in that changes in Capacity over changes in a voltage determined by this capacity be measured. 3. The method according to any one of claims 1 or 2, characterized in that Changes in capacity through changes in one by that capacity frequency of an oscillator can be measured. 4. The method according to any one of claims 1 or 2, characterized in that Changes in capacity through changes in capacity specified limit frequency of a filter (high pass, low pass) can be determined. 5. The method according to any one of claims 1 or 2, characterized in that changes in the capacitance are determined by changes in the bridge diagonal voltage of a measuring bridge ( 8 ) adjusted by means of this capacitance. 6. The method according to any one of claims 1 to 5, characterized in that line-shaped or flat measuring ( 2 , 3 ) and / or reference electrodes ( 5 , 6 ) are used. 7. The method according to claim 5, characterized in that an AC voltage source with a frequency of 1 to 100 kHz, preferably from 10 to 20 kHz, is used as the voltage source ( 1 ). 8. The method according to claim 7, characterized in that as an AC voltage a square wave or sine wave voltage is used. 9. Presence sensor for performing a method according to claim 1, in which in the vicinity of the location at least one measuring electrode is attached, which is connected to a voltage source, the measuring electrode (s) being connected to an evaluation circuit measuring capacitance changes, characterized that at least one further reference electrode ( 5 , 6 ) is attached outside the location ( 4 ) and is connected to a voltage source ( 1 ), and that the measuring ( 2 , 3 ) and reference electrode (s) ( 5 , 6 ) are connected to an evaluation circuit ( 7 ) measuring relative changes in capacitance. 10. Presence sensor according to claim 9, characterized in that the measuring ( 2 , 3 ) and reference electrode (s) ( 5 , 6 ) are connected to a voltage source ( 1 ) and are connected directly to a voltage measuring device. 11. Presence sensor according to claim 9, characterized in that the measuring ( 2 , 3 ) and reference electrode (s) ( 5 , 6 ) are each integrated as a capacitor in an oscillator. 12. Presence sensor according to claim 9, characterized in that the measuring ( 2 , 3 ) and reference electrode (s) ( 5 , 6 ) are each integrated as a capacitor in a filter (high and low pass). 13. Presence sensor according to claim 9, characterized in that the measuring ( 2 , 3 ) and reference electrode (s) ( 5 , 6 ) are integrated as capacitors in a measuring bridge ( 8 ). 14. Presence sensor according to one of claims 9 to 13, characterized in that the measuring ( 2 , 3 ) and / or reference electrode (s) ( 5 , 6 ) are linear or flat. 15. Presence sensor according to claim 14, characterized in that the lines adhering measuring ( 2 , 3 ) and / or reference electrode (s) ( 5 , 6 ) are arranged in a meandering shape. 16. Presence sensor according to claim 14, characterized in that at least two flat measuring ( 2 , 3 ) or reference electrodes ( 5 , 6 ) are arranged inclined to each other at certain surface angles. 17. Presence sensor according to claim 16, characterized in that the area angle is 0 °.