Shielding for capacitance sensing
Systems, methods, and apparatuses for measuring capacitance of a load. The method includes applying, via a conductor, a time-varying voltage signal with a DC offset to the load. At least a portion of the conductor is shielded with a shield and the time-varying voltage signal is applied to the shield. The conductor is coupled to the load and one or more power-related parameters of the conductor are monitored to monitor the capacitance of the load.
1 . A method for determining capacitance of a load, the method comprising:
producing a time-varying voltage signal by amplifying a low-voltage version of the time-varying voltage signal;
producing a DC source voltage;
injecting the low-voltage version of the time-varying voltage signal onto the DC source voltage to produce a low voltage version of the time-varying voltage signal with a DC offset;
amplifying the low voltage version of the time-varying voltage signal with the DC offset to produce a time-varying voltage signal with the DC offset;
applying, via a conductor, the time-varying voltage signal with the DC offset to the load;
shielding at least a portion of the conductor with a shield;
applying the time-varying voltage signal to the shield;
coupling the conductor to the load; and
monitoring one or more power-related parameters of the conductor to monitor the capacitance of the load.
2 . The method of claim 1 , wherein shielding the conductor comprises co-axially shielding the conductor.
3 . The method of claim 2 , wherein shielding the conductor includes shielding the conductor with a wrapped foil shield.
4 . The method of claim 1 , wherein at least a portion of the conductor comprises a trace on a printed circuit board, and wherein shielding the conductor comprises shielding the trace with conductive layers above and below the trace.
5 . The method of claim 1 , wherein monitoring comprises measuring current in the conductor and calculating the capacitance based upon the measured current.
6 . The method of claim 1 , wherein the injecting comprises injecting the low-voltage version of the time-varying voltage signal onto the DC source voltage with a summing amplifier.
7 . The method of claim 1 , wherein amplifying the low voltage version of the time-varying voltage signal with the DC offset comprises amplifying the low voltage version of the time-varying voltage signal with a gain to produce the time-varying voltage signal with the DC offset.
8 . The method of claim 1 , comprising:
coupling the conductor to an electrostatic chuck; and
monitoring one or more power-related parameters of the conductor to monitor a combined capacitance of the electrostatic chuck and a workpiece positioned on the electrostatic chuck.
9 . The method of claim 8 , comprising:
shielding at least a portion of the electrostatic chuck to nullify stray capacitances in the electrostatic chuck.
10 . An apparatus for sensing capacitance of a load, the apparatus comprising:
a DC source to provide a DC source voltage;
a varying signal source to provide a low-voltage version of a time-varying voltage signal;
a signal injector configured to inject the low-voltage version of the time-varying voltage signal onto the DC source voltage to produce a low voltage version of the time-varying voltage signal with a DC offset;
a first amplifier to amplify the low voltage version of the time-varying voltage signal with the DC offset to produce a time-varying voltage signal with the DC offset; and
a second amplifier to amplify the low-voltage version of the time-varying voltage signal to produce the time-varying voltage signal;
an output node to couple to a conductor;
a shield node to couple to a shield for the conductor;
power circuitry configured to:
apply the time-varying voltage signal to the shield node; and
apply, to the output node, the time-varying signal with the DC offset; and
a capacitance monitor to sense the capacitance of the load by monitoring one or more power-related parameters of the conductor.
11 . The apparatus of claim 10 , wherein the output node is coaxially arranged with the shield node.
12 . The apparatus of claim 11 , comprising a coaxial cable, the coaxial cable comprising an inner conductor configured to couple to the output node and a shield conductor configured to couple to the shield node.
13 . The apparatus of claim 12 , wherein the shield conductor comprises a wrapped foil.
14 . The apparatus of claim 12 , wherein the capacitance monitor comprises a current monitor to monitor current and determine the capacitance of the load based upon the monitored current.
15 . The apparatus of claim 14 , wherein the current monitor includes one or more of a flux gate sensor, a resistive shunt sensor, a hall effect sensor, and a current mirror circuit.
16 . The apparatus of claim 10 , comprising a printed circuit board, the printed circuit board comprising:
a trace coupled to the conductor; and
conductive layers above and below the trace.
17 . The apparatus of claim 10 , wherein the signal injector comprises a summing amplifier.