WO2012158004A1

WO2012158004A1 - Electromagnetic sensor of forces

Info

Publication number: WO2012158004A1
Application number: PCT/MA2011/000016
Authority: WO
Inventors: Abdelrhani NAKHELI; Mabrouk BENHAMOU; Seddik BRI
Original assignee: Universite Moulay Ismail
Priority date: 2011-05-16
Filing date: 2011-12-19
Publication date: 2012-11-22
Also published as: MA33790B1

Abstract

The proposed sensor is an electromagnetic sensor of forces. Its principle of operation relies on the phenomenon of influence by magnetic induction between two flat coils, of like diameter and comprising the same number of turns, situated a certain distance, x, apart, on one and the same axis passing through their centres. One of the coils is fixed on a horizontal support, and powered by a Wien oscillator, with precise conditions of phase and amplification. This coil generates a sinusoidal voltage of 84 Hz and of amplitude 2.5 V. The second flat coil is wound on an insulating cylinder, linked to the end of a spring. The other end of this same spring is attached to a fixed support. The second coil is therefore secured to the cylinder suspended vertically from the spring. The two coils, the spring and the cylinder are aligned on one and the same vertical axis. The cylinder plays the role of guidance, since it can move vertically and pass through an orifice on the contour of which is placed the fixed coil. At the lower end of the cylinder, we have fixed a hook making it possible to suspend masses or to attach a gauge for placing masses. The magnetic induction created by the fixed coil gives rise to an electromotive force at the terminals of the moving coil which depends on the distance x between the coils. When a mass is placed on the gauge, the spring elongates, the cylinder moves downwards, and the distance x between the coils decreases. This translates into an increase in the voltage across the terminals of the moving coil. The spring therefore plays the role of a force-displacement converter. The designed device is an electromagnetic sensor of forces, and can also be considered to be a displacement sensor: - sensor for forces: from 0 to 10 g, with an accuracy of 8 mg ≤ Δm ≤ 20 mg; - sensor for displacements: from 0 to 5 mm, with an accuracy of 4 μm ≤ Δx ≤ 10 μm. Under the best operating conditions, the accuracies of these sensors become: 4 mg ≤ Δm ≤ 10 mg and 2 μm ≤ Δx ≤ 5 μm. Finally, improvements could be made to this sensor, to make it more efficacious and conducive to numerous industrial applications.

Description

ELECTROMAGNETIC SENSOR OF FORCES

The proposed sensor is an electromagnetic force sensor. In the field of physical instrumentation, force sensors are generally electrodynamic, piezoelectric sensors or using strain gauges.

The operating principle of the proposed sensor is based on the fundamental laws of electromagnetism; which justifies its name "Electromagnetic force sensor". Such a sensor consists of a supply circuit (Cil, Photo3), a conditioning circuit (CI2, photo3)), of two identical flat coils (BF, BM, photo3), of the same diameter 1.8cm, and formed each of 30 turns, which are 0.1mm copper conductor wires, a solid support fixed to a marble base (SOC, photo3), an elastic spring (R, photo3), and a solid (CY, photo3) cylinder insulating diameter 1.8cm, height 5cm and magnetic permeability μ, two hooks (CR2, CR3, photo3) are attached to the centers of the two bases of the insulating cylinder.

The fixed support consists of four parts: A rectangular board (Pl, photo3) 3cm thick, 20cm long and 15cm wide, glued to the marble base (SOC, photo3); a board (P2, photo3) 30cm long, 6cm wide and 3cm thick is attached to the rectangular base board and perpendicular thereto, On this board 30cm held vertically, are fixed perpendicularly two boards (P3 , P4, photo3) of 15cm length each, same thickness 1cm and same width 6cm, these two boards of 15cm in length are parallel to each other and horizontally and 10cm apart from each other.On the board P3 is pierced with a hole (OR, photo3) of diameter 2cm, in which the cylinder (CY) is introduced and can move vertically almost without friction. On the board (P4) is fixed a hook (CRI, photo3), to which is fixed the upper end of the spring (R, photo 3), at the other end of the spring, is suspended the cylinder (CY) by the intermediate hook (CR2). On the cylinder is fixed the coil (BM). On the contour of the orifice (OR, photo3) is fixed fixed coil (BF, photo3).

The CRI hook, the spring, the hook CR2, the cylinder CY and the hook CR3 are aligned on the same vertical axis (Photo 3). The cylinder plays a guiding role and allows the moving coil (BM) which is integral with it to approach or move away from the fixed coil (BF), when a force is exerted on the hook (CR3, photo3 ) or by hanging a tare (T, photo4) for laying masses, or hang directly masses to the CFG hook.Le cylinder is likely to move vertically up or down, almost without friction when exerting a force at its lower end; which has the effect of lengthening or compressing the spring. The two flat coils (BF, BM) are connected by connection wires (FC, photo5) to the conditioning circuit (CI2, photo5) which is fed by the supply circuit (Cil, photo5). The output voltage of the conditioning circuit is routed to a digital voltmeter (V, photo3) of precision O.lmV.

The fixed coil (FIG. 1; BF) is fed by a Wien oscillator (FIG. 1), of frequency f ₀ = 84 Hz, whose phase and amplification conditions are satisfied (f0 = 1 / 2nRC, with R = 10kO and C = 0.2μΡ). The amplification condition A = 1 + R2 / R1 = 3 (R2 = 2.2kQ and R1 = lkO). A is the voltage gain. This oscillator is followed by two isolation follower amplifiers, to power the fixed coil, and consequently, it is traversed by a sinusoidal current creating a sinusoidal magnetic induction along its axis. The latter creates, through the voice coil, a variable flux, Φ, and a variable and measurable induced electromotive force. The maximum value of this induced ind depends naturally on the distance x separating the two coils, and the flux Φ is proportional to the magnetic induction, B, whose variation, as a function of x, is given by the following relation: B ( x) = p ^ NIR ^2/2 (R ² + x ² ) ^{3 2} , with I the current flowing through the coils, R their radius, N their number of turns, and x the distance separating the two coils. For x = 0, the preceding formula becomes simple, and we have: B (x) = μ ₀ NI / 2R.

When hooking a mass to the hook, the spring lengthens, the cylinder moves downward, and therefore the distance x between the two coils decreases; which results in an increase in the maximum voltage induced at the terminals of the voice coil. The latter being of low amplitude, it was necessary to provide amplifier circuits, rectifying and filtering (figl, RF), to make this voltage exploitable.

The conditioning circuit (figl, photo2) is powered by a stabilized symmetrical power supply, ± 15V (photo 1).

The electromagnetic force sensor proposed is characterized by its measuring range (Og at 10g) which depends on the mechanical characteristics of the spring (k = 2g / mm).

A hysteresis cycle occurs when a certain critical elongation is exceeded, and there is the appearance of a remanent deformation. The proposed sensor is limited to 10g. Under these conditions, there is no hysteresis cycle, and the measurements are reversible, from 0 to 10g. The sensitivity of the sensor has been set at S = 200 mg / mV, using an adjustment potentiometer (PO, photo 6), the sensitivity of the proposed sensor depends on the inter-coil distance x, but it is practically invariable, fixed distance.

The proposed sensor is characterized by a drift (of origin): When the sensor is turned on, there is a slow drift, and after about 60 minutes of operation, this drift becomes very low (Ιμν / min), and the output voltage of the sensor stabilizes at a constant.

The response of the sensor, V (m), is not linear, but it obeys a polynomial relation. The immediate consequence of this non-linearity is a variable sensitivity, which involves the distance x separating the two coils. The accuracy of this sensor obviously depends on the elements specific to the experimental device (coils, friction, inter-coil distance, number of turns, spring, and signal conditioning circuit), and the quality of the measuring apparatus used. Precision of measurements is Am = 10mg at 0g, and can reach 4mg at 10g. The voltmeter used gives voltages with a precision of O.lmV. The reading error is estimated at 0.05 mV, resulting in an error Am = 10mg at 0g under the best operating conditions, and can reach 4mg at 10g.

The characteristic curve of the sensor, V = f (m), is obtained, by hanging precision masses ranging from 0g to 10g (table below), and by raising the corresponding voltage, using a voltmeter of 0.1 mV accuracy. Analysis of this curve indicates that the response is not linear (Figure 2).

A polynomial adjustment of order 5, characterized by a standard deviation compatible with the experimental accuracy of the sensor (AV = ± 0.05 mV), seems suitable:

V (m) = A + Bi m + B ₂ m ² + B ₃ m ³ + B ₄ m ⁴ + B ₅ m ⁵ , with the adjusted coefficients:

A = 9.87028, Bi = 4.96455, B ₂ = 0.41911, B ₃ = -0.06976, B ₄ = 0.00956, B ₄ = -3.84615 × 10 ^-4. The characteristics of this polynomial fit are: r ² = 0.99999 and σ = 0.043, where r ² is the correlation coefficient and σ is the standard deviation.

Description of the figures:

Figure 1: Diagram of the conditioning circuit comprising the coils, the Wien oscillator (O), the amplifier stages, the rectifying and filtering circuit (RF), and the potentiometer for adjusting the sensitivity of the sensor.

Figure 2: Og sensor response at 10g: V = f (m). (Calibration curve) Photo 1: Reproduces the printed circuit board and the components of Figure 1. Photo 2: Stabilized symmetrical power supply (± 15V) of the sensor. Photo 3: Vacuum electromagnetic sensor (no load). SOC: Marble base

P1, P2, P3 and P4: Plates forming the fixed support OR: Orifice diameter 2 cm, located on P3 CY; Insulating cylinder CRI: Hook fixed on P4

CR2: Hook fixed in the center of the upper base of the Cylinder (CY) CR3: Hook fixed in the center of the lower base of the cylinder (CY) R: Spring

BF: Coil of 30 turns fixed on the contour of the orifice (OR) BM: Coil of 30 turns, fixed on the cylinder (CY) Cil: PCB of the supply circuit CI2: Electronic card of the conditioning circuit V: Digital display voltmeter Photo 4: Electromagnetic sensor, with tare Photo 5: Electromagnetic sensor, with tare loaded with a mass of 5g.

Photo 6: Electromagnetic force sensor (rear view)

FC: Connection wires of the coils with the circuit board.

Cl 1: Power supply circuit

CI2: Printed circuit board

PO: Potentiometer for adjusting the sensitivity of the sensor

Claims

Claims.

1. Device forming an electromagnetic force measurement sensor, characterized in that it comprises a fixed solid support, having an orifice (P3, OR, photo3) on the contour of which is placed a fixed coil (BF, photo3) and in which is inserted a cylinder (CY, photo3) integral with a moving flat coil (BM, photo3), and is connected to a spring (R, photo 3) fixed on the support. (P4, photo3), the two flat coils are connected to a conditioning circuit powered by a stabilized supply ± 15V, (photol) when a force is exerted on the cylinder, the spring extends and allows the measurement of the force exerted using the conditioning circuit (photo2) which delivers a voltage.

2. According to claim 1, the solid support is characterized in that it comprises four boards (PI, P2, P3, P4, photo3), on the board P4 is fixed a hook (CRI, photol), on the board P3 is pierced an orifice of diameter 2cm on the contour of which is fixed a flat coil of 30 turns (P3, BF, photo3).

3. According to claim 2, the cylinder (CY, photo3) is characterized in that it comprises a hook fixed on its upper base (CR2, photo3), a hook fixed on its lower base (CR3, photo3), and a flat coil of 30 turns (BM, photo 3) which is attached to it.

4. According to claims 1, 2 and 3, the spring (R, photo3) is characterized in that it is fixed to the hook (CRI, photo 3), and to which the cylinder (CY) is suspended by means of the hook (CR2, photo3).

5. According to claims 1,2, 3 and 4, the cylinder (CY) is characterized in that it is introduced into the orifice (OR, photo3) and can move vertically almost without friction when exercising a force on the hook (CR3, photo3).

6. According to claims 5, the cylinder (CY) is characterized in that it contains a hook (CR3, photo3) which is suspended masses, or by hanging a tare for laying masses, which causes the elongation spring, and results in the approximation of the coils.

7. According to claims 6, the conditioning circuit (Figl) is characterized in that it contains, a Wien oscillator (0, Figl) which supplies the fixed coil (BF), an amplification circuit of the sampled voltage at the terminals of the voice coil (BM), a circuit of straightening and filtering (RF, Figl), and a potentiometer for adjusting the sensitivity of the sensor (PO, photo6).

8. The electromagnetic force sensor is characterized by its range of use from 0g to 10g, its sensitivity S = 200mg / mV, its precision4mg <Am <10mg

9. The electromagnetic force sensor is characterized in that, The diameter of the coils, the number of turns of each coil, and the section of the wire can be modified. The choice of the constant k of the spring makes it possible to set the measuring range and the range of use of the sensor.

10. Device according to claim 1, characterized by its ability to measure displacements Knowing the characteristic of the spring, k = 2mg / ^ m and the relation

Δχ = Am / k.