Electrical capacitance proximity sensor

A method for operating an electrical capacitance proximity sensor which has a sensor section including a detecting electrode and a guard electrode coupled to a fist signal line and a second signal line, respectively, said method comprising:

• receiving a first oscillation signal from said detecting electrode through said first signal line;
• detecting a nearby object approaching said detecting electrode according to a variation in electrostatic capacitance in said first oscillation signal generated by said detecting electrode;
• supplying a second oscillation signal having the same phase and voltage as said first oscillation signal to said guard electrode through said second signal line in a normal operation mode;
• ceasing supplying said second oscillation signal to said guard electrode through said second signal line in a self-diagnostic mode;
• monitoring an input signal from said detecting electrode to detect a substantial change in said electrostatic capacitance before and after said ceasing supplying said second oscillation signal;
• and
• determining an incorrect connection of said sensor section when said substantial change is detected.

An electrical capacitance proximity sensor for detecting a nearby object, comprising:

• a sensor section which includes a first electrode used as a detecting electrode, a second electrode used as other than said detecting electrode, and a guard electrode provided between said first and second electrodes, said guard electrode being insulated from said first and second electrodes;
• a detecting circuit section which detects said nearby object by monitoring a variation in an electrostatic capacitance generated by said first electrode to output a detecting signal;
• a connecting cable which includes first and second wires to couple said sensor section to said detecting circuit section, wherein:
• said first electrode and said guard electrode are coupled to one end of said first and second wires, respectively;
• said detecting circuit section receives a first oscillation signal according to said electrostatic capacitance generated by said first electrode from the other end of said first wire and supplies a second oscillation signal having the same phase and the same voltage as said first oscillation signal to the other end of said second wire, said detecting circuit section taking a normal operation mode for detecting said nearby object and taking a self-diagnostic mode for making a diagnostic of a correct connection between said sensor section and said detecting circuit section, and
• wherein said detecting circuit section further includes:
• a voltage pattern applying circuit which supplies a predetermined voltage pattern, instead of said second oscillation signal, to said other end of said second wire in said self-diagnostic mode;
• and
• a control circuit which determines an incorrect connection between said sensor section and said detecting circuit section in said self-diagnostic mode based on changes in said detecting signal when said predetermined voltage pattern is supplied to said other end of said second wire.