Enhancing sensitivity and robustness of mechanical rotation and position detection with capacitive sensors
Described example user interface control apparatus includes a first structure, with a first side, conductive capacitor plate structures spaced along a first direction on the first side, a movable second structure with an auxiliary conductive structure, and an interface circuit to provide excitation signals to, and receive sense signals from, the conductive capacitor plate structures to perform a mutual capacitance test and a self-capacitance test of individual ones of the conductive capacitor plate structures to determine a position of the second structure or a user's finger relative to the first structure along the first direction.
1. An apparatus comprising:
a first structure, comprising capacitor plate structures on a first side of the first structure, wherein the capacitor plate structures are spaced along a first direction on the first side;
a second structure movable relative to the first structure along the first direction, the second structure having a second side facing the first side of the first structure;
an auxiliary conductive structure disposed on the second side of the second structure; and
an interface circuit coupled to the capacitor plate structures.
2. The apparatus of claim 1 , wherein:
the second structure includes an aperture configured to allow light to pass through the second structure.
3. The apparatus of claim 1 , further comprising:
an optical source disposed on the first side of the first structure, the optical source configured to direct a light away from the first side;
an optical sensor disposed on the first side of the first structure, the optical sensor configured to receive the light; and
a reflector disposed on the second side of the second structure, the reflector configured to reflect the light from the optical source to the optical sensor;
wherein the interface circuit is configured to provide a position signal based on the optical sensor.
4. The apparatus of claim 3 , wherein:
the optical source and the optical sensor each comprise a light emitting diode;
the optical source is forward biased by the interface circuit to produce the light at a first frequency; and
the optical sensor is reverse biased by the interface circuit to receive the light at a second frequency.
5. The apparatus of claim 4 , wherein:
the first frequency and the second frequency are different.
6. The apparatus of claim 4 , wherein:
the first frequency and the second frequency are equal.
7. The apparatus of claim 1 , wherein:
the auxiliary conductive structure is configured to overlap an outer portion of two neighboring capacitor plate structures on the first side of the first structure.
8. The apparatus of claim 1 , wherein:
the auxiliary conductive structure comprises a first auxiliary conductive structure; and
the first auxiliary conductive structure is positioned radially opposite of a second auxiliary conductive structure of an axis of the first structure.
9. The apparatus of claim 1 , further includes:
a transparent protective overlay over the capacitor plate structures on the first side of the first structure.
10. The apparatus of claim 1 , further comprising:
a processor coupled to the interface circuit and configured to perform matrix testing to determine both a mutual capacitance of groups of the capacitor plate structures and a self-capacitance of individual ones of the capacitor plate structures.
11. The apparatus of claim 10 , wherein:
the matrix testing to determine the mutual capacitance includes designating a multiplexer interconnection in the interface circuit between a first node and a second node of each of the groups of the capacitor plate structures.
12. The apparatus of claim 10 , wherein:
the matrix testing to determine the self-capacitance includes designating a multiplexer interconnection in the interface circuit between a first node and a second node of each of the individual ones of the capacitor plate structures.
13. The apparatus of claim 10 , wherein:
the matrix testing to determine the mutual capacitance includes designating an internal multiplexer of the processor coupling a first node and a second node of each of the groups of the capacitor plate structures.
14. The apparatus of claim 10 , wherein:
the matrix testing to determine the self-capacitance includes designating an internal multiplexer of the processor coupling a first node and a second node of each of the individual ones of the capacitor plate structures.
15. The apparatus of claim 11 , wherein:
the multiplexer interconnection in the interface circuit between the first node and the second node of each of the groups of the capacitor plate structures is associated with a corresponding general-purpose I/O (GPIO) terminal.
16. The apparatus of claim 12 , wherein:
the multiplexer interconnection in the interface circuit between the first node and the second node of each of the individual ones of the capacitor plate structures is associated with a corresponding general-purpose I/O (GPIO) terminal.
17. A method comprising:
performing a mutual capacitance test of a first capacitive plate structure of capacitive plate structures spaced along a first direction on a stationary first structure to determine a mutual capacitance measurement, wherein the stationary first structure faces a movable second structure, and wherein the movable second structure includes an auxiliary conductive structure;
performing a first self-capacitance test of the first capacitive plate structure to determine a first self-capacitance measurement; and
processing the mutual capacitance measurement and the first self-capacitance measurement to identify a relative position of the stationary first structure and a second structure, the second structure movable in the first direction.
18. The method of claim 17 , further comprising:
performing a second self-capacitance test of the first capacitive plate structure to determine a second self-capacitance measurement.
19. The method of claim 18 , further comprising:
determining a self-capacitance difference based on the first self-capacitance measurement and the second self-capacitive measurement.
20. The method of claim 19 , wherein:
the self-capacitance difference is a first value when the auxiliary conductive structure and a user's finger are proximate to the first capacitive plate structure;
the self-capacitance difference is a second value when the auxiliary conductive structure is proximate to the first capacitive plate structure and the user's finger is not proximate to the first capacitive plate structure;
the self-capacitance difference is a third value when the user's finger is proximate to the first capacitive plate structure and the auxiliary conductive structure is not proximate to the first capacitive plate structure;
the self-capacitance difference is a fourth value when neither the auxiliary conductive structure nor the user's finger is proximate to the first capacitive plate structure;
the first value is greater than the second value;
the second value is greater than the third value; and
the third value is greater than the fourth value.