Apparatus to indicate the spacing between a vehicle and a metal article

Apparatus mounted on a vehicle and arranged to provide a signal indicative of the spacing between the vehicle and a metal article being less than a predetermined amount, said predetermined amount being small compared with the length of the vehicle, the apparatus in combination with said vehicle comprising sensor units mounted to at least one fender and comprising at least one sensor unit mounted to a respective end portion of said at least one fender, of said vehicle, said sensor units being arranged in parallel with each other and each comprising:

• (a) a coil;
• (b) oscillator circuit means coupled to said coil and arranged in combination with said coil to generate a respective stray electromagnetic field, and to provide an output signal in response to any disturbance of said electromagnetic field as a result of the spacing between said vehicle and said metal article being less than said predetermined amount;
• (c) regulating circuit means arranged to receive said output signal and to compensate the disturbance of said electromagnetic field after a predetermined delay in response to said output signal, and
• (d) evaluating circuit means arranged to receive said output signal provided by said oscillator circuit means;
• said apparatus further comprising spacing indicating circuit means mounted internally of the vehicle, and a single conductor line and earth return means electrically connecting said parallel arrangement of said sensor units to the indicating circuit means, the evaluating circuit means of each one of said sensor units being arranged in response to the respective output signal to cause a change in current in said single conductor line.