Weighing force sensor

Weighing force sensor

Weighing force sensor is a device that transforms gravity into electrical signal. It is mainly used in electronic weighing to realize force-electrical conversion.

Common types include resistance strain type, electromagnetic force type and capacitance type, among which the resistance strain type is widely used because of its simple structure, high accuracy and wide application surface.

Principle
Resistance strain weighing sensor is based on such a principle: elastomer (elastic element, sensitive beam) under the action of external force elastic deformation, paste on the surface of the resistance strain gauge (conversion element) also with deformation, the resistance strain gauge] deformation, its resistance will change (increase or decrease), then the corresponding measurement circuit to the resistance change into electrical signal (voltage or current), to complete the process of converting external force into electrical signal.

It can be seen that resistance strain gauge, elastomer and detection circuit are several indispensable parts of resistance strain weighing sensor.

The following three aspects are briefly discussed.

One, resistance strain gauge resistance strain gauge is a resistance wire machinery distributed on the substrate made of an organic material, that is, become a strain gauge.

An important parameter of his own is the sensitivity coefficient, k.

Let's talk about its meaning.

There is a metal resistance wire, whose length is l, its cross section is circular of radius r, its area is marked as s, and its resistivity is recorded as ρ. The Poisson coefficient of this material is μ.

When the resistance wire is not affected by external force, its resistance value is r: r = ρ l / s (Ω) (2-1) When its ends are affected by f force, it will elongate, that is, deformation.

Let its elongation Δ l, its cross section area shrinks, that is, the radius of its section circle decreases Δ r.

In addition, it can be proved that the resistivity of the metal resistance wire will change after deformation, recorded as Δρ.

Formula (2- -1), that is, how much the resistance value is changed after the resistance wire changes.

We have: Δ r = Δρ l / s + Δ l ρ / s Δ s ρ l / s2 (2-2) with formula (2- -1) removal formula (2- -2) to get Δ r / r = Δρ / ρ + Δ l / l Δ s / s (2-3) in addition, We know that the cross-sectional area of the wire, s = π r2, Then Δ s = 2 π r * Δ r, So Δ s / s = 2 Δ r / r (2-4) from material mechanics we know Δ r / r = - μΔ l / l (2-5), When the minus sign indicates the elongation, The radius direction is reduced.

μ Is the Poisson coefficient of the lateral effect of the material.

Formula (2-4) (2-5) into (2- -3), Δ r / r = Δρ / ρ + Δ l / l + 2 μΔ l / l = (1 + 2 μ (Δρ / ρ) / (Δ l / l)) * Δ l / l = k * Δ l / l (2-6) where k = 1 + 2 μ + (Δρ / ρ) / (Δ l / l) (2-7) (2-6)) illustrates the relationship between resistance strain gauge resistance (resistance relative change) and resistance wire elongation (length relative change).

It needs to be noted that the size of the sensitivity coefficient k value is a constant determined by the properties of the metal resistance wire material, it has nothing to do with the shape and size of the strain gauge, the k value of different materials is generally between 1.7 and 3.6; the k value is a quantity, that is, it has no dimension.

In the mechanics of materials, Δ l / l is called as a strain, recorded as ε, with it to indicate that the elasticity is often too large, very inconvenient to often take its one million as a unit, recorded as με.

In this way, formula (2- -6) often write: Δ r / r = k ε (2- -8), two, elastomer elastomer is a structural part with a special shape.

It has two functions. First, it bears the external force of the weighing sensor, and produces reaction force on the external force to achieve relative static balance; secondly, it produces a high quality strain field (area), so that the resistance strain gauge pasted in this area is ideal to complete the conversion task of strain date electrical signal.

Take the elastomer of the sb series weighing sensor as an example to introduce the stress distribution.

A cuboid cantilever with a holes.

The bottom center of the hole is subjected to pure shear stress, but the upper and lower parts will have tensile and compressive stress.

The main stress direction is the pull god and the first is the compression. If the strain gauge is stuck here, the upper half of the strain gauge will be stretched and the resistance value increases, while the lower half of the strain gauge will be compressed and the resistance value is reduced.

The strain expression for the central point at the bottom of the hole is listed below and no longer derived.

ε = (3q (1 + μ) / 2 eb) * (b (h2-h2) + bh 2) / (b (h3-h3) + bh 3) (2-9) where: shear force on q- -section; e- -Young's modulus: μ - -Poisson coefficient; b, b, h, h- -is the geometric size of the beam.

It should be noted that the stress states analyzed above are all "local" cases, while the strain gauge actually feels the "average" state.

Three, the function of the detection circuit is to change the resistance of the strain gauge into voltage output.

Because the bridge has many advantages, such as can inhibit the influence of temperature changes, can inhibit the lateral force interference, can easily solve the compensation problem of the weighing sensor, so the bridge has been widely used in the weighing sensor.

Because the full bridge equal arm bridge has the highest sensitivity, the parameters of each arm are consistent, and the influence of various interference is easy to offset each other, so the weighing sensor adopts the full bridge equal arm bridge.

Service environment
The weighing load cell is actually an electrical signal output device that convera mass signal into a measurement.

To first consider the actual working environment of the sensor, which is crucial for the correct selection of the sensor, it is related to the normal operation of the sensor, its safety and service life, and even the reliability and safety of the whole weighing device.

In general, the high temperature environment causes the melting of coating material, solder joints and structural changes in the stress of elastomer; dust and moisture affect the sensor; the sensor elastomer is damaged or short circuit; the electromagnetic field interferes with the output of the sensor.

Under the corresponding environmental factors, we must choose the corresponding force weighing sensor to meet the necessary weighing requirements.

Compare
Resistance strain load weighing sensor is often called weighing sensor for various electronic weighing, and used to measure the experimental load or force is called force sensor, although the form of the two structure, production process is the same, but in use, precision calibration and requirements or there are a lot of specific differences.

For the zero drift of the scale sensor, the performance of long-term stability is generally not added, but in the long-term performance of the long-term stability.

When the resistance strain sensor is used as the reference load of the material testing machine, 1 / 100 of the rated load of the load sensor is usually taken as the scope of application of the material testing machine.

At this time, the indicating accuracy of the test machine can be guaranteed to be 1%]].