Linear absolute position sensing using capacitive sensing
The embodiments described herein are directed to systems and devices for electronically measuring the absolute position of one or more moving targets e.g., along the length of a metal beam using mutual capacitive sensing. The beam may be made of metal and may have a limited inset area to fit a position detection sensor device along its length. The moving targets may have no active elements and the position of multiple targets may be detected simultaneously along the beam. The systems and devices described herein do not utilize electronic position feedback and instead rely on an integrated ruler and minimize the total number of sensors required to support recalibration, thereby minimizing scan time (more sensors results in a linear increase in scan time).
1. An apparatus comprising:
a plurality of receive (Rx) sensors organized into a plurality of Rx columns, each Rx sensor of the plurality of Rx sensors driven by an Rx signal of a set of Rx signals;
a transmit (Tx) sensor, the Tx sensor configured to couple to Rx sensors of one or more Rx columns in response to a target moving into proximity of the one or more Rx columns;
a processing device operatively coupled to the memory, the processing device configured to:
scan each of the plurality of Rx sensors; and
determine a position of the target based on the scan of each of the plurality of Rx sensors using mutual capacitive sensing; and
a guard channel coupled between the plurality of Rx columns and the Tx sensor to collapse electric field lines generated by the Tx sensor when the target is not present.
2. The apparatus of claim 1 , wherein the Rx sensors of each Rx column are driven by a unique combination of Rx signals of the set of Rx signals based on a gray code.
3. The apparatus of claim 1 , wherein to determine the position of the target, the processing device is configured to:
for each Rx sensor scan:
compare a signal measured during the scan to a threshold; and
output a binary 1 if the measured signal is above the threshold and output a binary 0 if the measured signal is below the threshold; and
determine the position of the target based on a binary encoding of the output of the Rx sensors from each Rx column, wherein each of the plurality of Rx columns covers a distance such that each successive Rx column indicates a position.
4. The apparatus of claim 3 , wherein to determine the position of the target based on the binary encoding of the output of the Rx sensors from each Rx column, the processing device is further configured to:
determine an Rx column of the plurality of Rx columns whose corresponding Rx sensors each have an output of binary 1; and
determine as the position of the target, a position indicated by the Rx column whose corresponding Rx sensors each have an output of binary 1.
5. The apparatus of claim 3 , further comprising:
a current source configured to maintain a current balance of each of the plurality of RX sensors, and wherein the measured signal of an RX sensor scan corresponds to an amount of time the current source is active to maintain a current balance of the Rx sensor being scanned.
6. The apparatus of claim 1 , further comprising:
a second plurality of Rx sensors organized into a second plurality of Rx columns;
a Tx sensor configured to couple to Rx sensors of one or more of the second plurality of Rx columns in response to a second target moving into proximity of the one or more second Rx columns.
7. The apparatus of claim 6 , wherein the processing device is further configured to:
scan each of the second plurality of Rx sensors; and
determine a position of the second target based on the scan of each of the second plurality of Rx sensors using mutual capacitive sensing.
8. The apparatus of claim 1 , wherein the Tx sensor is turned off when being scanned.
9. The apparatus of claim 1 , wherein the plurality of Rx columns comprises an Rx sensor pattern.
10. The apparatus of claim 9 , further comprising:
one or more additional Rx sensor patterns and one or more additional Tx sensors.
11. A system comprising:
a measurement beam;
one or more targets configured to move along the measurement beam; and
a sensor device located within the measurement beam, the sensor device comprising:
a plurality of receive (Rx) sensors organized into a plurality of Rx columns, each Rx sensor of the plurality of Rx sensors driven by an Rx signal of a set of Rx signals;
a transmit (Tx) sensor, the Tx sensor configured to couple to Rx sensors of one or more Rx columns in response to a target of the one or more targets moving into proximity of the one or more Rx columns;
a processing device configured to:
scan each of the plurality of Rx sensors; and
determine a position of the target based on the scan of each of the plurality of Rx sensors using mutual capacitive sensing; and
a guard channel coupled between the plurality of Rx columns and the Tx sensor to collapse electric field lines generated by the Tx sensor when the target is not present.
12. The system of claim 11 , wherein the Rx sensors of each Rx column are driven by a unique combination of Rx signals of the set of Rx signals based on a gray code.
13. The system of claim 11 , wherein to determine the position of the target, the processing device is configured to:
for each Rx sensor scan:
compare a signal measured during the scan to a threshold; and
output a binary 1 if the measured signal is above the threshold and output a binary 0 if the measured signal is below the threshold; and
determine the position of the target based on a binary encoding of the output of the Rx sensors from each Rx column, wherein each of the plurality of Rx columns covers a distance such that each successive Rx column indicates a position.
14. The system of claim 13 , wherein to determine the position of the target based on the binary encoding of the output of the Rx sensors from each Rx column, the processing device is further configured to:
determine an Rx column of the plurality of Rx columns whose corresponding Rx sensors each have an output of binary 1; and
determine as the position of the target, a position indicated by the Rx column whose corresponding Rx sensors each have an output of binary 1.
15. The system of claim 13 , wherein the sensor device further comprises:
a current source configured to maintain a current balance of each of the plurality of RX sensors, and wherein the measured signal of an RX sensor scan corresponds to an amount of time the current source is active to maintain a current balance of the Rx sensor being scanned.
16. The system of claim 11 , wherein the sensor device further comprises:
a second plurality of Rx sensors organized into a second plurality of Rx columns;
a Tx sensor configured to couple to Rx sensors of one or more of the second plurality of Rx columns in response to a second target of the one or more targets moving into proximity of the one or more second Rx columns.
17. The system of claim 16 , wherein the processing device is further configured to:
scan each of the second plurality of Rx sensors; and
determine a position of the second target based on the scan of each of the second plurality of Rx sensors using mutual capacitive sensing.
18. The system of claim 11 , wherein the Tx sensor is turned off when being scanned.
19. The system of claim 11 , wherein the plurality of Rx columns comprises an Rx sensor pattern.
20. The system of claim 19 , wherein the sensor device further comprises:
one or more additional Rx sensor patterns and one or more additional Tx sensors.