Apparatus and methods for determining force applied to the tip of a probe

Abstract

An apparatus for determining a force F.sub.tip applied to a tip of an electrical impedance spectroscopy probe includes a load cell, accelerometer, and a processing means. The probe tip has a substantially planar distal end for contacting human or animal tissue. The load cell measures the force F.sub.loadcell applied axially along a longitudinal axis when the probe tip is in contact with human or animal tissue. The accelerometer measures a gravity vector A.sub.axial. The apparatus includes a means for compensating for the mass of the probe tip using the measured force and the gravity vector to produce a calibrated measurement force F applied to the probe tip.

Claims

1. An apparatus comprising an elongate probe, said elongate probe being an electrical impedance spectroscopy probe for investigating the electrical conductivity or bioimpedance of cervical tissue, the apparatus capable of determining a force F.sub.tip applied to the tip of said elongate probe, the apparatus comprising: a handle; said elongate probe comprising said probe tip attached to said handle, wherein the probe tip comprises an EEPROM within the probe tip and wherein a mass of the probe tip is stored on the EEPROM; a load cell located in said handle and capable of measuring a force F.sub.loadcell applied axially to said probe tip along a longitudinal axis thereof when said probe tip is in contact with said human or animal cervical tissue, wherein said load cell comprises four strain gauges in a bridge configuration, wherein the bridge configuration is excited by bursts of square wave pulses at a frequency of 1 kHz; an accelerometer located in the handle for measuring a gravity vector A.sub.axial, the accelerometer in fixed relationship with the probe tip and the load cell; and a microcontroller for compensating for the mass of the probe tip using said measured force and gravity vector to produce a calibrated measurement of said force F.sub.tip applied to said probe tip, the microcontroller generating a displayable output for indicating to a user the calibrated measurement of said force F.sub.tip; wherein said probe tip is an interchangeable probe tip removably attached to said handle.

2. The apparatus as claimed in claim 1, wherein microcontroller is further capable of determining an electrical conductivity of said human or animal cervical tissue to which the distal end of the probe tip is applied.

3. The apparatus as claimed in claim 1, wherein said accelerometer is an analogue tri-axial MEMS accelerometer.

4. The apparatus as claimed in claim 1, wherein the microcontroller includes analogue to digital converters to digitize outputs of said load cell and said accelerometer.

5. The apparatus as claimed in claim 1, wherein said calibrated measurement of force F.sub.tip=F.sub.loadcellA.sub.axial*(M.sub.tip+M.sub.load), where A.sub.axial is a component of an output of the accelerometer corresponding to the axial-aligned direction of the probe tip, M.sub.tip is the mass of the probe tip and M.sub.load is the free mass of the load cell and a connector for the probe tip.

6. The apparatus as claimed in claim 1, wherein said displayable output is capable of indicating real-time calibrated measurements of force applied to the probe tip.

7. The apparatus as claimed in claim 1, wherein said displayable output includes threshold indications configured for indicating whether too much or too little force is being applied to the probe tip.

8. The apparatus as claimed in claim 1, further including recording means for recording measurements produced by said load cell and said accelerometer to facilitate a repeatable application of force to the probe tip.

9. The apparatus as claimed in claim 1, wherein the elongate probe takes electrical impedance spectroscopy measurements only when the calibrated measurement of said force F.sub.tip is within acceptable limits.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawing in which:

(2) FIG. 1 is a schematic representation of an elongate probe for use in an embodiment of the invention.

(3) FIG. 2 is a schematic representation of an elongate probe and processing means for use as an embodiment of the invention.

DETAILED DESCRIPTION

(4) FIG. 1 shows an elongate probe 1 which has a handle 3 attached to a probe tip 2. The probe tip can be attached to the handle using a standard commercial connector arrangement 4. The probe tip 2 may be removeably attached to the handle 3 so that the clinician can select and interchange different probe tips for different patients. The probe tip 2 has a substantially planar distal end 5 which includes an arrangement of electrodes, for example a tetrapole arrangement of known type for use in electrical impedance spectroscopy.

(5) A load cell 6 is located in the handle 3. A triaxial MEMS accelerometer 7 is also provided in the handle 3 of the probe 1 and, hence, is in a fixed relationship to both the probe tip 2 and the load cell 6. The handle may also include a processor (microcontroller) 8, a display 9 and a data recorder 10 for recording measurements produced by said load cell and said accelerometer.

(6) The EIS technique involves placing the distal end 5 of the probe tip 2 against the tissue whose electrical conductivity it is desired to measure. The pressure applied by the clinician as the probe tip is placed on the tissue is important as differences in applied pressure significantly affect the results. The changes in cervical tissue measured as an indicator of pre-term birth are more subtle and hence more prone to being affected by the applied pressure than the pre-cancerous changes which are more usually measured by the EIS technique. Consequently it is not only important to be able to repeat the same applied pressure when taking sequential measurements but it is also important to apply pressure within predetermined thresholds.

(7) The apparatus described herein facilitates this by providing the clinician or other user with a display means 9 indicating the applied pressure and indicating whether that applied pressure is within a desired range. This could be done by a bar graph or a traffic light indicator, for example, with a green light displayed when the applied pressure is within a desired range. Alternatively or in addition, an audible alarm or other signal may be provided.

(8) The apparatus described herein is able to compensate for the mass of the probe tip 2 in order to measure (and display) a calibrated measurement of the applied force which is more accurate than a direct measurement of the force applied at the probe tip. The probe tip 2 has a known mass which, due to the action of gravity, could apply a force (dependent on the orientation of the probe) that significantly affects the accuracy of the direct measurement.

(9) It is only necessary to measure the applied force at the probe tip 2 in order to determine the applied pressure because the probe tip has a known area at its distal end 5 (and P=F/A).

(10) The load cell 6 measures the force applied axially along the longitudinal axis of the probe tip. This force is equal to the force applied to the probe tip together with the mass of the tip multiplied by gravity and resolved in the axial direction. The mass of the probe tip is known and the local gravity vector with respect to the probe A.sub.axial is measured using the accelerometer. It is therefore possible in this way to obtain a calibrated measurement of the force applied to the probe tip which compensates for the mass of the tip.

(11) In the illustrated embodiment, the load cell 6 comprises four foil strain gauges arranged in a bridge configuration. The bridge can be excited by bursts of square wave pulses at a frequency of 1 kHz which allows the detection of the small resistance change of the load cell bridge with both low power requirements and low sensitivity to DC drift. The output of the bridge can be amplified by and filtered with a Sallen and Key circuit, whose output can be sampled many times per cycle by a microcontroller's 8 analogue to digital converter circuit. Other methods of electrically measuring force would be suitable for this application and understood by the skilled reader.

(12) In the illustrated embodiment, the accelerometer 7 is an Analog Devices tri-axial MEMS device whose output is suitable for direct connection to the microcontroller's 8 analogue to digital converter circuit. The only signal processing required is simple linear calibration for zero and range. The calibrated tri-axial output of the accelerometer 7 is operated on by a rotation matrix so that it can be accurately aligned with the longitudinal axis of the probe 1. Other methods of measuring the gravity vector resolved to the longitudinal axis of the probe would be suitable for this application and understood by the skilled reader.

(13) The mass of the probe tip can be measured with a balance and stored in an EEPROM within the probe tip 2. Probe tips 2 can be easily interchangeable because the probe 1 can read the mass for each specific probe tip 2 from its EEPROM. The free mass of the load cell 6 can be found by a calibration process wherein the probe 1 is held in two orientations. It is possible to do a full automatic load cell calibration using the known mass of the probe tip 2 and the output of the accelerometer 7, by asking the user to hold the probe 1 in different positions.

(14) A display means 9 (gives the clinician feedback as to whether the force applied to the probe tip 2 is within acceptable limits. The display means could be a five LED bar graph wherein the central LED is highlighted to indicate the desired pressure and progressively more LEDs are lit as the pressure is increased. The range of pressure thresholds required to light each of the LEDS are programmable.

(15) The probe 1 can be set only to take EIS measurements when the pressure is within acceptable limits. An LED bar graph is intuitive to use and, under test, allowed the handheld probe 1 to maintain EIS measurements within the limits of +6% of the desired force.

(16) As illustrated in FIG. 2, the processing means are provided, for example, in the form of a PC 8, which can record patient information, e.g., via a recording means 10, guide the clinician through the measuring process, control the probe, analyse and save the results in a database.

(17) Although the description above is in relation to an EIS probe, the apparatus and force measurement technique described herein can be used in other applications. For example, the apparatus could be used to determine the force applied to the tip of a probe used in joint surgery to assess the quality of fit of a new joint. Other applications can be envisaged.