Calibration method and apparatus for an electronic weight indicator

• US 4,535,854 A
• Filed: 11/16/1982
• Issued: 08/20/1985
• Est. Priority Date: 11/16/1982
• Status: Expired due to Fees

• Current Assignee: Pennsylvania Scale Company
• Original Assignee: Pennsylvania Scale Company
• Inventors: Gard, Eric A., Bartsch, Kenneth E., Kautter, William J. Jr.
• Primary Examiner(s): Goldberg, E. A.
• Assistant Examiner(s): Foster, Patrick W.
• Application Number:
  US06/442,026
• Time in Patent Office: 1,008 Days
• Field of Search: 364/567, 177/1, 177/25, 177/50, 177/164
• US Class Current: 177/1
• CPC Class Codes: G01G 23/01 Testing or calibrating of weighing apparatus
• CPC Class Codes: G01G 23/163 involving digital counting

Abstract

An electronic weight indicator adapted to convert an analog output of a load cell into a digital display which, after initial calibration, automatically corrects for dead load, drift and other calibration errors introduced by the circuitry. The meter can also be programmed to count for creep. The weight indicator employs a microprocessor to control an analog-to-digital converter which converts the analog signal from the load cell to a digital display. The microprocessor employs a unique algorithm permanently retained in an associated memory to correct for drift and compensate for deadload changes and creep. Once calibrated, no further adjustments by the user are required. All adjustments are made automatically, without operator intervention. The weight indicator can be used with any load cell producing an analog signal in response to a force, e.g. a weight on a weighing platform connected to the load cell.

Claims

1. A method of calibrating and correcting for changes after calibration in an electronic weight indicator adapted to convert an analog output of a load cell into a digital display with the aid of a digital computer comprising:

(a) initially calibrating the weight indicator to display, in digital form, a range of weights, the analog signal of which is produced by the load cell including inputing the computer with a data base including at least,

1. a zero reference measurement;

2. a deadload correction offset measurement;

3. a span measurement of the analog and A/D circuitry;

4. the data base corresponds to;

##EQU6## if the deadload correction is positive, and ##EQU7## if the deadload correction is negative where;

M=corrected measurement of load cell output;

M Sig=uncorrected measurement of load cell output;

M Zero=zero reference measurement;

M Gref=2 ppm/°

C. reference measurement;

M Bias=measurement of deadload correction;

3. A method as claimed in claim 1, wherein the deadload correction offset is applied to a programmable instrument amplifier.

5. A method as claimed in claim 3, wherein the output of the counter is inputted to a digital-to-analog (D/A) converter which controls the ground reference of the instrumentation amplifier.

2. the deadload correction offset measurement is determined by a count contained in a counter;

(b) converting the analog output of the load cell to a digital value with the aid of an A/D converter;

(c) repetitively measuring the zero reference, the deadload correction offset, and the span of the analog and A/D circuitry;

(d) multiplexing the zero reference, deadload correction offset and span of the analog and A/D circuitry with the load cell output; and

(e) normalizing the analog input to the zero, deadload correction offset and span measurements to obtain a displayed weight independent of all analog and A/D offsets and gains.

4. A method as claimed in claim 2, wherein the counter is reset and advanced by a microprocessor.

6. A method calibrating and correcting for changes after calibration in an electronic weight indicator adapted to convert an analog output of a load cell into a display with the aid of a digital computer comprising the steps:

a. initially calibrating the weight indicator to display, in digital form, a range of weights the analog signal of which is produced by the load cell including inputting the computer with a data base including at least;

1. a zero reference measurement;

2. a deadlock correction offset measurement;

3. a span measurement of the analog and A/D circuitry;

b. applying the output of the load cell to an instrumentation amplifier;

c. applying a programmable offset to the instrumentation amplifier to provide automatic deadload compensation said programmable offset is determined by a count contained in a counter;

d. correcting for drift in accordance with the algorithm;

##EQU8## if the deadload correction is positive or;

##EQU9## if the deadload correction is negative.

7. A method calibrating and correcting for changes after calibration in an electronic weight indicator adapted to convert an analog output of a load cell into a display with the aid of a digital computer comprising the steps:

a. initially calibrating the weight indicator to display, in digital form, a range of weights the analog signal of which is produced by the load cell including inputting the computer with a data base including at least;

1. a zero reference measurement;

2. a deadload correction offset measurement;

3. a span measurement of the analog and A/D circuitry;

b. applying the output of the load cell to an instrumentation amplifier;

c. applying a programmable offset to the instrumentation amplifier to provide automatic deadload compensation;

d. correcting for drift in accordance with the algorithm;

##EQU10## if the deadload correction is positive or;

##EQU11## if the deadload correction is negative in which algorithm is contained in a read-only-memory (ROM) of a microprocessor; and

in which the microprocessor controls a counter containing a count for controlling the programmable offset applied to the instrumentation amplifier.

8. A method calibrating and correcting for changes after calibration in an electronic weight indicator adapted to convert an analog output of a load cell into a display with the aid of a digital computer comprising the steps:

a. initially calibrating the weight indicator to display, in digital form, a range of weights the analog signal of which is produced by the load cell including inputting the computer with a data base including at least;

1. a zero reference measurement;

2. a deadload correction offset measurement;

3. a span measurement of the analog and A/D circuitry;

b. applying the output of the load cell to an instrumentation amplifier;

c. applying a programmable offset to the instrumentation amplifier to provide automatic deadload compensation;

d. correcting for drift in accordance with the algorithm;

##EQU12## if the deadload correction is positive or;

##EQU13## if the deadload correction is negative in which the zero reference measurement is obtained from a load cell ground.

9. A method calibrating and correcting for changes after calibration in an electronic weight indicator adapted to convert an analog output of a load cell into a display with the aid of a digital computer comprising the steps:

a. initially calibrating the weight indicator to display, in digital form, a range of weights the analog signal of which is produced by the load cell including inputting the computer with a data base including at least;

1. a zero reference measurement;

2. a deadload correction offset measurement;

3. a span measurement of the analog and A/D circuitry;

b. applying the output of the load cell to an instrumentation amplifier;

c. appying a programmable offset to the instrumentation amplifier to provide automatic deadload compensation;

d. correcting for drift in accordance with the algorithm;

##EQU14## if the deadload correction is positive or;

##EQU15## if the deadload correction is negative in which the span reference (M Gref) is obtained from a resistor network.

12. An electronic weight indicator as claimed in claim 9, wherein the means for obtaining a zero reference measurement is a load cell ground point.

10. An electronic weight indicator adapted to convert an analog output of a load cell into a digital display comprising:

a. means converting the analog output of the load cell to a digital display including an analog-to-digital converter;

b. means to correct for changes after calibration in the analog and digital circuitry including temperature induced drift and drift produced by component changes, said means including;

1.

(a) A microprocessor having a read-only memory containing an algorithm for obtaining a deadload correction offset measurement,

(b) said microprocessor controls a counter containing a count for providing the deadload correction offset measurement.

2. means for obtaining a zero reference measurement,

3. means for obtaining a span measurement of the analog and digital circuitry,

4. means to multiplex the zero reference measurement, the deadload offset correction measurement and span measurement with the load cell output, and

13. An electronic weight indicator as claimed in claim 10 in which means for obtaining a span measurement is a resistor network.

11. means to correct the drift in accordance with:

##EQU16## if the deadload correction is positive, or ##EQU17## if the deadload correction is negative, where M=corrected measurement of load cell output,

M Sig=uncorrected measurement of load cell output,

M Zero=zero reference measurement,

M Gref=2 ppm/°

C. reference measurement,

M Bias=measurement of deadload correction.