Cross-coil measuring mechanism
The cross-coil measuring unit (also called quotient measuring unit ) is an analog, electrical measuring device and is used to display the ratio of two electrical currents I 1 and I 2 by converting this ratio into a corresponding pointer deflection. The cross- coil measuring mechanism is constructed similarly to the moving-coil measuring mechanism , but in contrast to it has two mechanically fixed, crossed moving coils and no return springs. Since there are no return springs, the pointer does not return to the zero position when the power is off.
Working principle
In the center are the two coils, which are twisted 90 degrees against each other, through which the two electrical currents I 1 and I 2 flow separately. The force F 1 and F 2 on the two current-carrying coils in the magnetic field caused by the permanent magnet results in a force proportional to the current ratios ( Lorentz force ). The geometrical arrangement of the coils generates a torque which causes the pointer to deflect. The cheese turns into the position in which the torque of one coil keeps the torque of the other in equilibrium. The force acting on a coil is maximum when the coil in question is exactly 90 degrees to the magnetic flux B caused by the permanent magnet . The force is minimal when the coil is parallel to the lines of magnetic flux.
application
Nowadays, the cross-wound measuring mechanism has largely been replaced by digital measuring devices in practical use . It is suitable e.g. B. for precise measurement of resistances . A coil with a very precise, known and temperature-stable measuring resistor is connected in series, and in parallel with this, the second coil with the resistance to be measured. If a measuring current supply is switched on (direct voltage), the currents through the two coils are inversely proportional to the size of the resistors, and a corresponding pointer deflection occurs. The deflection is independent of the size of the supply voltage within the scope of the current carrying capacity of the instrument, as long as a minimum current strength is reached for moving the pointer. However, the polarity determines the direction of the deflection from the balanced state. A cross-coil instrument can be used as a temperature measuring device by connecting two resistors with different temperature coefficients. The deflection then changes depending on the temperatures of the two resistors. Usually a constant resistor and a variable resistor with a known temperature coefficient, which is mounted as a measuring sensor, are used for this.
In motor vehicles ( instrument cluster ) and on boats in particular , displays with cross-coil measuring mechanisms are still often used for tank contents, oil pressure or engine temperature, since the measuring arrangement is quite insensitive to fluctuations in the supply voltage. There it is usually not the cross-wound bobbin that is designed to be rotatable, but the permanent magnet (cross-wound rotary magnet measuring mechanism). The restoring force for the pointer is generated with the help of a small additional fixed magnet inside the coil housing, which acts on the rotary magnet.
