Balance operation

Balance operation

Agentek seminarTel Aviv,

10/01/2011

Agenda

Physics

Electromagnetic compensation cell

Strain gauges with resistive element

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Agenda

Physics



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Physics reminderThe weight, that means the gravity attractive force, is proportional to the mass m and to the acceleration of the gravity g:

The gravity g depends on altitude and latitude.

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gmFW ⋅==

Gravity compensation (1)To compensate the gravity from the manufacturing place to the place of use, two possibilities:

- Compensate with a known mass

- Compensate the gravity changeDisplay of the balance

Load

2000,00 g

1000.00 g + m

1000.00 g

1000 g 2000 g

Linear characteristics

0.000 0 g

Standard weight of 1000 g Adjustment weight of 2000 g

1000.00 g ‐ m

0 g

‐ 1 mg + 1 mg

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Gravity compensation (2)What happens if the standard is wrong?

The manufacturer always provides one of these two adjustment (compensation) devices

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Agenda

Physics



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Electromagnetic compensation cell (1)The principle of electromagnetic force compensation is based on the electrodynamics conversion principle.

This figure represents a system in which a wire is placed between the poles of a permanent magnet.

S N

Force FCurrent I

Battery B

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Electromagnetic compensation cell (2)With the source B, a current I runs through the wire, it appears to an electrodynamics force F that has the tendency to take the direction of the arrow.

This method of measure gives a relation between the current I and the force F that the magnetic field and the force of the thread in this field stay constant.

If F becomes equal to the weight of the thread, this one floats in the magnetic field. While increasing the current I, we can compensate the supplementary forces acting on the wire henceforth also.

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Force FCurrent I

Battery B

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Electromagnetic compensation cell (3)To become a balance, additional installations are necessary.

We need a weighing pan to place the items to weigh, and a position indicator to control the location of the wire in the magnetic field.

We need also a current regulator and an instrument to measure the current.

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Force F

Current regulator

Battery B

0.3 A

Position indicator

Measuring instrument

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Electromagnetic compensation cell (4)With no load on the pan, the system receives just enough current from the current regulator to keep the two pointers of the position indicator on the same level.

The current indicated on the measuring instrument corresponds to the zero position, that is to say, the zero point of the balance. After that, a weight of, for example, 100g is placed on the weighing pan. As a result, the weighing system is pushed down.

The change in the position of the compensation system is clearly shown on the position indicator.

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Electromagnetic compensation cell (5)By adjusting the current regulator, the system receives more current until the 100 g weight is compensated.

The two pointers of the position indicator are again at the same level as before the 100 g weight was placed on the pan.

The instrument indicates the difference in the current between the loaded and the unloaded balance.

The difference is proportional to the weight on the pan.

Running I0 in the no load state = 0.3 A

Running I1 in the load state = 1.3 A

Difference of current for 100 g = 1 A

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Electromagnetic compensation cell (6)

The instrument measure can be graduated in grams.

0.3 A corresponds to 0 g and 1.3 A to 100 g.

If we used an instrument graduated by 1000 d, we could weigh weights of 0…100 g then close to 0.1 g

100 g/1000 d = 0.1 g/d

However, the system that has just been described is not only of complicate use, but more limited precision, for example, by the precision of the instrument and the pin indicator.

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F

Current regulator

Battery B

1.3 A

Position indicator

Measuring instrument100 gCurrent I

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Industrial Application

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Agenda

Physics



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Strain gauges with resistive element (1)The gauges with resistive element are passive sensors translating in resistance change their own distortion which is in principle equal to the one of the studied structure.

The resistance change is measured practically always with a Wheatstone bridge. This mounting is suitable for the compensation of the important temperature effects by the introduction, in the wanted branch of the bridge, of a thermal compensation gauge, identical to the active gauge, glued on a piece of the same metal at the same temperature (but unsolicited).

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Examples of gauge shapes

The strain gauge is glued to the deformable bar

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Resistance change

The load placed on the beam creates a deflection and changes the length of the strain gauge (+, -) so its resistance.

The Wheatstone bridge measures the change.

A good knowledge of the mechanical properties of the metal (its elasticity) allows having load cells of good quality.

The balance resolution is limited by the value of the resistance of the strain gauge

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Example of load cells

This type of sensor sensitive to the lateral efforts requires having mounting pieces to protect completely the sensor. It is suitable for small ranges from 20 kg to 200 kg.

For capacities until 50 tons, the used sensors are constituted by a simple rod made of steel for the distorting body. We measure the dilation of the surface (flare).

This type of sensor is sensitive to lateral efforts and to the application point of the load, requires having mounting pieces to protect the sensor from lateral load. It is suitable for the weighing of silos and weighing-bridges of large capacity.

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ConclusionThe electromagnetic force compensation gives the best resolution (World record = 640 000 000 points)

The electromagnetic force compensation is used for class I and II balances (gold and jewelery balances)

The strain gauges cells is well fitted for the retail balances and the industrial scales (class III balances)

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Balance operation

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