Temperature compensated measuring system

• US 4,464,725 A
• Filed: 10/06/1983
• Issued: 08/07/1984
• Est. Priority Date: 05/19/1981
• Status: Expired due to Term

• Current Assignee: Setra Systems Incorporated (Fortive Corp.)
• Original Assignee: Setra Systems Incorporated (Fortive Corp.)
• Inventors: Briefer, Dennis K.
• Primary Examiner(s): Chin, Gary
• Application Number:
  US06/539,227
• Time in Patent Office: 306 Days
• Field of Search: 364/557, 364/558, 364/567, 364/568, 364/571, 364/466, 177/25, 177/211, 177/226
• US Class Current: 702/101
• CPC Class Codes: G01G 21/244 combined with flexure-plate fulcrums flexure-plate fulcrums G01G21/07
• CPC Class Codes: G01G 23/06 Means for damping oscillations, e.g. of weigh beams
• CPC Class Codes: G01G 7/06 by electrostatic action

Abstract

A temperature-compensated measuring system employs an internal signal-processor to provide high accuracy measurements, correct over a wide range of temperatures. In a calibration mode, the system employs the signal-processor to provide mathematical constants used to generate a temperature-compensation function. In a measurement mode, signals representing the uncompensated quantity and the system temperature are supplied to the signal-processor, which uses them to generate a temperature-compensation function from which it produces a high-accuracy temperature-corrected value of the measured quantity. In various forms, the invention can be used to measure temperature-compensated force, pressure or acceleration.

Claims

1. A digital scale with a temperature compensation system, comprising:

a scale mechanism,

first transducer means for sensing the force on the scale mechanism due to the presence of an object to be weighed and for generating digital force signals indicative thereof,

second transducer means adjacent said scale mechanism for sensing the current ambient temperature and for generating digital temperature signals indicative thereof,

means for generating a series of data indicative of n predetermined discrete ambient temperatures within an operating range,

a set of data indicative of m known weights, where m and n are integers,

means for applying said weights to the scale mechanism in a predetermined sequence at each of said ambient temperatures,

data storage means for storing the digital force values obtained with each known weight at each discrete temperature, along with the corresponding digital temperature signal value,

computer means for obtaining the calibration constants for said scale by using the known weights as the values of W and the corresponding stored digital force values as the values of F to evaluate the expression ##EQU5## at each temperature and solving the resulting n sets of m simultaneous equations for each of the coefficient terms a₁ (T), a₂ (T) . . . a_m (T), and then substituting the numerical values of the coefficient terms for each a_i (T) in the set of expressions ##EQU6## where k_ij are the calibration constants, and substituting the stored corresponding digital temperature signal values for T, and solving each of the resulting m sets of n simultaneous equations for each k_ij,

said data storage means having means for storing the computer k_ij values as a set of m×

n calibration constants,

means for computing a numerical value indicative of the corrected weight of an unknown object using the formula ##EQU7## by employing the stored calibration constants which are the computer polynomial coefficients within each a_i (T) expression, and

means for generating an output signal based on the said computer numerical value representing the corrected weight of the object on the scale mechanism

whereby the effect of temperature change on the scale mechanism, transducer means and other electrical components of the scale is taken into account.

2. The system of claim 1, wherein m is 4 and n is 3.

3. A temperature compensation method for a digital scale having a scale mechanism force transducer means producing digital force signals representative of the force on the scale mechanism due to an object to be weighed, comprising the steps of

(a) generating a digital temperature signal representative of the current ambient temperature,

(b) in a calibration mode, applying a predetermined series of m known weights to said scale mechanism at a first discrete fixed ambient temperature in the nominal operating temperature range of the scale,

(c) recording the values of the digital force signal and digital temperature signal,

(d) changing the ambient temperature to subject said scale to a predetermined sequence of n discrete temperatures in its operating range, each time repeating steps a and b, to obtain a set of m×

n stored digital force and temperature values, m and n being integers,

(e) using the known weights as the values of W and the corresponding stored digital force values as the values of F to evaluate the expression ##EQU8## at each temperature and solving the resulting n sets of m simultaneous equations for each of the coefficient terms a₁ (T), a₂ (T), . . . a_m (T),

(f) substituting the numerical values of the coefficient terms for each a_i (T) in the set of expressions ##EQU9## where k_ij are the calibration constants, and substituting the stored corresponding digital temperature signal values for T, and solving each of the resulting m groups of n simultaneous equations for each k_ij,

(g) storing the computer K_ij values as a set of n×

m calibration constants,

(h) in the post calibration measurement mode, to weigh a given object of unknown weight, computing a numerical value indicative of the corrected weight for the unknown object using the formula ##EQU10## by employing the stored calibration constants which are the computer polynomial coefficients within each a_i (T) expression, and

(i) generating an output signal based on the said computer numerical value representing the corrected weight of the object on the scale,

whereby the effect on the scale mechanism, transducer and associated electronics of variations in ambient temperature is taken into account.

4. The method of claim 3, wherein m is 4 and n is 3.