Apparatus containing a cylinder with a displaceable piston therein and a measuring transducer arranged essentially within the cylinder

An apparatus containing a cylinder with a piston displaceably guided therein and positionally adjustable by a hydraulic fluid medium. A capacitive measuring transducer is arranged in the cylinder. The measuring transducer comprises a tube attached at the piston and arranged in a longitudinal opening thereof. The tube is electrically conductively connected with the cylinder. Apart from the tube, the measuring transducer further comprises a rod protruding into the tube and insulatingly secured at the cylinder. This rod is connected by means of an electrical bushing or lead with the input of an electronic unit attached externally at the cylinder. The electronic unit comprises an oscillator having a connection electrically coupled with the cylinder and a connection coupled with the non-inverting input of a capacitive feedback differential amplifier. The inverting input of the differential amplifier is electrically connected with the rod which is secured so as to be electrically insulated at the cylinder.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus containing a cylinder, a 
piston displaceable in the cylinder, and a measuring transducer arranged 
essentially within the cylinder. 
In its more specific aspects, the invention relates to an apparatus 
comprising a cylinder, a piston displaceably guided in such cylinder, the 
piston being positionally shiftable by a suitable flow medium and having 
an elongate or longitudinal opening. Further, there is provided a 
measuring transducer arranged essentially within the cylinder and having 
two parts which are displaceable towards one another, one such part being 
attached at the cylinder and protruding into the longitudinal opening of 
the piston, and the other such part being secured at the piston or being 
constituted by the piston itself. 
Such cylinder arrangements are used, by way of example, for lifting loads, 
for shutting off closures or for positioning tools of building and machine 
tools. 
Swiss Pat. No. 488,999 and the corresponding U.S. Pat. No. 3,654,549, 
disclose an apparatus containing a cylinder and a measuring transducer 
installed within such cylinder. With one embodiment of the 
state-of-the-art equipment there is used an inductive measuring transducer 
having one part secured at the cylinder and the other part at the piston. 
The part of the measuring transducer which is secured at the cylinder 
comprises two hollow tandemly arranged coils disposed essentially 
coaxially with respect to the cylinder. The measuring transducer part 
attached at the piston comprises a ferromagnetic core, which, depending 
upon the position of the piston, penetrates to a greater or lesser extent 
into both coils. Both of these coils are connected with an 
alternating-current source and a bridge circuit, from which there can be 
tapped-off a signal constituting a measure for the piston position. 
The coils required for the heretofore known equipment are relatively 
expensive. This is particularly then the case if the piston has a large 
displacement stroke. A further drawback of the prior art equipment resides 
in the fact that, at least three coil connections must be led out of the 
cylinder by means of electrically insulating bushings or equivalent 
structure. These bushings are particularly then expensive and prone to 
disturbance if large pressures are used in the cylinder. It is furthermore 
to be mentioned that an apparatus working with inductive measuring 
transducers can be sensitive to external magnetic fields and ferromagnetic 
parts of other machines, so that measurement errors can easily arise. 
SUMMARY OF THE INVENTION 
Therefore, it is a primary object of the present invention to provide 
apparatus of the previously mentioned type which is not susceptible to the 
aforementioned drawbacks and limitations of the prior art constructions. 
Another and more specific object of the present invention aims at the 
provision of an apparatus which can be fabricated at low costs, is 
insensitive to disturbing or extraneous influences caused by other 
equipment and machines, and wherein particularly there is only required a 
single electrically insulated bushing or lead. 
Now in order to implement these and still further objects of the invention, 
which will become more readily apparent as the description proceeds, the 
apparatus of the present development comprises a cylinder and a piston 
having an elongate or longitudinal opening and displaceably guided in such 
cylinder. The piston can be positionally shifted within the cylinder by 
means of a suitable flow medium. Further, a measuring transducer is 
arranged essentially within the cylinder. This measuring transducer 
comprises two parts or elements which can be displaced towards one 
another. One of these parts is attached at the cylinder and protrudes into 
the longitudinal opening of the piston, whereas the other part is secured 
at the piston or formed by the piston itself. The measuring transducer is 
a capacitive measuring transducer. One of both parts of the measuring 
transducer is electrically conductively connected with metallic parts of 
the cylinder and piston and the other of the two parts of the measuring 
transducer is provided with an electrical connection leading out of the 
cylinder and electrically insulated with respect to the cylinder and the 
piston. 
Since the one electrode of the measuring transducer is electrically 
conductively connected by means of the piston and the cylinder with the 
electronic unit, only a single electrically insulated bushing or lead, or 
equivalent structure, must be led out of the interior of the cylinder. The 
cylinder, essentially formed of metal and connected with the ground 
terminal or connection of the electronic unit, additionally forms a 
Faraday cage which extensively eliminates disturbances caused by external 
or foreign electrical fields. The capacitance of the measuring transducer 
therefore can be measured while using an alternating-current having a 
relatively low frequency. This, in turn, again renders possible measuring 
the capacitive changes by means of a capacitive negative feedback 
differential amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Describing now the drawings, in FIG. 1, reference character 1 designates a 
cylinder and reference character 3 a piston displaceably guided within 
such cylinder 1. The cylinder 1 essentially consists of metallic, 
electrically conductive parts or elements, and, in particular, contains a 
metallic jacket or shell 5 which is provided at both opposed ends with 
likewise metallic caps or closures 7 and 9. Inserted into the cap or 
closure 7 is a metallic bushing 11 equipped with different not 
particularly referenced sealing rings and forming a sealed sliding guide 
for the metallic piston rod 13 at the piston 3. Threaded into the cap or 
closure 9 is a metallic bushing 15 possessing external threading or 
threads. This bushing 15 is provided with a collar 15a and by means of the 
latter, in conjunction with a seal 16, is tightly sealed with respect to 
the cap 9. The longitudinal or elongate opening 15b of the bushing 15 is 
provided at its end confronting the internal chamber or compartment of the 
cylinder 1 with a widened portion 15c. Further, the caps 7 and 9 are 
provided with connection openings 7a and 9a, respectively, for the infeed 
and withdrawal of an electrically insulating flow medium, for instance, 
hydraulic oil, which serves to pressure impinge the piston 3 and to 
displace the latter. It is possible to work, by way of example, with a 
pressure up to 300 bar. 
At the end of the piston rod 13 located internally of the cylinder 1, there 
is sealingly secured a metallic ring 17 by means of a nut 19 threaded onto 
the rod 13, a spring ring 21 and a seal 22. The ring 17 forms the head of 
the piston 3 and is displaceably and sealingly guided within the jacket 5. 
Ring 17 is provided with a recess at the region of its cylindrical sliding 
surface, within which there is arranged a resilient sliding contact 18 
forming an electrically conductive connection between the ring 17 and the 
jacket 5. The piston 3, or more exactly stated, the rod 13, is equipped 
with a continuous longitudinal or elongate opening 13a formed by a bore 
coaxially arranged with respect to the cylinder jacket 5. Within the 
longitudinal opening 13a there is arranged a metallic, electrically 
conductive cylindrical tube or pipe 23 which is coaxially arranged with 
respect to the cylinder jacket 5. This tube 23 has, in particular, an 
exactly cylindrical inner surface. The one end of the tube 23 is flushly 
connected with the inner piston end, i.e., with that end of the rod 13 at 
which there is secured the ring 17. The other end of the tube 23 is 
provided with a plug 25 or equivalent structure, which is threaded into 
the outer end of the rod 13 in a widened portion of the longitudinal 
opening 13a and tightly closes the same. The tube 23 is provided at the 
region of its attached end with at least one opening for the pressure 
equalization of the hydraulic fluid medium. The tube 23 forms the one 
electrode of a capacitive measuring transducer and is electrically 
conductively connected with the piston 3 which is metallic, except for the 
sealing rings, and by means of such piston 3 conductively connected with 
the metallic parts of the cylinder 1 and particularly the cylinder jacket 
5. 
The other electrode of the capacitive measuring transducer is formed by a 
metallic, electrically conductive, round rod 27 arranged within and 
essentially coaxially with respect to the cylinder 1. This round or 
circular rod 27 is provided with a jacket or sheath 29 formed of 
electrically insulating, fluoro-containing plastic, for instance 
polytetrafluoroethylene. The one end of the rod 27 is seated in widened 
portion 15c of the bushing longitudinal opening 15b, as particularly 
clearly apparent from the showing of FIG. 2. The rod 27 is provided with 
an essentially ring-shaped groove 31a at that location where it protrudes 
out of the bushing 15. Into this groove there is inserted a spring ring 31 
which, in turn, is retained by a metallic ring 33 which is fixedly 
threaded by screws or equivalent fastening expedients at the bushing 15. 
The jacket or sheath 29, which extends up to the one end face of the rod 
end seated in the bushing 15, insulates the rod 27 with respect to the 
bushing 15 and also the spring ring 31 and at the same time serves as a 
seal. Hence, the rod 27 is secured so as to be immobile and sealingly at 
the cylinder 1 and is electrically insulated with regard to such cylinder. 
Rod 27 protrudes into the tube or pipe 23 and extends up to the region of 
the end of the cylinder jacket 5 facing away from the cap or closure 9. 
The rod 27, for each possible position of the piston 3, thus partially 
protrudes into the longitudinal opening 13a and the pipe 23. The jacket 29 
fixedly seated upon the outer surface of the rod 27 and the inner surface 
of the tube 23 collectively form a sliding seat which is essentially free 
of play. During operation of the cylinder 1 hydraulic oil or any other 
suitable fluid medium flows between the jacket 29 and the inner surface of 
the tube 23, so that there is formed an oil film. The dielectric of the 
capacitive measuring transducer is formed by the jacket or sheath 29 and 
this oil film. The jacket or sheath 29 is constructed in such a fashion 
that its thickness is approximately 3 to 10 times greater than that of the 
oil film. Between the outer surface of the tube 23 and the inner surface 
of the longitudinal opening 13a, while discounting the attachment location 
at the plug 25, there is present a free ring-shaped or annular gap. 
Furthermore, the tube 23 consists of a relatively thin-wall, resiliently 
flexible material, so that it can be automatically centered by the rod 27. 
At the cylinder 1 there is attached an electronic unit or electronic 
portion 43 having a sealed metallic housing 44, the attachment being 
accomplished, for instance, by screws 39 or equivalent fastening devices 
which threadably connect the electronic unit 43 at the bushing 15. At that 
location where the edge of the electronic unit 43 bears against the end 
surface of the cap 9, there is arranged a sealing ring 37. The one end of 
an electrically conductive pin 35 is secured at the rod 27. This pin 35 
piercingly extends through the bushing 15 without contact and, while 
insulated with respect to the housing 44, extends into the electronic unit 
43. This electronic unit 43 is connected, as will be explained more fully 
hereinafter, by means of a two-wire cable 50 with further elements. 
The tube 23 attached at the piston 3 and the rod 27 secured to the cylinder 
1, as already mentioned, collectively form a capacitive measuring 
transducer whose capacitance is dependent upon the piston position. If the 
piston 3 is located in the illustrated position, where it is introduced as 
far as possible into the cylinder 1, then the capacitance of the measuring 
transducer assumes its maximum value. In the other piston position the 
capacitance of the measuring transducer, to which there is added also the 
capacitance of the lead or bushing, assumes its minimum value. If the 
piston 3 is displaced, in the showing of FIG. 1, from the left towards the 
right, then the capacitance of the measuring transducer linearly 
increases. The electronic unit 43, during operation, produces an 
electrical signal, namely an impressed current constituting a measure for 
the piston position and the piston displacement. 
The construction and mode of operation of the electronic unit 43 will now 
be explained on the basis of the circuit shown in FIG. 3. Here, reference 
character 41 schematically represents the capacitive measuring transducer. 
The electrode of the measuring transducer 41, formed by the tube 23, is 
electrically conductively connected with the cylinder 1 by means of the 
piston 3 and, in particular, the sliding contact 18. The cylinder 1, in 
turn, is connected by means of the screws 39 or the like with the metallic 
housing 44 and with the ground connection 45 of the electronic unit 43. 
The electrode of the measuring transducer 41, and which electrode is 
formed by the rod 27 attached at the cylinder 1, is conductively 
connected, by the pin 35, with the input or measuring connection 47 of the 
electronic unit 43. The input connection 47 is coupled by means of a 
capacitor 49 with the inverting input 51a as well as by means of a 
feedback capacitor 53 with the output 51c of a differential amplifier 51. 
The inverting input 51a and the output 51c are furthermore connected by 
means of a high-ohm resistance 55 with one another and which in 
conjunction with the capacitor 49 serves for stabilizing the operating 
point. 
The non-inverting input 51b of the differential amplifier 51 is connected 
with the output 57a of an oscillator 57 which also has an input and output 
connection 57b coupled with the ground connection 45. The output 57a of 
the oscillator 57 is furthermore connected by means of an amplifier 59 and 
a capacitor 61 with the input connection or terminal 47. 
The output 51c of the differential amplifier 51 is connected by means of a 
resistor 63 with the non-inverting input 65b of a differential amplifier 
65. The input 65b is furthermore connected by means of a resistor 67 with 
the ground connection or terminal 45. The inverting input 65a of the 
differential amplifier 65 is connected by means of a resistor 69 with the 
oscillator output 57a and by means of a resistor 71 with the differential 
amplifier-output 65c. The resistors or resistances 63 and 69 have the same 
resistance value. The resistors 67 and 71 likewise have the same 
resistance value, and the latter amounts to, for instance, the ten-fold of 
the resistance value of the other resistances 63 and 69. 
The inverting input 73a of a differential amplifier 73 is connected by 
means of a resistor 75 with the differential amplifier-output 65c, by 
means of a trimmer resistor 77 with the oscillator output 57a and by means 
of a trimmer resistor 79 with the differential amplifier-output 73c. The 
non-inverting input 73b of the differential amplifier 73 is electrically 
speaking at ground. 
The differential amplifier-output 73c is connected by means of two 
resistors 81 and 83 having the same resistance value, with the inverting 
input 85a and the non-inverting input 85b, respectively, of a differential 
amplifier 85. The inverting input 85a is connected by means of a feedback 
resistance 87, having the same resistance value as the resistors 81 and 
83, with the differential amplifier-output 85c. The non-inverting input 
85b is connected by means of the source-drain path of a field-effect 
transistor 89 with the ground connection 85. The gate of the field-effect 
transistor 89 is connected with the output 57c of the oscillator 57. 
The differential amplifier-output 85c is connected with the input 89a of a 
voltage-current converter 89. The connection or terminal 89b of the 
converter 89 is coupled with the ground connection 45 and its output 89c 
with a current supply device 91 as well as with the connection 93 of the 
electronic unit 43. The connection 95 of the electronic unit 43 is 
likewise connected with the current supply device 91 and with the ground 
connection 45. The current supply device 91 is furthermore connected by 
the lines or conductors, which have been generally indicated schematically 
by the arrows, with the different active elements of the electronic unit 
43 and supplies thereto, during operation, the requisite supply voltages. 
The connection 93 is coupled by means of a line or conductor of the cable 
50 and an indicator or display instrument 97 with the one terminal or 
connection of a direct-current voltage source 99, the other terminal of 
which is connected by means of the other line or conductor of the cable 50 
with the connection or terminal 95. As will be further more fully 
explained hereinafter, the connections or terminals 93 and 95, on the one 
hand, form the signal outputs and, on the other hand, the supply voltage 
connections of the electronic unit or section 43. 
In the description to follow there will be explained the mode of operation 
of the electronic unit 43. During operation the oscillator 57 produces a 
sinusoidal alternating-current voltage of a magnitude U.sub.o and a 
frequency of less than 10 kHz, for instance 1 kHz. This 
alternating-current voltage which appears between the output 57a and the 
connection 57b is delivered to the non-inverting differential 
amplifier-input 51b. Furthermore, this voltage is doubled, without any 
phase shift, by the amplifier 59 and thus delivered in-phase with the 
voltage infed to the input 51b, by means of the capacitor 61, to the input 
connection or junction 47. The measuring transducer 41 has a capacitance C 
during the momentary piston position. When the piston 3 is located in its 
terminal position, illustrated in FIG. 1, then the capacitance of the 
measuring transducer 41 assumes the maximum value C.sub.max. On the other 
hand, if the piston 3 has assumed its other terminal or end position, at 
the cylinder end located at the left-hand side of the illustration of FIG. 
1, and thus, protrudes as far as possible out of the cylinder 1, then the 
measuring transducer 41 together with its infeed lines has the minimum 
capacitance C.sub.min. 
Furthermore, the feedback capacitor 53 has a capacitance C.sub.53 and the 
capacitor 61 the capacitance C.sub.61. At the output 51c of the 
differential amplifier 51 there is then present, in relation to ground, an 
alternating-current voltage having the magnitude U.sub.a. The output 
voltage then assumes the value 
EQU U.sub.a =U.sub.o (1+(C-C.sub.61)/C.sub.53). 
Now if the capacitance C.sub.61 of the capacitor 61 is made equal to the 
minimum capacitance C.sub.min, then the voltage U.sub.a, with the piston 
position where the measuring transducer has the capacitance C.sub.min, 
just assumes the value U.sub.o. Now if the piston 3, starting from the 
aforementioned terminal position, is displaced deeper into the cylinder 1, 
i.e., towards the right of FIG. 1, then the voltage U.sub.a linearly 
increases as a function of the piston displacement path. By means of the 
differential amplifier 65 there is formed and amplified the difference 
between the potentials or voltages U.sub.a and U.sub.o. At the output 65c 
of the differential amplifier 65 there thus appears as alternating-current 
voltage, the magnitude of which is proportional to the capacitance 
increase C-C.sub.min of the measuring transducer, which is formed when the 
piston 3, starting from its previously mentioned terminal position, is 
shifted towards the right of FIG. 1. In the event that the capacitance of 
the capacitor 61 should not exactly coincide with the minimum capacitance 
of the measuring transducer, then this can be compensated by means of the 
trimmer resistor 77. By means of the trimmer resistor or resistance 79 it 
is possible to adjust the gain of the differential amplifier 73. The 
alternating-current signal present at the output 73c of the differential 
amplifier 73 is now delivered to both of the inputs of the differential 
amplifier 85. The oscillator 57 delivers to the gate of the field-effect 
transistor 89 a square wave-a.c. voltage which is in synchronism with the 
sinusoidal a.c.-voltage present at the oscillator output 57a. Hence, the 
transistor 89 is thus, in synchronism with the alternating a.c.-voltage, 
turned-on and turned-off between its blocking and conductive states. 
Consequently, the non-inverting differential amplifier-input 85b is 
alternately switched back and forth between the potential of the 
differential amplifier-output 73c and electrical ground. The differential 
amplfier 85 thus alternately functions as a non-inverting and as an 
inverting amplifier, so that it demodulates the alternating-current 
voltage infed thereto. The voltage-current converter 89 then generates an 
impressed current or receives such. Such is proportional to the magnitude 
of the infed demodulated voltage, and thus, to the capacitance difference 
C-C.sub.min and varies between the values O and I.sub.max. The current 
supply device 91 is structured such that it consumes a constant current 
having the magnitude I.sub.o. The current I, displayed by the indicator or 
display instrument 97, and infed to the connection 93 and 95 by the 
voltage source 99, thus varies between the values I.sub.o and I.sub.o 
+I.sub.max and constitutes a measure for the piston position. The 
indicator or display instrument 97 and the voltage source 95, which are 
only connected by means of two lines with the electronic unit 43, 
therefore can be readily arranged at a considerable distance from the 
electronic unit 43. 
Of course, the apparatus can be modified in a number of different ways 
without departing from the teachings and underlying principles of the 
present invention. For instance, the tube 23 can be omitted, so that the 
one electrode of the capacitive measuring transducer is directly formed by 
the piston, or stated more exactly, by its piston rod. 
With certain fields of application, for instance in the case of building or 
construction machines, it can be necessary that the cylinder be hingedly 
connected, at the location of the cap or closure 7 for pivotal movement 
with the frame of a machine. In this case, the electronic unit 43, instead 
of being arranged at the end face of the cylinder 1, also can be located 
at the region of the cylinder end at a peripheral location of the 
cylinder. Instead of using the pin 35 there then can lead radially out of 
the cylinder an electrical lead or bushing which is electrically insulated 
with respect to such cylinder. 
Also, the input portion of the electronic unit 43 can be modified. For 
instance, the amplifier 59 and the capacitor 61 can be omitted. In the 
event that there is desired a range switching, this can be realized in a 
most simple manner in that, instead of the feedback capacitor 53, there 
are provided a number of switchable feedback capacitors. 
Moreover, the current I, instead of being used for indicating the piston 
position, additionally can be employed for controlling or regulating any 
given control or regulating elements for an operating procedure. 
Moreover, the electronic device or unit 43 also can be readily constructed 
such that instead of producing an impressed current it generates a 
voltage, whose value constitutes a measure for the piston position. 
Additionally, it is also possible to produce a digital electrical signal 
which constitutes a measure of the displacement of the piston. 
While there are shown and described present preferred embodiments of the 
invention, it is to be distinctly understood that the invention is not 
limited thereto, but may be otherwise variously embodied and practiced 
within the scope of the following claims.