JPS638401B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring the thickness of a non-metallic coating on a metallic arcuate surface.

For the most economical use of coating or sheathing materials, it is desirable to maintain the thickness of the sheath to a minimum amount consistent with specification requirements. However, due to variations in the extrusion process, the thickness of the extruded material may vary locally and also over the entire circumference of the cable or other structure. Therefore, it is necessary to provide a means for measuring the sheath thickness at various locations around the cable and providing an indication of the thickness at the measurement location or the amount of variation from the reference thickness.

Capacitive measuring instruments are widely used for these purposes. Typically, the measuring head is rotated about the axis of the cable and also held stationary in a predetermined angular position around the cable for a predetermined short period of time. Thus, for example, the measuring head can be rotated in steps and held stationary in several positions spaced apart from each other by 45 degrees.

However, capacitive types have certain drawbacks. That is, the gauge or measuring head is extremely sensitive to water present on the sheath. Even a small amount of water present on the sheath will significantly change the signal from the measuring head. Also, any changes in the coating material will affect the measuring head and require recalibration. The above point is extremely important since the materials to be extruded often change due to current developments and changes in the supply of raw materials and the prices of raw materials. Even if the changes in the raw material are extremely subtle, the output signal will fluctuate significantly. The introduction of new products also poses the problem of requiring a new calibration chart or table for each new product. Whereas in the past, products and their coverings remained unchanged for long periods of time, for example, years, today the aforementioned changes in materials and the introduction of new products occur frequently.

The present invention provides an apparatus for measuring the coating thickness of a metal arcuate surface that compensates for variations in the outer diameter of the arcuate surface plus the coating. When the coating thickness of a thin, non-conductively coated cylindrical member changes, the cylindrical member moves due to the variation, and the distance between the cylindrical member and the detection head changes, and the effect of this changes depending on the diameter of the cylindrical member. Change. The present invention is based on the fact that if the cylindrical member is positioned relative to the detection head so that each member emits a signal of the same magnitude regardless of its diameter, the effect of thickness variation will be approximately equal, independent of the diameter of the cylindrical member. It is based on

The measuring device of the invention compensates for variations in diameter from a predetermined reference value and also allows the detection head to be used opposite arcuate surfaces of various diameters without the need for recalibration. The detection head is arranged by one or more supporting or positioning members having a surface of a predetermined shape directed towards the outer surface of the coating.
Supported against a metal arcuate surface.

The surface of said predetermined shape is V-shaped with convex arcuate sides when viewed parallel to the axis of said metal arcuate surface, said arcuate sides being of equal size from said detector to each mandrel. formed by sequentially positioning a series of cylindrical metal mandrels of different diameters, with the mandrel axes initially lying in a common plane perpendicular to the detector plane, such that each arcuate side is a curve tangential to the outer circumference of the mandrel.

The measuring device of the invention is particularly suitable for electrically inductive detection heads, but can also be used for other types of detection heads.

The present invention will be easily understood from the following description with reference to the accompanying drawings.

Figures 1 and 2 show the probe or support member itself, a part of which is actually in contact with or in close proximity to the surface of the structure to be measured or monitored. The support member has a stem portion 10 and a head portion 11. The stem portion 10 is adapted to be placed within the measuring head and, in this embodiment, has a coil located within the groove 12 to form the sensing head, as will be explained in more detail below. do. The head portion 11, in this embodiment, is annular in plan view, as shown in FIG. 2, and when viewed from the side in a direction coinciding with the axial direction of the cable or other structure to be measured. , has a contact surface 13 which is shaped in a predetermined manner.
Although the term "contact surface" is used for convenience, a small gap may remain between the surface 13 and the cable or other object. Hole 14 extends axially through the support member. The bore 14 may receive a tuning element such as a slug or tube of a suitable material such as ferrite. Due to these synchronization factors,
Individual detection heads can be tuned, allowing the heads to be coordinated in systems using multiple heads, such as those described herein. The tuning element can change the sensitivity of the head and change the resonant frequency of the head.

From a calibration line in such a manner that the reluctance of the cooperating magnetic circuit for a given wall thickness remains substantially constant over a given range of cable diameters, the shape of the contact surface 13 can be determined. obtain. Since the magnetic influence on the detection head is a function of the area and/or mass of the metal surface and the distance from the metal surface, curved surfaces of relatively small diameter must be It should be placed closer to the head than a curved surface with a larger diameter. According to the invention, a cable of smaller diameter has a contact surface 1
“Go into” or “fall down” into the shape of 3.
This is illustrated in FIG. 1, where two cables or similar structures are indicated by dashed lines 15a and 15b. The smaller diameter structures 15a touch the contact surface 13 more deeply than the larger diameter structures 15b.
It is inside the shape of.

The shape of surface 13 is obtained from calibration data.
To obtain calibration data, a flat detector surface with a flat contact surface 13 is provided with a support member as shown in FIG. 1, and a set of mandrels of different known diameters are first placed on the surface. One after the other they are positioned so that they touch, and then they are pulled apart by a predetermined small step.
In this way, a metal mandrel, e.g. . Then, the mandrel is made such that each step is
They are separated in small steps of 0.005 inch (0.127 mm) and a set of readings is obtained. The same process is performed on further mandrels, such as 0.75 inch (19.05 mm) and 1.0 inch (25.4 mm).

In this way, a table called a calibration table is obtained. From these tables it can be ascertained at what distance from the detector each of the mandrels must be positioned to give the same signal magnitude reading. These positions can be graphically represented and depicted by curves tangent to or representing each of the mandrels. This is illustrated in FIG. The position of the three mandrels is 16 at a position from the flat detector surface 17, where the magnetic influence cooperating with the oscillator circuit of the various mandrels on the detection head is the same.
a, 16b and 16c. The curves 18a and 18b correspond to the mandrels 16a and 16b.
and 16c. These two curves form plane 1 for the support member.
Form the shape of 3.

A typical shape is shown enlarged in FIG. Typical values for species parameters are; a = 0.884 inches (22.5 mm); b = 0.575 inches (14.6 mm); c =
0442 inches (11.2mm); d=0.550 inches (14.0
mm); e=0.175 inch (4.45 mm); f=0.025 inch (0.635 mm);

The curves have been described as being obtained graphically, as shown in Figure 3, but this is a relatively time consuming and not very accurate method and is provided for illustrative purposes only. . Current computer technology is capable of calculating curves 18a and 18b of FIG. 3 from the calibration data and table information referenced above. Also, once various readings are obtained with the various mandrels relative to the detector plane, a closer space calibration table can be obtained through the use of a computer.

The bottom 19 of the surface 13 is not important. It is only necessary that the bottom not touch the surface of the smallest cable being measured. Typical materials for the support member are ceramic materials having good wear resistance and low friction properties; one such ceramic material is commercially available under the trade name "Henium". Because machining is difficult, manufacturing issues tend to restrict the shape of non-critical parts of the shape.

A calibration table such as that referred to above for obtaining the shape of surface 13 can be used to create a set of curves for such a table. Figure 6 shows a set of curves, with as the horizontal axis the centering or movement of the arcuate surface along an axis perpendicular to the axis of the arcuate surface, and as the vertical axis the thickness W of the layer covering the arcuate surface. Using. The curve in Figure 6 is 0.5 inch (17.7 mm); 0.6
For equivalent mandrels of inch (15.2 mm); 0.8 inch (20.3 mm); 1.0 inch (25.4 mm); 1.2 inch (30.5 mm); 1.5 inch (38.1 mm) and 2.1 inch (53.3 mm) diameters It is. Mathematically, these curves can be very closely approximated. The corresponding equations are derived using the least squares method.

where: C = proportional term of the oscillation _frequency of the oscillator An can be considered constant.

The mathematical relationship representing the shape of the support member can be expressed as follows with reference to FIG.

Wa=√(++) ² − ² −√(+) ² − ²
......(2) Here, F = radius of the arcuate surface R = radius of curvature of the shape H = abscissa of the center of the radius of curvature of the shape W = thickness of the layer covering the arcuate surface Wa = direction perpendicular to the axis Movement of the arcuate surface in Equation (1), if X=Wa, then Here, A ₀ to An are constant within a predetermined range of the radius of the arcuate surface, and C = constant within a predetermined range of the radius of the arcuate surface for a predetermined thickness of the layer covering the arcuate surface. It is.

As shown in FIGS. 7, 8, 9 and 10, the measuring head 20 includes an elongated housing 21.
and has two probes or support members 22 and 23 inserted at each end of the haftsung 21. One of the probes is active, generates an output signal, and is coupled to a support or positioning member and a detection head. In this embodiment, probe 22 is active. The other probe 23 is inactive, forms a balanced structure, is aligned with the detection head, and acts only as a support or positioning member. The head is mounted on a structure having a sheath to be measured, for example a cable 24. Conveniently, each of the probes has a coil 25 formed on the stem 10, but only the coil cooperating with the probe 22 is incorporated into the oscillator circuit via a terminal 27. The probe 23 can be replaced by other shaped members and its purpose is solely for support, guidance and counterbalancing.

The measuring head 20 is attached to a support rod 28 via a flexible diaphragm 29. Diaphragm 29 is retained within a circular recess 30 in housing 21 by a ring 31, which in the illustrated embodiment is secured to a small screw 31a, although other retention means may be used. It is thus held in place. screw 32
extends through the center of the diaphragm into a threaded hole formed in the end of rod 28.
Washers 34 are positioned on both sides of the diaphragm. The arrangement is such that the attachment point of the measuring head to the support rod 28 is at the center of gravity of the measuring head.

The rod 28 is attached to a slider 35 that slides axially on an axle 36 extending from the main support structure 37. The rod is pivotally mounted by a pin 38 and held adjacent the pivot point by a thin shear pin or wire 39. A spring 40 extends between the fixed bracket 41 and the slider 35 and acts to force the slider toward the outer end of the rod 28. A brake 42 extends between the main support structure 37 and the slider 35 and acts to brake the movement of the slider.

The diaphragm is made of an elastomer, such as neoprene or rubber, and provides three degrees of freedom at the point of connection to the rod 28: pitch, roll and yaw. Additionally, the diaphragm provides damping and damping due to the materials used and also due to the general flexible properties of the diaphragm. Diaphragm compliance
The diaphragm does not allow the head to rotate or flop and prevents the head from following the product downstream. When the head is retracted from the cable or other object,
The diaphragm maintains the head in the correct orientation and makes it ready for return to the surface of the cable. The compactness of the diaphragm makes it possible to bring the pivot point very close to the surface on which the measuring head is engaged. This limits toppling movements which would otherwise lead to a lack of stability of the measuring head, causing it to jump and/or oscillate.

Stops or movement limiters may be provided to prevent excessive movement of the head and excessive deflection of the diaphragm. That is, as particularly shown in FIG.
5 extends in the radial direction. The end of the projection 45 travels within an enclosure 46 attached to the housing 21. In the event of extreme movement of the measuring head 20 and/or extreme deflection of the diaphragm 29, the protrusion 45 contacts the wall of the enclosure 46 and prevents further movement of the head relative to the rod 28.

The properties of the diaphragm can be varied, for example, by changing its thickness, hardness, shape of the non-constraint area and mounting location.

Although the above description refers to the positioning of a measuring head constructed as a single unit from a detector or measuring member and a probe or support member of a predetermined shape, it is also possible to separate the above-mentioned members separately. It is possible. For example, a probe or support member of a predetermined shape may be separated from a detector or measurement member. As discussed with respect to FIGS. 7-10, in the FIG. 7 arrangement, one probe is inactive, acting only as a support member. Other probes include detection or measurement means. In such an arrangement, both probes may be made inactive and the detection or measurement means may be mounted separately on the housing 21. The sensing or measuring means need not be of the dielectric type, but may be of any suitable type, and may also perform functions other than measuring or indicating the thickness of the sheath. An example of such an arrangement is illustrated in FIG. In the arrangement of FIG. 11, two support and positioning members 5
0 is held within housing 51, and detection head 52 is also positioned within the housing. Detection head 52 may be of various forms, including:
For example, it may be of a form using the dielectric principle as described above, or other forms may be used depending on the particular application. The use of a specially shaped surface on the support and positioning member 50 allows for the precise positioning of the housing 51 with respect to the arcuate surface on which the specially shaped surface is positioned, and thus the Accurate positioning of the parts is ensured. Accordingly, in its broadest concept, the present invention provides a specific method for accurately positioning other members associated with support and positioning members on an arcuate surface and for compensating for variations in the radius of the arcuate surface. A support and positioning member having a shaped surface is provided.

In the mounting position of the rod 47 of the brake 42, each end of the rod is in the form of a ball and socket joint to permit easy movement of the slider 35. Furthermore, this allows for a limited amount of lateral movement of the head.

The measuring head used and its mounting structure are, for example, hollow structures 53 made of plastic material.
surrounded by. The material of the hollow structure 53 is typically Plexiglas. The hollow structure 53 and the main support structure 37 mount the measuring head on the device. Due to some overload of the measuring head,
If shear pin 39 is sheared, the head will
8, the rod 28 is guided into the slot 43 in the hollow structure 53.

One form of apparatus incorporating a measuring head as shown in FIGS. 7 to 10 is shown in FIGS. 12 and 13.
Illustrated in the figure. Although four measuring heads 20 are provided, only three measuring heads are shown in FIG. 12. Heads 20 are supported around cable 24 at 90 degree intervals. The head is attached to the main support structure 3 by a cantilever 54 attached to a radially slidable rack 55 slidably mounted within a bracket 56.
7 and a hollow structure 53.
Bracket 56 is attached to gripping ring 57. From the grip ring 57, four shafts 5
8 is extended. Each of the shafts 58 has a pinion 59 at its forward end which engages the rack 55.

Each of the shafts 58 has a chain gear at its rear end, and a chain moved by a sector gear and a motor passes through each chain gear. FIG. 13 schematically shows the arrangement of chain 60, chain gear 61, sector gear 62 and chain gear 63 from a motor (not shown). When the motor operates, the chain gear 63 is rotated in a fixed direction or in the opposite direction,
Sector gear 62 and therefore chain 60 are moved. When the chain 60 moves, the gear 6
1, the shaft 58 is rotated, and the pinion 5
9 is rotated, and the rack 55 is moved radially into and out of the measuring head 20. Such movements occur during initial setting or stopping of the device or to avoid cable ridges. A ridge detector can be provided upstream of the device, by means of which the motor can be activated to retract the measuring head 20.

Grip ring 57 is mounted within outer ring 70 and is rotatable within this outer ring over a limited range. Grip ring 57 is provided with a gap 71 and outer ring 70 is provided with a similar gap 72. Once gap 71 and gap 72 are aligned, support 73 allows the entire device to be lowered around the cable. Of course, it is also possible to raise the entire device around the cable. The cable 24 is supported on both sides of the gripping ring 57 by movable supports 74, one of which is shown in FIG.

As mentioned above, the coil on the probe is part of an oscillator whose frequency changes due to variations in the inductive effect of the probe as a result of variations in the distance of the metal surface from a reference point on the probe. do. This variation in frequency can be utilized to generate a signal indicating variations in the thickness of the non-metallic coating to means such as a dial indicator and/or chart recorder.

A practically useful method of directing thickness variations is
The goal is to generate a display that represents a cross-section of the product being monitored.

FIGS. 14 and 15 respectively illustrate a circuit for visually displaying a pseudo cross section of a cable sheath and a displayed curved waveform.

In the detailed description below, four equivalent channels are utilized to generate four dc control voltages. Equivalent elements in each of the channels are referred to by the same reference number with an additional reference letter. However, if a specific element is not cited, only the reference number is cited.

Referring to FIG. 14, the converter includes a cable 1 having a core 111 and a sheath 112.
Functions to visually display the pseudo thickness and eccentricity of a portion of 10. The converter includes a circuit, generally designated 115, for generating a dc control voltage proportional to the thickness of the sheath 112 at four points on the circumference. Furthermore, the converter is connected to circuit 115
includes a quadrature oscillator 116 and circuit 117 for multiplying and summing d-c voltages from the oscilloscope 116 to produce an output voltage that drives the quadrature oscillator 116 and the oscilloscope 118. The detailed structure of the converter will become apparent from the following description detailing its function and operation.

In typical applications, cable 110 includes:
It is advanced between four inductive probes 120 spaced 90 degrees from each other around the cable. Inductive probe 120 is 1.25M
Frequency controlled oscillator 121 with a nominal frequency of Hz
It forms part of the frequency determining part of. The thickness of cable sheath 112 adjacent probe 120 creates varying eddy currents and/or reluctance within the probe, which causes the frequency of each of oscillators 121 to vary independently. The output signal of each oscillator 121 is sent to a discriminator 122 which produces a d-c voltage that varies about the nominal reference thickness of the cable sheath 112. Thus,
If the cable armor 112 has a thickness variation of ±20 mils, this will appear at the output of the discriminator 122 as a voltage of ±5 volts. To eliminate instantaneous variations in the thickness of the sheath 112, the varying d-c output voltage from the discriminator 122 is coupled to a sample and hold circuit 123 with a selectable sample period varying from 2 seconds to 80 seconds. Sent.

The varying d from sample and hold circuit 123
The -c output voltage is provided to the input of gate 125. Each of the gates 125 is controlled by a 60 cycle square wave oscillator 126 with a 90% duty cycle. This oscillator 126
The square wave signal from is used to open gate 126 and send the output signal of sample and hold circuit 123 to the inverting input of operational amplifier 127. The non-inverting input of operational amplifier 127 is connected to a nominal reference voltage source 128 of ±10 volts. Operational amplifier 127 also uses large negative feedback (represented by unnumbered resistors) to maintain its unity gain. The input signals applied to the inverting input of this amplifier are −10 volts and +
Can vary between 10 volts. Therefore, amplifier 12
The output signal of 7 can vary between 0 and +20 volts. These output signals are fed to the input of gate 129, which is controlled by the output of a 4KHz nominal square wave oscillator 130 with a 50% duty cycle.

The output of gate 129 is Zener diode 1
31 and series connected resistors 132 and 133 connected to a nominal voltage source 134 of +10 volts to clamp to a minimum voltage of 5 volts. Therefore, when gate 129 is open, its output is the greater of the output of operational amplifier 127 or the Zener diode voltage.

Said quadrature oscillator 116 comprises a 2KHz sine wave oscillator 140, one of whose outputs
It is sent through a phase shift circuit 141 that shifts the signal by 90 degrees. Furthermore,
Another output of oscillator 140 and phase shift circuit 141
The outputs of are each half-wave rectified by a diode 142 to generate four half-wave signal voltages. The two positive half-wave signals from diodes 142A and 142B are sent to the inputs of multipliers 143A and 143B, respectively, while diodes 142C and 14
The two negative half-wave signals from 2D are sent to the inputs of multipliers 143C and 143D, respectively. These half-wave signals are passed through gate 129 within multiplier 143.
to produce four half-wave output signals (two positive going and the other two negative going). These output signals are
It is offset by 90 degrees and is proportional to the magnitude of the signal from gate 129. The half-wave signals from multipliers 143A and 143C, which are 180 degrees out of phase with each other and have opposite polarities, are sent to the input of summing amplifier 144. Similarly, multipliers 143B and 1
The half-wave signal from 43D is sent to the input of summing amplifier 145. The outputs of summing amplifiers 144 and 145 provide the necessary Y and X drive signals to oscilloscope 118, respectively.

In order that the operation of the converter described above may be better understood, reference will now be made to FIG. 15, which shows representative waveforms displayed on oscilloscope 118. (1) to (4) below show each of these waveforms.

(1) Nominal reference thickness of the cable armor (2) Minimum allowable cable armor thickness (3) Actual cable armor thickness (4) Maximum allowable cable armor thickness For each of these waveforms, the nominal reference thickness (1) is determined by the nominal voltage source 128 and the minimum allowable thickness
(2) is determined by the voltage across the Zener diode 131, the actual thickness (3) is determined by the voltage generated by the sample and hold circuit 123,
And the maximum allowable thickness (4) corresponds to the maximum diameter of the oscilloscope image plane.

Waveform A shows the situation when the actual thickness (3) is greater than the nominal reference thickness (1). Waveform B is the actual thickness
The state when (3) is smaller than the nominal standard thickness (1) is shown. As mentioned above, the square wave oscillator 130 free-runs at a frequency approximately four times the oscillation frequency of the sine wave oscillator 140. However, when square wave oscillator 130 is synchronized to four times the frequency of oscillator 140 by closing switch 146, waveform A
and B appear as waveforms C and D, respectively.
These waveforms represent a pseudo-cross section of the cable sheath 112 with critical portions expanded to show small variations in actual thickness. This pseudo-cross section consists of four probes 1 equally spaced around the cable.
20. In a typical embodiment, the gains and magnitudes of the various voltage sources are adjusted so that the radius of the arc or circle represents the nominal reference thickness of the cable sheath ±20 mils. Therefore, circles 2 and 3 coincide when the cable armor thickness is nominally -20 mils. Conversely, when the cable sheath thickness is nominal +20 mils, the circumference of circle 3 coincides with the periphery 4 of the oscilloscope's image plane. Similarly, when the actual thickness of the cable armor matches the required nominal reference thickness,
Circles 1 and 3 match. In this way, the operator can judge whether the sheath 112 attached to the cable core 111 is larger than or smaller than a predetermined size and whether the sheath is eccentric or not by just looking at the image on the oscilloscope 118. can do.

The waveform shown in FIG. 15 is developed as follows. When gate 129 is closed, the output of gate 129, which is applied to multiplier 143, is returned to +5 volts across Zener diode 131. Since the square wave oscillator 130 has a duty factor of 50%, this forms a relatively strong trajectory 2. When gate 129 is open, sine wave oscillator 1
The half-wave voltage obtained from 40 is multiplied by the varying d-c voltage obtained from operational amplifier 127. During the 10% of the time that gate 125 is closed and its output is zero, the output of amplifier 127 is taken from voltage source 128 and is therefore +10 volts giving a nominal reference thickness (1). However, during the remaining 90% of the time that gate 125 is open, amplifier 127
The output voltage of is the output voltage of the circuit 123 and the voltage source 12
There will be a voltage difference between 8 and 8. The output voltage of the circuit 123 is
The output of amplifier 127 can vary between 0 and +20 volts as it can vary within a range of -10 to +10 volts representing the range of cable armor 112 thicknesses. However, since the output of gate 129 is clamped to a minimum voltage of +5 volts by Zener diode 131, the output cannot fall below this minimum voltage. This represents a -20 mil tolerance for cable armor 112. Conversely, if the output voltage of amplifier 127 is +15 volts, this corresponds to the maximum diameter of the oscilloscope's image plane, which is +20 mils above the nominal reference thickness of cable sheath 112.
The 90% impulse factor of the square wave oscillator 126 is chosen such that the intensity at the nominal reference thickness (1) displayed on the oscilloscope is much smaller than the intensity at the actual thickness (3).

In this example, four separate circuits 115 are used to form the dc sample and hold voltages that control the quadrature oscillator 116. In another embodiment, the output is the sample and hold circuit 12
The probe 120 can be switched periodically to one oscillator via one discriminator that is switched to three at the same time. In yet another embodiment, rather than using four probes 120, only one probe 120 can be rotated around cable 110. Also, the output of the single discriminator is periodically switched to the sample and hold circuit 123 as the probe rotates.

[Brief explanation of the drawing]

FIG. 1 is a side view of a specific example of a measurement head probe. FIG. 2 is a plan view of the contact surface of the probe of FIG. 1; FIG. 3 is a diagram schematically showing a method for obtaining the shape of the probe. FIG. 4 is an enlarged view of the shape of the probe. FIG. 5 is an enlarged view of the shape with various parameters attached. FIG. 6 is a diagram showing a series of calibration curves. FIG. 7 is a perspective view of the measuring head and its associated support structure. FIG. 8 is a longitudinal sectional view of the head, support structure and hollow structure. Figure 9 shows
9 is a bottom view of the measuring head seen from the direction of arrow A in FIG. 8; FIG. FIG. 10 is a sectional view taken along line XX in FIG. 9. FIG. 11 is a longitudinal sectional view of a modification of the measuring head. FIG. 12 is a perspective view of a specific example of an apparatus incorporating the probe of the form shown in FIGS. 1 to 4. FIG. 13 is an end view of the device of FIG. 12 showing the actuation gear. FIG. 14 is a block circuit diagram of a converter for visually displaying the thickness and eccentricity of a cable sheath in a pseudo cross section. FIG. 15 is a diagram showing typical waveforms displayed on an oscilloscope supplied with signals from the converter shown in FIG. 14. 10...Stem part, 11...Head part, 2
1 and 54...housing, 53...hollow structure, 24...cable, 29...diaphragm,
36... Mandrel, 50... Support member, 52... Detection head.

Claims (1)

Claims: 1. A device for measuring and indicating the thickness of a non-metallic coating on a metallic arcuate surface, comprising: at least one detection head located opposite the coating; a support member supporting the detection head; a detector in each detection head adapted to emit a signal indicative of the distance between the detector and the metal arcuate surface; is V-shaped with convex arcuate sides when viewed parallel to the axis of the mandrel, the arcuate sides initially aligning the axis of the mandrel so that each mandrel receives a reference signal of the same magnitude from the detector. formed by sequentially positioning a series of cylindrical metal mandrels of different diameters lying in a common plane perpendicular to the detector plane, each arcuate side tangential to the outer circumference of each mandrel. A device for measuring the thickness of a coating on a metal arc-shaped surface by comparing a reference signal with a detector signal caused by variations in thickness. 2. The device of claim 1, wherein the detection head is positioned between two support members. 3. Device according to claim 1 or 2, in which the detection head is integrated into a support member. 4. The device of claim 1, wherein the detector comprises an electrical induction device forming part of an oscillating circuit. 5. The device of claim 4, wherein the guiding device has a hole in the direction of the axis of the detector and has a tuning element in the hole. 6. A plurality of sensing heads spaced around a cable or other similar structure for measuring and indicating the thickness and eccentricity of a non-metallic coating on a metal surface of the cable or other similar structure. The device according to item 1. 7 a gripping ring arranged around the axis of the cable, a plurality of brackets installed on the gripping ring, a bracket for each of the detection heads, and supporting the detection head with respect to each bracket; Claim 6 comprising an installation means for
Apparatus described in section. 8. The mounting means of each of the sensing heads comprises a cantilever beam connected at one end to a bracket and at the other end to an axle of the main support structure, the axle extending radially towards the axis of the cable. 7. The device according to claim 7. 9 includes a rack slidable in the radial direction of the gripping ring in each bracket, to which the cantilever is attached, and further for sliding the rack in the radial direction of the gripping ring to detect the detection head 9. Apparatus according to claim 8, including means for radially moving. 10. The apparatus of claim 8, further comprising a hollow structure surrounding each of the mandrel and the detection head, a slider for sliding in the axial direction of the mandrel, and a damping means for damping the movement of the slider. 11. The apparatus of claim 6 including means for displaying the signal from the detection head. 12 means for generating, in response to the output from the sensing head, a d-c voltage proportional to the constant thickness of the coating at each sensing head, and four half-wave voltages offset by 90° from each other; a quadrature oscillator, means for applying a d-c voltage to each of the four half-wave voltages to generate four half-wave control voltages, and a means for driving the x and y inputs of the oscilloscope;
and means for generating two full-wave control voltages by applying the half-wave control voltages offset by 180 degrees. 13. Apparatus according to claim 12, including first means for periodically switching each of the separate d-c voltages to a predetermined first reference level representative of the minimum allowable thickness of the coating. 14. The apparatus of claim 13 including second means for periodically switching each of the separate d-c voltages to a predetermined second reference level representative of the normal reference thickness of the coating. 15 The V-shape is expressed by Wa=√(++) ² −√(+) ² − ² , where Wa is the displacement of the arcuate surface in the direction perpendicular to the axis of the arcuate surface, and W is the displacement of the arcuate surface in the direction perpendicular to the axis of the arcuate surface. Claim 1, wherein the thickness of the non-metallic coating, H is the abscissa of the center of the radius of curvature of the V-shape, R is the radius of curvature of the V-shape, and F is the radius of the arcuate surface. Apparatus described in section.