Electrostatic occupant detection system

An electrostatic occupant detection system includes an electrostatic sensor and an electronic control unit. The electronic control unit is switchable between an occupant determination state in which the electronic control unit outputs a sine wave having a constant amplitude and a diagnosis state in which the electronic control unit maintains a voltage of the electrostatic sensor at a constant level. The electronic control unit gradually changes at least one of an amplitude and a frequency of the sine wave either when the electronic control unit switches from the occupant determination state to the diagnosis state and/or when the electronic control unit switches from the diagnosis state to the occupant determination state.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority to Japanese Patent Applications No. 2010-109576 filed on May 11, 2010, and No. 2011-058204 filed on Mar. 16, 2011, the contents of which are incorporated in their entirety herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to automobiles and other vehicles, and, more particularly, to an electrostatic occupant detection system.

2. Description of the Related Art

JP-A-11-271463 discloses an electrostatic occupant detection system that includes a mat-shaped electrostatic sensor and an occupant detection electronic control unit (ECU). The electrostatic sensor is supplied with a sine wave from the occupant detection ECU and generates an electric field between a main electrode disposed in a vehicle seat and a vehicle body. The electrostatic sensor outputs an electrical current or a voltage to the occupant detection ECU in accordance with a change in the electric field, and the occupant detection ECU determines the presence of an occupant based on the electrical current or the voltage. When the electrostatic occupant detection system performs a self diagnosis, in general, the occupant detection ECU stops supplying the sine wave to the electrostatic sensor and switches to a predetermined direct current (DC) voltage or the ground level only at an initial check during vehicle start-up.

The above-described occupant detection ECU supplies only the sine wave to the electrostatic sensor during the normal operation. It is preferred to correct variation in a detected capacitance of the electrostatic sensor based on the diagnosis of the electrostatic occupant detection system and a change in an environmental temperature even during the normal operation. In such a case, the sine wave supplied to the electrostatic sensor is stopped once and is switched to the predetermined DC voltage or the ground level. If the sine wave is rapidly switched to the DC voltage or the ground level, noise may be generated, the noise may adversely affect other electronic devices including a radio, and radio noise may be output.

SUMMARY OF THE INVENTION

In view of the foregoing problems, it is an object of the present invention to provide an electrostatic occupant detection system that can reduce noise having frequencies other than a sine wave frequency when a sine wave supplied from an electronic control unit to an electrostatic sensor is changed to a predetermined DC voltage or a ground level.

An electrostatic occupant detection system includes an electrostatic sensor and an electronic control unit. The electrostatic sensor includes an electrode disposed in a vehicle seat. The electrostatic sensor outputs an electrical current or a voltage as an output value in accordance with an intensity of an electric field or a change in an electric filed generated by the electrode. The electronic control unit outputs a sine wave for generating the electric field and determines the presence of an occupant based on the output value. The electronic control unit is switchable between an occupant determination state where the electronic control unit outputs a sine wave having a constant amplitude and a diagnosis state where the electronic control unit maintains a voltage of the electrostatic sensor at a constant level. The electronic control unit gradually changes at least one of an amplitude and a frequency of the sine wave in at least one of a case where the electronic control unit switches from the occupant determination state to the diagnosis state and a case where the electronic control unit switches from the diagnosis state to the occupant determination state.

In the electrostatic occupant detection system, the electronic control unit gradually changes at least one of the amplitude or the frequency of the sine wave in at least one of the case where the electronic control unit switches from the occupant determination state to the diagnosis state and the case where the electronic control unit switches from the diagnosis state to the occupant determination state. Thus, the electrostatic occupant detection system can reduce noise having frequencies other the frequency of the sine wave and can restrict an adverse effect of noise on other electronic devices including a radio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

An electrostatic occupant detection system10according to a first embodiment of the present invention will be described with reference toFIG. 1. The electrostatic occupant detection system10includes an occupant detection ECU11and an electrostatic sensor apparatus12.

The electrostatic sensor apparatus12is disposed in a vehicle seat33. The vehicle seat33includes a seat cushion34on which an occupant sits and a seat back35against which an occupant leans. In a bottom portion of the seat cushion34, a seat-cushion frame34ais disposed. In the seat back35, a seat-back frame35ais disposed. The seat-cushion frame34aand the seat-back frame35aare electrically coupled with a vehicle body (vehicle ground)32. In the seat cushion34, an electrostatic sensor31is also disposed. The electrostatic sensor31is located opposite the seat-cushion frame34aat a distance from the seat-cushion frame34a. The electrostatic sensor31is coupled with the occupant detection ECU11through a connector wiring36such as a wire harness. The electrostatic sensor31includes a main electrode, a sub electrode, and a guard electrode.

The occupant detection ECU11includes a sine wave control portion51. The sine wave control portion51can function as a sine wave attenuating portion and a sine wave amplifying portion. The sine wave control portion51includes a sine wave generator53and an amplitude voltage control portion54. The amplitude voltage control portion54includes a DC voltage output portion56, a linear waveform output portion57, a switch element58, and a switching control portion59. The switch element58is a two-input one-output switch. An equivalent circuit of a detected object such as a human body and liquid detected by the electrostatic sensor apparatus12can be expressed by a parallel circuit of a resistance RMX (real term: conductance) and a capacitance CMX (imaginary term: susceptance) as shown inFIG. 2. Thus, rather than detecting a capacitance, the electrostatic sensor apparatus12detects an impedance Z having a real term R and an imaginary term C as shown inFIG. 3. The occupant detection ECU11applies a sine wave VSG1shown inFIG. 4Ato the detected object. When the sine wave VSG1is applied, the switching control portion59outputs a control signal to the switch element58so that the switch element58connects the DC voltage output portion56and the sine wave generator53. The above-described state is an occupant determination state. The sine wave generator53generates the sine wave VSG1and outputs the sine wave VSG1to the electrostatic sensor31. The sine wave VSG1has an amplitude corresponding to a constant DC voltage and has a predetermined constant frequency.

When the sine wave VSG1is applied to the electrostatic sensor31, a potential difference is generated in a current detection resistor9in the occupant detection ECU11in accordance with an impedance of a detected object. If the impedance of the detected object includes only a real term R, the potential difference generated at the current detection resistor9does not include a phase-lead element with respect to the sine wave VSG1. Thus, when the potential difference generated at the current detection resistor9is sampled based on a real-term sampling signal shown inFIG. 4B, which is in the same phase as the sine wave VSG1, an output signal depending on the magnitude of only the real term R can be obtained as shown inFIG. 4D.

If the impedance of the detected object includes only an imaginary term C, the potential difference generated at the current detection resistor9includes a phase-lead element with respect to the sine wave VSG1. Thus, when the potential difference generated at the current detection resistor9is sampled based on an imaginary-term sampling signal shown inFIG. 4C, whose phase leads by 90 degrees from the sine wave VSG1, an output signal depending on the magnitude of only the imaginary term C can be obtained as shown inFIG. 4E. An impedance of an actual detected object includes a real term R and an imaginary term C. Thus, an impedance Z having various phases is measured, and the occupant detection ECU11determines the detected object based on the impedance Z.

The occupant detection ECU11receives the impedance Z of the detected object from the electrostatic sensor apparatus12when the electrostatic occupant detection system10is in a normal operation after vehicle start-up. The occupant detection ECU11determines that the vehicle seat33is vacant, the vehicle seat33is equipped with a child restraint system (CRS), or the vehicle seat33is occupied by an adult based on the impedance Z.

The linear waveform output portion57selectively outputs a first linear waveform LW1and a second linear waveform LW2. The first linear waveform LW1inclines in a decreasing direction from a level of the DC voltage output from the DC voltage output portion56to a zero level at a constant inclination angle. The second linear waveform LW2inclines in an increasing direction from the zero level to the level of the DC voltage at a constant inclination angle.

The switching control portion59outputs a control signal to the linear waveform output portion57so that the linear waveform output portion57outputs one of the first linear waveform LW1and the second linear waveform LW2. The switching control portion59also outputs a control signal to the switch element58so that the switch element58is switched between a DC side and a linear side. When the switch element58is switched to the DC side, the switch element58connects the sine wave generator53with the DC voltage output portion56. When the switch element58is switched to the linear side, the switch element58connects the sine wave generator53with the linear waveform output portion57.

When the occupant detection ECU11switches from the occupant determination state to a diagnosis state, the switching control portion59outputs a control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the first linear waveform LW1and outputs a control signal to the switch element58so that the switch element58is switched from the DC side to the linear side. While the electrostatic occupant detection system10is in the occupant determination state, the sine wave generator53outputs the sine wave VSG1having a constant amplitude. When the occupant detection ECU11switches from the occupant determination state to the diagnosis state, the sine wave generator53outputs a sine wave VSG2whose amplitude is attenuated from the constant amplitude of the sine wave VSG1to zero at a constant inclination angle, that is, at an attenuation rate depending on the first linear waveform LW1while a DC bias voltage level of the amplitude is maintained. The DC bias voltage level may be referred to as a center level. Eventually, the sine wave generator53outputs a DC voltage DC1at the center level of the amplitude of the sine wave VSG1.

When the occupant detection ECU11switches from the diagnosis state to the occupant determination state, the switching control portion59outputs a control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the second linear waveform LW2. Then, the sine wave generator53outputs a sine wave VSG3whose amplitude is amplified from zero to the constant amplitude of the sine wave VSG1at the same inclination angle as the sine wave VSG2, that is, at an amplification rate in accordance with the second linear waveform LW2while the center level of the amplitude is maintained. After that, the switching control portion59outputs a control signal to the switch element58so that the switch element58is switched from the linear side to the DC side, and the sine wave generator53outputs the sine wave VSG1.

The attenuation rate of the sine wave VSG2and the amplification rate of the sine wave VSG3are small so that when the amplitude of the sine wave changes at the attenuation rate or the amplification rate, noise having frequencies other than the sine wave frequency are not output.

Next, a process performed by the occupant detection ECU11when the electrostatic occupant detection system10makes a diagnosis will be described with reference toFIG. 6.

At S1, the sine wave generator53outputs the sine wave VSG1having the constant amplitude to the electrostatic sensor31when the electrostatic occupant detection system10is in the normal operation state after vehicle start-up. The switching control portion59outputs the control signal to the switch element58so that the switch element58is switched to the DC side, and the sine wave generator53outputs the sine wave VSG1that has the constant amplitude corresponding to the DC voltage output from the DC voltage output portion56. The present state is the occupant determination state.

At S2, when the occupant detection ECU11determines to make a diagnosis, the process proceeds to S3. At S3, the switching control portion59outputs a control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the first linear waveform LW1. At S4, the switching control portion59outputs the control signal to the switch element58so that the switch element58is switched to the linear side. Then, at S5, the sine wave generator53outputs the sine wave VSG2whose amplitude is attenuated from the constant amplitude of the sine wave VSG1to zero at the constant inclination angle, that is, at the attenuation rate depending on the first linear waveform LW1while the center level of the amplitude is maintained.

Eventually, the sine wave VSG2becomes the DC voltage DC1at the center level of the amplitude of the sine wave VSG1. Thus, at S6, the sine wave generator53outputs the DC voltage DC1. The present state is the diagnosis state. At S7, the occupant detection ECU11executes a diagnosis process. At S8, the switching control portion59outputs the control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the second linear waveform LW2. Then, at S9, the sine wave generator53outputs the sine wave VSG3whose amplitude is amplified from zero to the constant amplitude of the sine wave VSG1at the constant inclination angle, that is, at the amplification rate depending on the second linear waveform LW2while the center level of the amplitude is maintained. At S10, the occupant detection ECU11determines whether the amplitude of the sine wave VSG3is the same level as the constant amplitude of the sine wave VSG1. If so, at S11, the switching control portion59outputs the control signal to the switch element58so that the switch element58is switched to the DC side. Then, the process returns to S1and the sine wave generator53outputs the sine wave VSG1having the constant amplitude. In other words, the electrostatic occupant detection system10returns to the occupant determination state.

As described above, the electrostatic occupant detection system10includes the electrostatic sensor31and the occupant detection ECU11. The electrostatic sensor31includes the electrodes disposed in the vehicle seat33and the electrodes generate a weak electric field. The electrostatic sensor31outputs an electrical current or a voltage in accordance with an intensity of the weak electric field or a change in the weak electric field. The occupant detection ECU11outputs the sine wave for generating the weak electric field to the electrostatic sensor31and determines an occupant based on the voltage or the current output from the electrostatic sensor31. When the occupant detection ECU11switches from the occupant determination state to the diagnosis state, the sine wave control portion51in the occupant detection ECU11gradually attenuates the amplitude of the sine wave to zero while maintaining the center level of the amplitude. In contrast, when the occupant detection ECU11switches from the diagnosis state to the occupant determination state, the sine wave control portion51gradually amplifies the amplitude of the sine wave to the constant amplitude of the sine wave in the occupant determination state while maintaining the center level of the amplitude. Because the amplitude of the sine wave is gradually attenuated or amplified, noise having frequencies other than the sine wave frequency can be reduced. Thus, an adverse effect of noise on other electronic devices including a radio can be restricted.

The sine wave control portion51includes the DC voltage output portion56, the linear waveform output portion57, the switch element58, the switching control portion59, and the sine wave generator53. The DC voltage output portion56outputs the constant DC voltage. The linear waveform output portion57selectively outputs the first linear waveform LW1and the second linear waveform LW2. The first linear waveform LW1inclines from the level of the DC voltage output from the DC voltage output portion56to zero at a constant decreasing rate. The second linear waveform LW2inclines from zero to the level of the DC voltage at a constant increasing rate that is reverse of the decreasing rate. The switch element58selects one of the output voltage of the DC voltage output portion56and the output waveform of the linear waveform output portion57. The switching control portion59outputs the control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the first linear waveform LW1or the second linear wave form LW2. The switching control portion59also outputs the control signal to the switch element58so that the switch element58is switched to the DC side or the linear side. The sine wave generator53outputs one of the sine wave VSG1having the constant amplitude depending on the DC voltage output from the DC voltage output portion56, the sine wave VSG2whose amplitude is attenuated from the constant amplitude of the sine wave VSG1to zero at the attenuation rate depending on the first linear waveform LW1while the center level of the amplitude is maintained, the DC voltage DC1at the center level of the amplitude of the sine wave VSG1, and the sine wave VSG3whose amplitude is amplified from zero to the constant amplitude at the amplification rate depending on the second linear waveform LW2while the center level of the amplitude is maintained. In other words, the switching control portion59outputs the control signals to the linear waveform output portion57and the switch element58so that the sine wave generator53outputs the sine wave VSG1, the sine wave VSG2, the DC voltage DC1at the center level, the sine wave VSG3, and the sine wave VSG1in this order.

Therefore, in a case where the electrostatic occupant detection system10makes a diagnosis, firstly, the sine wave generator53outputs the sine wave VSG1having the constant amplitude. Next, the sine wave generator53outputs the sine wave VSG2whose amplitude is gradually attenuated from the constant amplitude to the center level of the constant amplitude. Then, the sine wave generator53outputs the DC voltage DC1at the center level of the constant amplitude. The occupant detection ECU11executes the diagnosis process with the DC voltage DC1. Because the sine wave is gradually attenuated into the DC voltage, noise having frequencies other than the sine wave frequency can be reduced. When the sine wave VSG3is amplified from the center level to the constant amplitude of the sine wave VSG1, the sine wave VSG3is gradually amplified based on the second linear waveform LW2. Thus, noise having frequencies other than the sine wave frequency can be reduced. Thus, the electrostatic occupant detection system10can restrict generation of noise when the sine wave supplied from the occupant detection ECU11to the electrostatic sensor31is changed to the DC voltage. Therefore, an adverse effect of noise on other electronic devices including a radio can be restricted.

Second Embodiment

An electrostatic occupant detection system10aaccording to a second embodiment of the present invention will be described with reference toFIG. 7. The electrostatic occupant detection system10aincludes an occupant detection ECU11aand the electrostatic sensor apparatus12.

The occupant detection ECU11aincludes a sine wave control portion51a. The sine wave control portion51aincludes a sine wave generator53a, an amplitude voltage control portion54a, and a clock generator60.

The clock generator60generates a predetermined-period clock signal and outputs the clock signal to the amplitude voltage control portion54aand the sine wave generator53a.

The sine wave generator53aoutputs one of a sine wave VSG4, a sine wave VSG5, a DC voltage DC2, and a sine wave VSG6as shown inFIG. 8. First, the sine wave generator53aoutputs the sine wave VSG4having a constant amplitude at a frequency depending on the clock signal. Next, the sine wave generator53aoutputs the sine wave VSG5whose amplitude is attenuated for an N-period of the sine wave VSG4from the constant amplitude to zero by 1/N of the constant amplitude per one period while the center level of the amplitude is maintained (“N” is a natural number). Then, the sine wave generator53aoutputs the DC voltage DC2at the center level. After that, the sine wave generator53aoutputs the VSG6whose amplitude is amplified for the N-period of the sine wave VSG4from zero to the constant amplitude by 1/N of the constant amplitude per one period while the center level of the amplitude is maintained.

The amplitude voltage control portion54acontrols the amplitude of the sine waves VSG4-VSG6output from the sine wave generator53a. When the sine wave generator53aoutputs the sine wave VSG4having the constant amplitude, the amplitude voltage control portion54acontrols the sine wave generator53so that the sine wave generator53outputs the sine wave VSG5whose amplitude is attenuated for the N-period from the constant amplitude of the sine wave VSG4to zero by 1/N per one period. When the sine wave generator53aoutputs the DC voltage DC2, the amplitude voltage control portion54acontrols the sine wave generator53aso that the sine wave generator53outputs the sine wave VSG6whose amplitude is amplified for the N-period from zero to the constant amplitude of the sine wave VSG4amplitude by 1/N per one period.

Next, a process performed by the occupant detection ECU11awhen the electrostatic occupant detection system10amakes a diagnosis will be described with reference toFIG. 9.

At S11, the sine wave generator53aoutputs the sine wave VSG4having the constant amplitude to the electrostatic sensor31at a clock frequency when the electrostatic occupant detection system10ais in a normal operation state after vehicle start-up.

At S12, when the occupant detection ECU11adetermines to make a diagnosis, the process proceeds to S13. At S13, the amplitude voltage control portion54acontrols the sine wave generator53aso that the amplitude of the signal output from the sine wave generator53ais attenuated for the N-period from the constant amplitude of the sine wave VSG4to the center level of the sine wave VSG4by 1/N per one period. Thus, the sine wave generator53aoutputs the sine wave VSG5whose amplitude is attenuated from the constant amplitude to zero by 1/N per one period while the center level of the amplitude is maintained. At S14, the sine wave generator53aoutputs the DC voltage DC2at the center level.

In the present state, the occupant detection ECU11aexecutes a diagnosis process at S15. After that, the amplitude voltage control portion54acontrols the sine wave generator53aso that the amplitude of the signal output from the sine wave generator53ais attenuated for the N-period from the center level, that is, the DC voltage DC2to the constant amplitude of the sine wave VSG4by 1/N per one period. Thus, the sine wave generator53aoutputs the sine wave VSG5whose amplitude is attenuated by 1/N per one period at the frequency depending on the clock signal.

At S17, when the amplitude of the sine wave VSG6becomes the same level as the constant amplitude of the sine wave VSG4, the process returns to S11, and the sine wave generator53aoutputs the sine wave VSG4having the constant amplitude.

As described above, the electrostatic occupant detection system10aincludes the occupant detection ECU11aincluding the sine wave control portion51a. First, the sine wave control portion51aoutputs the sine wave VSG4at the constant amplitude. Next, the amplitude of the sine wave is attenuated for the N-period from the constant amplitude to the center level of the sine wave VSG4by 1/N per one period. Then, the sine wave control portion51aoutputs the DC voltage at the center level. After that, the amplitude of the DC voltage is amplified for the N-period from the center level to the constant amplitude of the sine wave VSG4by 1/N per one period.

Thus, when the electrostatic occupant detection system10amakes the diagnosis after vehicle start-up, the amplitude of the sine wave is attenuated for the N-period from the constant amplitude to the center level by 1/N per one period. The diagnosis is performed with the DC voltage at the center level. Because the amplitude of the sine wave is gradually attenuated from the constant amplitude to the center level, noise having frequencies other than the sine wave frequency can be reduced. Also when the amplitude is amplified from the center level to the constant amplitude, the amplitude is increased by 1/N per period. Thus, noise having frequency other than the sine wave frequency can be reduced. Thus, the electrostatic occupant detection system10acan restrict generation of noise when the sine wave supplied from the occupant detection ECU11ato the electrostatic sensor31is changed to the DC voltage. Therefore, an adverse effect of noise on other electronic devices including a radio can be restricted.

Third Embodiment

An electrostatic occupant detection system10baccording to a third embodiment of the present invention will be described with reference toFIG. 10. The electrostatic occupant detection system10bincludes an occupant detection ECU11band the electrostatic sensor apparatus12.

The occupant detection ECU11bincludes the sine wave control portion51described in the first embodiment, a linear waveform output portion62, a DC voltage output portion63, a switch element65, and an output switching control portion66. The switch element65is a three-input one-output switch.

The switch element65has three input terminals, and the input terminals are coupled with output terminals of the sine wave generator53, the linear waveform output portion62, and the DC voltage output portion63, respectively. The switch element65selects an output signal from one of the output terminals based on a control signal from the output switching control portion66and outputs the selected signal to the electrostatic sensor31through the connector wiring36.

The linear waveform output portion62selectively outputs a third linear waveform LW3and a fourth linear waveform LW4by a slew rate control. The third linear waveform inclines in a decreasing direction from the center level of the sine wave VSG1, to which the amplitude of the sine wave VSG2is attenuated, to a predetermined level L3at a constant inclination angle. The fourth linear waveform LW4inclines in an increasing direction from the predetermined level L3to the center level at a constant inclination angle. The third linear waveform LW3and the fourth linear waveform LW4are line symmetry

The DC voltage output portion63outputs a DC voltage DC3at the predetermined level L3.

The output switching control portion66outputs a control signal to the linear waveform output portion62so that linear waveform output portion62outputs the third linear waveform LW3or the fourth linear waveform LW4. Furthermore, the output switching control portion66outputs a control signal to the switch element65so that the switch element65selects the output signal of the sine wave generator53. As shown inFIG. 11, the sine wave VSG1and the sine wave VSG2are output in order. When the amplitude of the sine wave VSG2is attenuated to the center level, the linear waveform output portion62outputs the third linear waveform LW3, and the switch element65selects the third linear waveform LW3. When the third linear waveform LW3decreases to the predetermined level L3, the switch element65selects the DC voltage DC3output from the DC voltage output portion63. After a predetermined time, the linear waveform output portion62outputs the fourth linear waveform LW4, and the switch element65selects the fourth linear waveform LW4. When the fourth linear waveform LW4increases to the center level, the switch element65selects the output signal from the sine wave generator53. Accordingly, the sine wave VSG3and the sine wave VSG1are output in order.

Next, a process performed by the occupant detection ECU11bwhen the electrostatic occupant detection system10bmakes a diagnosis will be described with reference toFIG. 12andFIG. 13.

At S21, the sine wave generator53outputs the sine wave VSG1having the constant amplitude to the electrostatic sensor31when the electrostatic occupant detection system10bis in a normal operation state after vehicle start-up.

At S22, when the occupant detection ECU11bdetermines to make a diagnosis, the process proceeds to S23. At S23, the switching control portion59outputs a control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the first linear waveform LW1. In addition, the switching control portion59controls the switch element58so that the switch element58selects the first linear waveform LW1. Accordingly, the sine wave generator53outputs the sine wave VSG2attenuated at the constant inclination angle.

At S24, when the occupant detection ECU11bdetermines that the sine wave VSG2becomes the center level of the amplitude of the sine wave VSG1, the process proceeds to S25. At S25, the occupant detection ECU11bperforms a slew rate control where the voltage decreases from the center level to the predetermined level L3based on the third linear waveform LW3. The output switching control portion66outputs a control signal to the linear waveform output portion62and the switch element65so that the linear waveform output portion62outputs the third linear waveform LW3and the switch element65selects the third linear waveform LW3.

At S26, the output switching control portion66outputs a control signal to the switch element65so that the switch element65selects the DC voltage DC3output from the DC voltage output portion63. Accordingly, the DC voltage DC3at the predetermined level L3is output to the electrostatic sensor31. In the present state, the occupant detection ECU11bexecutes a diagnosis process at S27.

At S28shown inFIG. 13, the occupant detection ECU11bperforms the slew rate control for increasing the voltage from the predetermined level L3to the center level. In this process, the output switching control portion66outputs control signals to the linear waveform output portion62and the switch element65so that the linear waveform output portion62outputs the fourth linear waveform LW4and the switch element65selects the fourth linear waveform LW4. At S29, the occupant detection ECU11bdetermines whether the voltage becomes the center level. If so, at S30, the switching control portion59outputs a control signal to the linear waveform output portion57so that the linear waveform output portion57outputs the second linear waveform LW2. Then, the sine wave generator53outputs the sine wave VSG3whose amplitude is amplified from the center level at the amplification rate depending on the second linear waveform LW2. The output switching control portion66outputs a control signal to the switch element65so that the switch element65selects the sine wave VSG3. Accordingly, the occupant detection ECU11boutputs the sine wave VSG3whose amplitude is amplified at the constant inclination angle.

At S17, when the amplitude of the sine wave VSG3becomes the same level as the constant amplitude of the sine wave VSG1, the process returns to S21, and the switch element65selects the sine wave VSG4having the constant amplitude and output from the sine wave generator53through the DC voltage output portion56and the switch element58.

Thus, the electrostatic occupant detection system10baccording to the present embodiment includes the occupant detection ECU11bincluding a DC shift control portion. The DC shift control portion decreases the output voltage from the center level of the amplitude of the sine wave attenuated by the sine wave control portion51to the predetermined level by the slew rate control, outputs the DC voltage at the predetermined level, and increases the voltage to the center level by the slew rate control. The DC shift control portion includes the linear waveform output portion62, the DC voltage output portion63, the switch element65, and the output switching control portion66. The DC shift control portion and the sine wave control portion51provide a diagnosis state transition portion and an occupant determination state transition portion.

In the electrostatic occupant detection system10baccording to the present embodiment, after the sine wave is attenuated to the center level in such a manner that noise are not generated, the voltage is decreased to the predetermined level by the slew rate control and the voltage is increased from the predetermined level to the center level by the slew rate control. Thus, the voltage is changed between the center level and the predetermined level while generation of noise is restricted. Thus, the electrostatic occupant detection system10bcan restrict generation of noise when the sine wave supplied from the occupant detection ECU11bto the electrostatic sensor31is changed to the DC voltage at the predetermined level.

In the above-described example, the voltage is decreased to the predetermined level L3, as an example. The voltage may also be decreased from the center level to the ground level. In the present case, after the voltage is decreased to the earth level by a linear wave from the linear waveform output portion62, the ground level is maintained for a predetermined time, and then, the voltage is increased to the center level. Thus, the DC voltage output portion63may be ground. Instead of the sine wave control portion51shown inFIG. 10, the sine wave control portion51ashown inFIG. 7may also be used.

Fourth Embodiment

An electrostatic occupant detection system10caccording to a fourth embodiment of the present invention will be described with reference toFIG. 14. The electrostatic occupant detection system10bincludes an occupant detection ECU11cand the electrostatic sensor apparatus12. The occupant detection ECU11cincludes a sine wave control portion51c.

The sine wave control portion51cincludes the DC voltage output portion56and the sine wave generator53described in the first embodiment, the linear waveform output portion62described in the third embodiment, a switching control portion67, and a multiplication portion68.

The switching control portion67controls the linear waveform output portion62so that the linear waveform output portion62outputs the third linear waveform LW3or the fourth linear waveform LW4to the multiplication portion68or stops outputting the waveform.

The multiplication portion68multiplies the sine wave VSG1having the constant amplitude and output from the sine wave generator53and the third linear waveform LW3or the fourth linear waveform LW4output from the linear waveform output portion62and outputs an attenuated sine wave VSG8or an amplified sine wave VSG9obtained by the multiplication to the electrostatic sensor31through the connector wiring36. While the linear waveform output portion62stops outputting the waveform, the multiplication portion68does not performs a multiplication process and outputs the sine wave VSG1from the sine wave generator53.

The attenuated sine wave VSG8is obtained by multiplying the sine wave VSG1and the third linear waveform LW3. The attenuated sine wave VSG8is attenuated from the constant amplitude of the sine wave VSG1while decreasing to the predetermined level L3in a constant inclination angle depending on the third linear waveform LW3.

The amplified sine wave VSG9is obtained by multiplying the sine wave VSG1and the fourth linear waveform LW4. The amplified sine wave VSG9is amplified to the constant amplitude of the sine wave VSG1while increasing from the predetermined level L3in a constant inclination angle depending on the fourth linear waveform LW4.

Next, a process performed by the occupant detection ECU11cwhen the electrostatic occupant detection system10cmakes a diagnosis will be described with reference toFIG. 16.

At S41, the sine wave generator53outputs the sine wave VSG1having the constant amplitude to the electrostatic sensor31when the electrostatic occupant detection system10cis in a normal operation state after engine start-up and the linear waveform output portion62does not output the waveform.

At S42, when the occupant detection ECU11cdetermines to make a diagnosis, the process proceeds to S43. At S43, the switching control portion67outputs the control signal to the linear waveform output portion62so that the linear waveform output portion62outputs the third linear waveform LW3. At S44, the multiplication portion68multiplies the sine wave VSG1and the third waveform LW3and outputs the attenuated sine wave VSG8.

At S45, when the attenuated sine wave VSG8decreases to the predetermined level L3, the DC voltage DC3at the predetermined level L3is output to the electrostatic sensor31. In the present state, the occupant detection ECU11cexecutes a diagnosis process at S46.

At S47, the switching control portion67outputs the control signal to the linear waveform output portion62so that the linear waveform output portion62outputs the fourth linear waveform LW4. At S48, the multiplication portion68multiplies the sine wave VSG1and the third waveform LW4and outputs the amplified sine wave VSG9.

At S49, when the occupant detection ECU11cdetermines that the amplitude of the amplified sine wave VSG9becomes the constant amplitude of the sine wave VSG1, the process returns to S41, and the sine wave VSG1having the constant amplitude is output.

As described above, the electrostatic occupant detection system10cincludes the occupant detection ECU11cincluding the sine wave control portion51c. The sine wave control portion51cmultiplies the sine wave VSG1having the constant amplitude by the third linear waveform LW3or the fourth linear waveform LW4. The third linear waveform LW3decreases from the amplitude level of the sine wave VSG1to the predetermined level L3at the constant inclination angle. The fourth linear waveform LW4increases from the predetermined level L3to the amplitude level of the sine wave VSG1at the constant inclination angle.

Thus, when the sine wave VSG1having the constant amplitude is multiplied by the third linear waveform LW3to provide the attenuated sine wave VSG8, the center level of the sine wave VSG8decreases to the predetermined level L3while being attenuated constantly. At the predetermined level L3, the attenuation of the sine wave VSG8ends and the DC voltage DC3at the predetermined level L3is output. Because the sine wave VSG8is gradually attenuated when the output signal is changed from the sine wave VSG1to the DC voltage DC3at the predetermined level, noise having frequencies other than the sine wave frequency can be reduced. When the sine wave VSG1is multiplied by the fourth linear waveform LW4to provide the attenuated sine wave VSG9, the center level of the sine wave VSG9increases from the predetermined level L3to the center level of the sine wave VSG1while being amplified constantly. Also in the present case, because the sine wave VSG9is gradually amplified, noise having frequencies other than the sine wave frequency can be reduced. Thus, the electrostatic occupant detection system10ccan restrict generation of noise when the sine wave supplied from the occupant detection ECU11cto the electrostatic sensor31is changed to the DC voltage. Therefore, adverse effects of noise on other electronic devices including a radio can be restricted.

Fifth Embodiment

In each of the above-described embodiments, the amplitude of the sine wave is changed when the occupant detection ECU11switches between the occupant determination state and the diagnosis state. In an electrostatic occupant detection system10daccording to a fifth embodiment of the present invention, a frequency of a sine wave is changed. The electrostatic occupant detection system10dwill be described with reference toFIG. 17. The electrostatic occupant detection system10dincludes an occupant detection ECU11dincluding a sine wave control portion51d. The sine wave control portion51dincludes a clock generator70in addition to the configuration of the sine wave control portion51described in the first embodiment.

The clock generator70generates a clock signal and can change a clock frequency of the clock signal. The clock signal generated by the clock generator70is input to the sine wave generator53. The sine wave generator53generates a sine wave having a frequency depending on the clock signal. Thus, by controlling the frequency of the clock signal generated by the clock generator70, the frequency of the sine wave generated by the sine wave generator53can be controlled.

The clock generator70receives a diagnosis state transition signal that orders a transition from the occupant determination state to the diagnosis state and an occupant determination state transition signal that orders a transition from the diagnosis state to the occupant determination state. The signals may be generated when a predetermined diagnosis time arrives and the signals may be generated based on an operation by a user.

When the clock generator70receives the diagnosis state transition signal, the clock generator70gradually decreases the frequency of the clock signal from the frequency in the occupant determination state to a predetermined frequency with a predetermined time. When the clock generator70receives the occupant determination state transition signal, the clock generator70gradually increases the frequency of the clock signal from the frequency in the diagnosis state to the frequency in the occupant determination state with a predetermined period.

FIG. 18Ais a diagram showing a change of the sine wave when the electrostatic occupant detection system10dswitches from the occupant determination state to the diagnosis state andFIG. 18Bis a diagram showing a change of the sine wave when the electrostatic occupant detection system10dswitches from the diagnosis state to the occupant determination state. As shown inFIG. 18A, the frequency is constant before time t0when the diagnosis state transition signal is input. After the time t0, the frequency is gradually decreased while a center level of the amplitude is maintained. As shown inFIG. 18B, after the time t1when the occupant determination state transition state is input, the frequency is gradually increased while the center level of the amplitude is maintained. Time t2inFIG. 18Bindicates a time when the frequency becomes the frequency in the occupant determination state.

(First Modification of Fifth Embodiment)

The electrostatic occupant detection system10daccording to the fifth embodiment changes the frequency while maintaining the center level of the amplitude of the sine wave, as an example. As shown inFIG. 19AandFIG. 19B, the electrostatic occupant detection system10dmay also change the center level of the amplitude while changing the frequency.FIG. 19Ais a diagram showing a change of the sine wave when the electrostatic occupant detection system10dswitches from the occupant determination state to the diagnosis state in the first modification of the fifth embodiment andFIG. 18Bis a diagram showing a change of the sine wave when the electrostatic occupant detection system10dswitches from the diagnosis state to the occupant determination state in the first modification of the fifth embodiment.

(Second Modification of Fifth Embodiment)

Alternatively, both the frequency and the amplitude may also be changed.FIG. 20Ais a diagram showing a change of the sine wave when the electrostatic occupant detection system10dswitches from the occupant determination state to the diagnosis state in the second modification of the fifth embodiment andFIG. 20Bis a diagram showing a change of the sine wave when the electrostatic occupant detection system10dswitches from the diagnosis state to the occupant determination state in the second modification of the fifth embodiment.

Other Embodiments

In each of the above-described embodiments, the amplitude or the frequency is gradually changed in both a case where the occupant detection ECU switches from the occupant determination state to the diagnosis state and a case where the occupant detection ECU switches from the diagnosis state to the occupant determination state. However, not limited to the above-described embodiments, the amplitude or the frequency may also be gradually changed only at one of the transitions and the amplitude or the frequency may also be changed rapidly at the other one of the transitions.