Method and circuit arrangement for determining an inductance of a measuring coil
An example includes: switching on and off a coil current at switch-on times and switch-off times specified by means of a clock signal; generating a PWM signal with a duty cycle representative of the inductance of the measuring coil with rising and falling PWM signal edges, triggered at the same time as the switching on or switching off of the coil current and triggered as soon as a value of the coil current reaches a specified trigger threshold, or both of which are triggered as soon as a value of the coil current reaches a specified trigger threshold; evaluating the PWM signal to determine the duty cycle and the inductance; detecting a steady-state maximum value of the coil current after switching on and before subsequently switching off the coil current; and specifying the trigger threshold depending on the detected steady-state maximum value of the coil current.
1 . A method for determining an inductance of a measuring coil, the method comprising:
switching on and switching off a coil current flowing through the measuring coil at switch-on times and switch-off times that are specified by means of a clock signal;
generating a PWM signal with a duty cycle representative of the inductance of the measuring coil by generating the PWM signal with rising and falling PWM signal edges;
which on the one hand are triggered at the same time as the switching on or switching off of the coil current and on the other hand are triggered as soon as a value of the coil current reaches a specified trigger threshold, or
both of which are triggered as soon as a value of the coil current reaches a specified trigger threshold;
evaluating the PWM signal to determine the duty cycle thereof and from this the inductance of the measuring coil;
for compensation of a temperature dependency of the duty cycle of the PWM signal due to a temperature dependency of an ohmic resistance of the measuring coil,
detecting a steady-state maximum value of the coil current after switching on and before subsequently switching off the coil current, and
specifying the trigger threshold depending on the detected steady-state maximum value of the coil current.
2 . The method as claimed in claim 1 , wherein compensating for the temperature dependency of the duty cycle is carried out after each time the coil current is switched on.
3 . The method as claimed in claim 1 , wherein compensating for the temperature dependency of the duty cycle is carried out after each nth time the coil current is switched on, where n is an integer in the range of 2 to 100.
4 . The method as claimed in claim 1 , wherein compensating for the temperature dependency of the duty cycle is commanded by a control signal as required.
5 . The method as claimed in claim 1 , wherein the measuring coil comprises a winding made of metallic material or by a conductor track made of metallic material on a circuit carrier board.
6 . The method as claimed in claim 1 , wherein the coil current is conducted via a measuring resistor arranged in series with the measuring coil and the detection of the value of the coil current includes detecting a voltage dropping across the measuring resistor.
7 . A circuit arrangement for determining an inductance of a measuring coil, the arrangement comprising:
a current feed device for switching on and switching off a coil current flowing through the measuring coil at switch-on times and switch-off times that are predefined by means of a clock signal;
a PWM signal generating device for generating a PWM signal with a duty cycle that is representative of the inductance of the measuring coil by generating the PWM signal with rising and falling PWM signal edges;
which on the one hand are triggered at the same time as the switching on or switching off of the coil current and on the other hand are triggered as soon as a value of the coil current reaches a specified trigger threshold, or
both are triggered as soon as a value of the coil current reaches a specified trigger threshold;
a trigger specification device to specify the trigger threshold;
a detection and comparison device for measuring a value of the coil current and for comparing this value with the specified trigger threshold;
an evaluating device for evaluating the PWM signal to determine the duty cycle thereof and from this the inductance of the measuring coil;
wherein, for compensation of a temperature dependency of the duty cycle of the PWM signal due to a temperature dependency of an ohmic resistance of the measuring coil, the PWM signal generating device further comprises
a detection and specification update device for detecting a steady-state maximum value max of the coil current after switching on and before subsequently switching off the coil current and for specifying the trigger threshold depending on the detected steady-state maximum value of the coil current.
8 . The circuit arrangement as claimed in claim 7 , wherein the detection and comparison device comprises:
a measuring resistor arranged in series with the measuring coil when determining the inductance of the measuring coil and via which the coil current is conducted; and
a comparator for comparing a voltage dropping across the measuring resistor with a trigger threshold specified as trigger voltage.
9 . The circuit arrangement as claimed in claim 8 , wherein the detection and specification update device comprises a sample-and-hold circuit for sampling and holding a voltage dropping across the measuring resistor.