Self calibrating micro-fabricated load cells

1. A self calibrating load cell, comprising:

a resonant double ended tuning fork force sensor; and

a phase locked loop circuit for detection of frequency changes upon external load application to the resonant double ended tuning fork force sensor;

wherein the resonant double ended tuning fork sensor has a zero-load resonance frequency (ω n,o,op ) and a zero-load scale factor (α o,op ); and

wherein, during calibration, a load on the resonant double ended turning fork force sensor is obtained according to a calibration curve equation: F appl =(ω 2n,0p −ω 2 n,o,0p )/(2*α o,op *ω n,o,0p ).

2. The self calibrating load cell of claim 1 , wherein the resonant double ended tuning fork force sensor comprises

at least one sense electrode;

at least one drive electrode;

a resonant tuning fork;

a spring supported roller; and

a load cell tip for application of the external load.

3. The self calibrating load cell of claim 2 , wherein the spring supported roller is situated between the resonant tuning fork and the load cell tip.

4. The self calibrating load cell of claim 1 , wherein the phase locked loop circuit comprises

a pre-amplifier stage;

a phase detector;

a controller; and

a voltage controlled oscillator (VCO).

5. The self calibrating load cell of claim 1 , wherein the resonant double ended tuning fork force sensor and a phase locked loop circuit are assembled on a printed circuit board (PCB).

6. The self calibrating load cell of claim 5 , wherein the PCB dimensions are 65 mm×52 mm.

7. The self calibrating load cell of claim 1 , wherein the resonant double ended tuning fork force sensor is implemented with a silicon-on-insulator (SOI) process with 100 μm silicon structural layer.

8. The self calibrating load cell of claim 7 , wherein the silicon-on-insulator (SOI) process comprises:

etching a device layer of a wafer;

removing photoresist residues from the wafer;

attaching the wafer to a secondary handle wafer and etching the backside of the wafer;

removing the secondary handle wafer from the device wafer;

cleaning the device wafer; and

separating the devices without dicing.

9. The self calibrating load cell of claim 1 , wherein the resonant double ended tuning fork force sensor has a resolution of up to 7 nN and a compressive load range of up to 0.085N, exceeding a dynamic range of 140 dB (100 parts per billion).

10. The self calibrating load cell of claim 1 , wherein the resonant double ended tuning fork force sensor has a scale factor of 216 kHz/N, a Q-factor greater than 60,000 at 3 mTorr ambient pressure and a zero load resonant frequency of up to 47.6 kHz.

11. A method of characterization, comprising:

calibrating a resonant double ended tuning fork sensor, wherein the calibrating comprises

performing a load test with the resonant double ended tuning fork force sensor prior to its use to obtain a zero-load resonance frequency (ω n,o,op ) and a zero-load scale factor (α o,op ); and

during application of external force, obtaining the zero-load resonance frequency (ω n,o,op ) and zero-load scale factor (α o,op ) as a function of detected frequency change;

applying external axial force (compressive or tensile) to a resonant double ended tuning fork force sensor, wherein a natural frequency of tines of resonant double ended tuning fork force sensor decreases or increases in response to the applied external axial force; and

detecting the decrease or increase in natural frequency by means of a circuitry comprising a phase locked loop circuit;

wherein, during calibration, a load on the resonant double ended turning fork force sensor is obtained according to a calibration curve equation: F appl =(ω 2n,0p −ω 2 n,o,0p )/(2*α o,op *ω n,o,0p ).

12. The method of claim 11 , wherein the resonant double ended tuning fork force sensor comprises:

at least one sense electrode;

at least one drive electrode;

a resonant tuning fork;

a spring supported roller; and

a load cell tip for application of the external load.

13. The method of claim 11 , wherein the phase locked loop circuit comprises:

a pre-amplifier stage;

a phase detector;

a controller; and

a voltage controlled oscillator (VCO).

14. The method of claim 11 , wherein the phase locked loop circuit and resonant double ended tuning fork force sensor are assembled on a printed circuit board (PCB).

15. A load cell assembled on a printed circuit board (PCB), comprising:

a force sensor; and

a circuit for detection of frequency changes upon external load application to the force sensor;

wherein the force sensor has a zero-load resonance frequency (ω n,o,op ) and a zero-load scale factor (α o,op ); and

wherein, during calibration, a load on the resonant double ended turning fork force sensor is obtained according to a calibration curve equation: F appl =(ω 2n,0p −ω 2 n,o,0p )/(2*α o,op *ω n,o,0p ).

16. The load cell of claim 15 , wherein the force sensor is a resonant double ended tuning fork force sensor.

17. The load cell of claim 15 , wherein the load cell is self calibrating.

18. The load cell of claim 15 , wherein the force sensor is implemented using a silicon-on-insulator process.