GB2040470A

GB2040470A - Differential Transformer Metering Displacement

Info

Publication number: GB2040470A
Application number: GB7941703A
Authority: GB
Original assignee: SANDRETTO SpA
Current assignee: SANDRETTO SpA
Priority date: 1978-12-19
Filing date: 1979-12-03
Publication date: 1980-08-28
Also published as: DE2948440A1; JPS5583801A; IT1108522B; FR2444924B1; IT7869893A0; NL7908848A; FR2444924A1; ES487083A1; BE880723A

Abstract

An apparatus for determining the position of a mobile member (12) relative to a fixed support member (11) comprises a transformer (21) containing respectively a ferromagnetic core (13) mobile with said mobile member (12), a primary winding (20) which can be connected to supply means, and a secondary winding (19) which can be connected to processing means, said primary and secondary windings being mounted mechanically rigid with said fixed support member (11) and being disposed coaxial to each other and wound in solenoid form so as to define in their interior a tubular duct arranged to receive said mobile core (13). <IMAGE>

Description

SPECIFICATION Apparatus for Determining the Position of a Mobile Member Relative to a Fixed Support Member This invention relates to an apparatus for deterining the position assumed by a mobile member relative to a fixed support member.

In particular, this invention relates to an apparatus for determining the position assumed by a mobile carriage of a plastics moulding machine, of a machine tool or of a hydraulic press etc., relative to a fixed support part on the machine.

In carrying out the aforesaid position determination, certain problems have generally to be solved which are not strictly connected with the accuracy with which the determination is required to be made, but derive instead from the working conditions under which the apparatus is required to operate. In this respect, such apparatus are constantly subject to considerable vibration and often have to operate in the presence of dust and/or liquid lubricant spray.

Optical measuring apparatus are at present commercially available comprising photodetector elements (photodiodes, phototransistors etc.) arranged to cooperate with corresponding luminous tracks. These photodetector elements and luminous tracks are disposed respectively on two members moving relative to each other, and cooperate in such a manner as to supply electrical signals proportional to the relative position of the two said members to each other.

Potentiometric measuring apparatus are also known, in which the movements of the mobile member are transmitted to a potentiometer slider, and consequently generate signals having an amplitude proportional to the extent of the movements.

The aforesaid optical or potentiometric apparatus have certain drawbacks which make their continuous use in a workshop not particularly advantageous. In this respect, the elements which generate the electrical signals are in the case of both types always in movement (photodiodes etc., or the potentiometer slider) as they are mounted on the mobile member. They are therefore subject to wear and consequently require continuous maintenance, this being aggravated by the fact that the action of the dust or liquid lubricants can change their technical characteristics.

Furthermore, the cost of such apparatus is always high, such as not to make it possible to use them on a large scale.

The aforesaid drawbacks are partly obviated by a further measuring apparatus which is constituted substantially by a differential transformer with its primary and secondary windings connected to a fixed part of the machine, and a ferromagnetic core connected to the mobile member. This core couples the transformer windings in such a manner as to give an output signal depending on the position of the mobile member relative to the fixed part. However, in such a case, the transformer has to have a length which is double the stroke to be measured, because of which, when the stroke is of the order of one or two metres, the transformer structure is excessively bulky.

The object of the present invention is to provide an apparatus for determining the position of a mobile member relative to a fixed support member, which is free from the aforesaid drawbacks of known apparatus.

The present invention provides an apparatus for determining the position of a mobile member relative to a fixed support member, comprising a transformer containing respectively a ferromagnetic core mobile rigidly with said mobile member, a primary winding which can be connected to supply means, and a secondary winding which can be connected to processing means, said primary and secondary windings being mounted mechanically rigid with said fixed support member and being disposed coaxial to each other and wound in solenoid form so as to define in their interior a tubular duct arranged to receive said mobile core; each position of said mobile member relative to said fixed member determining a corresponding coupling factor between said primary and secondary windings, and a consequent voltage signal obtainable from said secondary winding which is directly proportional to said relative position.

The present invention will be more apparent from the description of a preferred embodiment given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which: Figure 1 is a sectional elevation of the sensor element of the apparatus according to the invention; Figure 2 is a partial perspective enlarged view of a detail of Figure 1; and Figure 3 shows the functional electrical schematic of an apparatus constructed according to the present invention.

Figure 1 shows a fixed plate 11 and mobile plate 1 2 connected respectively, in a manner not illustrated, to a support member and a mobile carriage of a moulding machine.

The mobile plate 12 is arranged to move perpendicular to the plate 11, and supports a tubular ferromagnetic element 13 mounted perpendicular to the plate 1 2 on the side facing the plate 11, and comprising a longitudinal slot 14. During the movement of the plate 12 relative to the plate 1 the tubular element 13 slides in a through bore 1 6 in the plate 11, and in a tubular member 17 constructed for example of bakelite and having one end fixed to the plate 11, in a manner not shown.

A secondary winding 1 9 and a primary winding 20 are wound coaxially about the tubular member 1 7 so that they become coupled together in order to constitute a transformer 21. The secondary winding 1 9 is wound uniformly from the first end to the second end of the tubular member 17, whereas the primary winding 20, starting from the first end of the tubular member comprises a first portion 22 wound in a first direction, for example clockwise, and a second portion 23 connected in series electrically with the portion 22 but wound in the opposite direction.

The secondary winding 1 9 and primary winding 20 of the transformer 21 become coupled magnetically by way of the tubular element 13, which constitutes substantially a mobile core, and can move so that coupling takes place only between the portion 22 of the primary winding 20 and a corresponding facing portion of the secondary winding 19. In addition, the portions 22 and 23 have a number of turns such as to generate in air two respective magnetic fluxes of opposite value. Finally the portion 22 has a winding pitch which compensates any response nonuniformity of the transformer 21, and gives a substantially linear transfer function (movement of the core 13, voltage induced across the secondary winding 19) between the primary winding 20 and secondary winding 19.

Around the tubular member 1 7 and secondary winding 19 and primary winding 20 there is disposed a jacket core 24, which is connected to the plate 11 by its flange portion 25, and is constructed of a ferromagnetic material in order to close externally the magnetic flux generated by the transformer 21. A screw 27 is screwed into the end wall 26 of the jacket core 24 at the other end to the flange portion 25, and carries at one end an auxiliary magnetic core 28. This latter is disposed in the second end of the tubular member 1 7 in a position corresponding with the second portion 23 of the primary winding 20, and its position can be adjusted axially by the screw 27 to give a widely adjustable coupling coefficient between the portion 23 and the facing portion of the secondary winding 19.

Figure 3 shows a functional schematic diagram relating to the supply to the transformer 21 and the processing of the electrical signals produced thereby. A signal is fed to the connection terminals 31 and 32 of the primary winding 20 from an oscillator 33, preferably a sinusoidal oscillator having an output frequency of the order of some units of KHz. Connection terminals 34 and 35 for the secondary winding 1 9 are connected respectively to a first and second input of a rectifier circuit 36, of which a first output is connected to earth and a second output connected to the input of an amplifier 37, particularly a d.c. voltage amplifier, provided with means for adjusting the gain and zero level of the output signal.The amplifier 37 is also connected to earth and its output is connected to a connection terminal 39 to which is connected a first terminal of a d.c. voltage indicator, e.g. a moving coil voltmeter, the second terminal of which is connected to earth.

The transformer 21, oscillator 33, rectifier circuit 36, amplifier 37 and indicator 40 overall constitute an apparatus 44 for determining and indicating the position assumed by the mobile plate 12 relative to the fixed plate 11. The operation of the apparatus 44 is as follows.

The apparatus is firstly calibrated in order to establish a relationship between the positions assumed by the plate 12 relative to the plate 11, and the respective d.c. voltages measured at the terminal 39 by the indicator 40. This is done by withdrawing the plate 1 2 from the plate 11 so that the free end of the tubular core 13 is in the bore 1 6 in the plate 11, in a position such as not to have any magnetic influence on the primary winding 20 and secondary winding 19 of the transformer 21. These windings are therefore aircoupled, and any signal fed to the primary winding 20 by the oscillator 33 induces in the portions 22 and 23 of the primary winding two respective magnetomotive forces of equal and opposite value.The flux generated by the primary winding 20 is therefore zero, and consequently no electrical signal is measured at the terminals 34 and 35 of the secondary winding 19 by the instrument 40. If this electrical signal is not exactly zero, the reading on the instrument 40 can be zeroed by adjusting the axial position of the magnetic core 28 by means of the support screw 27. This varies the coupling coefficient between the portion 23 of the primary winding 20 and the corresponding portion of the secondary winding 19, until the two said magnetomotive forces have been completely compensated.

If the plate 12 now approaches the plate 11, the mobile core 1 3 becomes inserted into the tubular member 17 to give rise to a closer coupling between part of the portion 22 of the primary winding 20 and a corresponding part of the secondary winding 1 9 by way of the tubular core 1 3. Consequently, the magnetomotive force deriving from the portion 22 becomes greater than the amount deriving from the portion 23, and thus an alternating electrical signal is available at the terminals 34 and 35 of the secondary winding 19 and has an amplitude proportional to the axial portion of the core 13 inserted into the tubular member 17.This signal isrectified and amplified in the rectifier circuit 36 and amplifier respectively, and is fed to the terminal 39 and measured by the indicator 40, the scale of which can be directly calibrated in mm or cm of movement of the plate 1 2 relative to the plate 11. As the portion 22 of the primary winding 20 has a pitch which substantially allows a linear transfer function between the primary terminals 31 and 32 and secondary terminals 34 and 35 to be obtained as the axial position of the tubular core 30 varies, the voltage measured at the terminal 39 varies linearly from 0 to a maximum value, for example 10 Volts, as the mobile plate 12 moves from a maximum distance to a minimum distance from the fixed plate 11.

From an examination of the characteristics of the present invention, it is apparent that the measuring and indicating apparatus 44 attains the aforesaid objects.

In this respect, if it is assumed that the frequency of the signal fed by the oscillator 33 is 1 KHz and the plate 12 has a speed of one metre per second relative to the plate 11, it is possible to determine the movements of the plate 12 with an accuracy of the order of 0.5 mm, which corresponds to the average accuracy required by the machines to which the apparatus 44 can be applied.

In comparison with known apparatus comprising a differential transformer, the jacket core 24 of the transformer 21 extends longitudinally substantially only over the same length as the stroke to be measured, the overall size therefore being reduced to one half that of said apparatus. Besides closing the magnetic flux generated by the primary winding 20, the core 24 also acts as a screen against any electrical disturbances which could be picked up by the windings of the transformer 21 and introduce measurement errors. Finally, the jacket core 24 offers good mechanical protection to the windings of the transformer 21 against any knocks which the transformer could receive during the normal use of the machine to which it is fitted.

The tubular core 1 3 can be constructed of any length, and for example of normal soft iron, so that its cost is very low. The longitudinal slot 14 considerably reduces losses due to parasite currents introduced by said core, and improves overall efficiency.

As the relative movements to be measured increase in length from a few tens of centimetres to a few metres, no substantial modifications are required to the winding system for the primary winding 20 and secondary winding 19 of the transformer 21, or to the various associated electronic supply and measuring devices. This is because as the length of movement to be measured changes, the diameter and number of turns of the conducting wires for said windings can be varied such that at the terminals 31 and 32 of the primary winding 20 there is always the same input impedance, and at the terminals 34 and 35 of the secondary winding 19, there is not only always the same output impedance, but there is also a voltage which when rectified and amplified always varies between the same minimum value (e.g. 0 V) and the same maximum value (e.g. 10 V).In this manner, it is possible to use the same type of oscillator, rectifier circuit, amplifier and indicator for any length of movement of the apparatus 44.

As the amplifier 37 is of adjustable gain, and can also be provided with means for adjusting its zero level, it is possible to adapt a measuring apparatus provided for measuring one particular movement, for example one metre, so that it can measure a smaller movement length, for example half a metre, or alternatively preferably use an intermediate part of the transfer function for measuring movements lying for example between 20 cm and 80 cm. By suitably adjusting the gain of the amplifier 37 and compensating its output signal zero level, it is possible to obtain at the terminal 39 a direct current electrical signal having a value always lying between the same minimum and maximum values and measurable by the indicator 40.

In comparison with optical or potentiometric measuring apparatus, the measuring apparatus 44 according to the present invention does not comprise electrical signal generating elements which are continuously in movement.

Consequently no wear occurs, which would require specific maintenance.

As the system by which the primary winding 20 and secondary winding 1 9 are wound is substantially the same whatever the movement length to be measured, and as it is not necessary to construct any part of the measuring apparatus 44 with a high degree of precision, the costs of the measuring apparatus 44 are low.

Finally, modifications can be made to the embodiment of the present invention without leaving the scope of the inventive idea.

Claims

1. An apparatus for determining the position of a mobile member relative to a fixed support member, comprising a transformer containing respectively a ferromagnetic core mobile rigidly with said mobile member, a primary winding which can be connected to supply means, and a secondary winding which can be connected to processing means, said primary and secondary windings being mounted mechanically rigid with said fixed support member and aing disposed coaxial to each other and wound in solenoid form so as to define in their interior a tubular duct arranged to receive said mobile core; each position of said mobile member relative to said fixed member determining a corresponding coupling factor between said primary and secondary windings, and a consequent voltage signal obtainable from said secondary winding which is directly proportional to said relative position.

2. An apparatus as claimed in claim 1, wherein said primary winding of said transformer has a first and second portion which are wound in opposite directions and disposed axially adjacent; said first portion being wound with a winding pitch substantially less than said second portion, and said mobile core being able to be inserted into said tubular duct only over a first length corresponding to the axial extension of said first portion.

3. An apparatus as claimed in claim 2, wherein said first and second portion generate, in air, magnetomotive forces of equal and opposite sign, and that said voltage signal obtainable from said secondary winding is substantially zero when said mobile core is outside said first portion of said primary winding.

4. An apparatus as claimed in claim 2 or 3, wherein an auxiliary ferromagnetic core, of axially adjustable position, is disposed in said tubular duct in a position corresponding with said second portion of said primary winding.

5. An apparatus as claimed in one of the preceding claims, wherein said mobile core is of tubular structure.

6. An apparatus as claimed in claim 5, wherein said core of tubular structure comprises an axial slot.

7. An apparatus as claimed in one of the preceding claims, wherein said solenoid-wound primary and secondary windings are disposed inside a ferromagnetic tubular jacket core.

8. An apparatus as claimed in one of the preceding claims, wherein said supply means for said primary winding comprise an oscillator circuit.

9. An apparatus as claimed in claim 8, wherein said oscillator circuit provides at its output an electrical signal having a frequency of the order of units of KHz.

10. An apparatus as claimed in claim 8 or 9, wherein said oscillator is sinusoidal.

11. An apparatus as claimed in one of the preceding claims, wherein said processing means comprise a rectifier circuit for said voltage signal obtainable from said secondary winding.

12. An apparatus as claimed in claim 11, wherein said processing means comprise an amplifier circuit for direct current signals disposed in series with the output of said rectifier circuit and comprising gain adjustment means.

13. An apparatus as claimed in claim 12, wherein said amplifier circuit is provided with means for adjusting the output signal voltage level.

14. An apparatus for determining the position of a mobile member relative to a fixed support member, as described with reference to the accompanying drawings.