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Calibration of Weight Sensors Using Dead Weights - AN3044
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GENERAL ENGINEERING TOPICS

MEASUREMENT CIRCUITS


SENSOR SIGNAL CONDITIONERS

Keywords:
weight sensor calibration, signal compensation, compensator, transducer, force sensors, torque
sensors, calibration, weight sensors

Feb 18, 2004

APPLICATION NOTE 3044
Calibration of Weight Sensors Using Dead Weights
Abstract: The proper procedure for calibrating sensors using weights is shown with descriptions of typical faults
resulting in increased error. Gauge study performance advantages can be gained.
Calibration and temperature compensation of strain-gauge-based weight sensors using the MAX1452 and
MAX1455 Precision Sensor Signal Conditioner integrated circuits is becoming increasingly popular. Leading
automotive suppliers of safety products are increasingly turning to the use of force sensors in their quest to
optimize airbag deployment forces appropriate for the mass of the occupant and severity of the deployment
situation.

Although the MAX1452 and MAX1455 incorporate very fine trim and calibration resolution with the 16-bit delta-
sigma digital to analog converters (DACs), they cannot compensate for a poor calibration procedure and test
setup. Significant points to consider when calibrating a weight sensor are highlighted here along with key test
operator functions. A typical dead-weight test system for characterization and initial prototype production will be
discussed since that is the type generally employed for characterization and prototype production.

A typical weight sensor characterization requires the use of a dead-weight test stand (also sometimes called a
creep-tester) in order to obtain accurate and repeatable sensor loading. Several points must be observed in
order to achieve the desired results.
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Fixture Orientation for Positive and Negative Force Loads on the Test Stand
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Sensor and Cable Orientation
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Torque of the Sensor Mounting Bolts
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Weight Numbering
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Shaft-to-Weight Clearance (When using an automatic weight lift for removing weights from load)
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Pre-conditioning Sensor and Fixtures After Sensor Mounting
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Monotonic Application and Removal of Weights
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Golden Samples and Data Tracking (SPC)
Safety
The first point to be made regards safety. Weights used for weight sensor calibration can be dangerous when not
handled properly. Recommended handling practice should include these points:
q

Always wear steel-toed shoes or steel shoe covers.
r

These are available at any industrial clothing or shoe supply outlet. These weights and fixtures
are more than heavy enough to fracture foot bones if dropped accidentally.
q

Always alternate the weight mounting slot orientation every 3-4 weights when stacking.
r

Weights stacked in the same orientation can tip and fall off the hanger onto the operator's feet
by tipping during the removal of a weight. The weights are designed to be alternately stacked in
this way for safety.
q

Always use the leg muscles to lift and lower weights.
r

Bending the back to lift a 20LB weight can cause approximately 720 IN-LB of torque to be
exerted by the lower back bones and muscle in addition to the compressive weight load itself on
the same joints.
q

Always secure fixtures and rods with the maximum allowable thread engagement. r

Engaging threaded joints only one or two threads (or turns) provides insufficient screw thread
engagement and may strip away under load causing the weights and fixtures to fall.
Unfortunately, safety precautions as outlined above generally go unheeded until an operator or bystander suffers
a severe blow to the foot requiring emergency room treatment. Be cautious, don't be that person.

Fixture Orientation
The proper fixture orientation for a typical doubly-constrained S-bend weight sensor under positive weight is
shown in Figure 1. The gage marked "A" is load in tension, and the gage marked "B" is in compression.

The "No-load" condition is actually not zero, but a positive value resulting from the weight of the cantilevered
sensor and the weight of the hanger (hanger includes threaded rod, chain, and weight platter).


Figure 1. Fixture orientation for positive weight application.

The proper fixture orientation for negative weight is shown in Figure 2. The gage marked "A" is now in
compression, and the gage marked "B" is now in tension. This produces a negative sensor output signal. The
orientation of the sensor does not change. However, the fixtures are now approximately 1 inch longer. Removing
the C-brackets, flipping them vertically, and rotating them horizontally produce this fixture orientation.

Figure 2. Fixture orientation for negative weight application.

The "no-load" condition is again not zero, but a negative value resulting from the weight of the cantilevered
sensor and the weight of the hanger assembly. This no-load condition is exactly the same value as the positive
no-load condition, but reversed in polarity.

There will always be 4 load conditions when cycling from no-load to +load, and no-load to -load. In fact, we shall
define the no-load conditions to be +noload and -noload.

An improper fixture arrangement is shown in Figure 3, where the sensor has been flipped and rotated, but the
fixtures remain unchanged. At first glance this may appear to apply negative weight, however close inspection of
the gages "A" and "B" reveal that they are in the same tension/compression configuration as Figure 1 above and
produce the same output signal polarity.


Figure 3. Improper fixture orientation for negative weight application.

Sensor and Cable Orientation
Keeping the sensor in the same orientation for all tests is important for a couple of reasons. First, the sensor
body is a cantilevered load. Although the doubly constrained S-bend loading tends to cancel common mode
cantilever effects, it is best practice not to rely on a sensor property for cancellation when simple attention to
fixture orientation eliminates the issue altogether.

Second, the cable weight and tension in the cable will present a load. Although it contributes a minor error to the
no-load conditions, it is an error that need not be a concern at all by properly constraining the cable. Proper
constraint is achieved when the cable is "hanging on" or supported by "non-active" load side of the sensor. The
non-active load side of the sensor is the side where the C-bracket is attached to the upper mount of the test
stand.

Figure 4 shows both proper, and improper cable constraint for test. A simple clothespin type clip mounted on
the upper shaft allows a quick connect method of cable constraint.

Figure 4. Proper and improper cable constraint, proper is shown at top.

Weight Numbering
All weights are not created equal. Generally dead weights are traceable to NIST primary standards via
secondary, or terserary traceable standards by qualified calibration laboratories. Sometimes inexpensive weights
are not traceable at all and can vary in weight by as much as 10% from each other. In any case, the weights
should be marked with a unique identification number on the edge and top surfaces such that they can be
applied and removed in a repetitive sequence. The non-equal weights will result in a slight non-monotonic
application of force to the sensor. When such weights are applied in any random order, the result is a sensor
output curve that is non-repeatable with respect to linearity. This is shown in an exaggerated sense in Figure 5.
As stated before, although this random variance error is minor, it does not have to be concerned about at all if
the weights are applied in the same sequence each time.

Figure 5. Sensor output variance caused by unequal weights applied in random sequence.

Shaft-to-Weight Clearance
When using an automatic weight lift mechanism, careful control of the shaft position within the weight slots is
required. Figure 6 shows weight with a slot for loading and unloading on the hanger. When the weight lift is
raised to remove the weights from the hanger, any contact between the threaded-rod shaft and the weight slot
will result in a hysteresis effect and/or a misleading measurement of the no-load condition.



















Figure 6. Weight contacting hanger shaft.

When using the dead-weight tester for creep testing, this aspect is again important. Creep testing of a force
sensor is intended to measure the amount of semi-permanent offset shift that occurs when a sensor is subjected
to a load in a given direction for an extended period of time or over temperature extremes. Creep is shown
graphically in Figure 7. A test of creep should include the no load measurement (after proper preconditioning),
the continuous monitoring of the test load measurement, and the final no load measurement and subsequent
relaxation characteristics.


Figure 7. Creep test measurement requires the accurate application of no load condition.

Pre-Conditioning Sensor and Fixtures After Mounting
The sensor and fixture assembly must be repeatedly