Self calibrating micro-fabricated load cells

• US 9,228,916 B2
• Filed: 03/15/2013
• Issued: 01/05/2016
• Est. Priority Date: 04/13/2012
• Status: Active Grant

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Valdevit, Lorenzo, Azgin, Kivanc
• Primary Examiner(s): Rogers, David A
• Application Number:
  US13/842,152
• Publication Number:
  US 20130276510A1
• Time in Patent Office: 1,026 Days
• US Class Current: 1/1
• CPC Class Codes: G01L 1/00 Measuring force or stress, in general measuring force due to impact G01L5/00
• CPC Class Codes: G01L 1/10 by measuring variations of frequency of stressed vibrating elements, e.g. of stressed strings using resistance strain gauges G01L1/22
• CPC Class Codes: G01L 25/00 Testing or calibrating of apparatus for measuring force, torque, work, mechanical power, or mechanical efficiency
• CPC Class Codes: G01N 33/2025 Gaseous constituents

Abstract

Self calibrating micro-fabricated load cells are disclosed. According to one embodiment, a self calibrating load cell comprises 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.

Claims

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 μ

m 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.