EP0562462A1

EP0562462A1 - Linear pneumatic actuator with braking and micrometric displacement device

Info

Publication number: EP0562462A1
Application number: EP93104449A
Authority: EP
Inventors: Luciano Migliori
Original assignee: Univer SpA
Current assignee: Univer SpA
Priority date: 1992-03-25
Filing date: 1993-03-18
Publication date: 1993-09-29
Also published as: ITMI920708A1; ITMI920708A0; IT1254848B

Abstract

A fluid pressure operated actuator comprising a cylinder (11), a reciprocable nut and piston member (12, 16) through which a freely rotatable screw shaft (15) extends; the screw shaft (15) engages the nut (16) to be rotated by the reciprocating movement of piston member (12). A fluid pressure operated turbine motor (25) is provided inside the cylinder (11) which is connectable to the screw shaft (15) through a gear reducer (43, 46, 47) to rotate the same shaft (15) before completation of a stroke of the piston member (12); an accurate stoppage and positioning of the piston member (12) and load device connected thereto, with micrometric precision under the control of a processing unit (CPU, 23); actuable coupling means (29, 30) are provided in said cylinder (11) to connect the screw shaft (15) to the turbine motor or to prevent the sliding movement of the piston member (12) in a stopped position;

Description

The present invention relates to a fluid pressure operated or linear pneumatic actuator of the type comprising a cylinder and a piston member which is reciprocable in the cylinder; the piston member comprises a nut engaging a freely rotatable screw-shaft which is made to rotate by the reciprocating movement of the piston; an electric signal generator is connected to the screw-shaft to output control signals indicative of the direction and speed of rotation of the said shaft and hence the sliding movement of the piston. The screw shaft is in turn connected to a braking and locking device which can be actuated to allow or to prevent sliding of the piston in any desired position along the actuator stroke.
As is known, linear pneumatic actuators are widely used in many application fields owing to their high working speed as well as their great flexibility in terms of use.
However, the pneumatic actuators traditionally employed, owing to the compressibility of the pressurised fluid used as power source, are unable to ensure a high precision degree for positioning of the load; therefore they cannot readily be used in automated systems or which require sophisticated apparatus for programming the working cycles.
Although, on the one hand, the use of safety brakes or fluid pressure actuated locking devices, in combination with code signal generators, allowed a certain control degree of the actuator, on the other hand, it has not resolved entirely the problem of a correct positioning of the load since the positioning accuracy is negatively influenced by many parameters and is well below the operational standards required in most applications. This lack of positioning accuracy is attributable to the pneumatic system per se, in addition to lags in the activation of the load braking or locking devices connected to the actuators.
Although encouraging results have been obtained with the currently known pneumatic actuators, the low degree of positioning accuracy still represents an unresolved problem.
In order to partly solve the abovementioned drawbacks and increase the positioning accuracy of fluid pressure operated actuators, EP-A- 0469253 proposes a fluid pressure powered linear actuator provided with a special load braking and locking device having extremely short operation times, enabling errors to be reduced to a limit acceptable for certain applications. However, the pneumatic actuators proposed hitherto are not yet able to guarantee an extremely precise positioning of the load or allow functional control by computerised systems such as to ensure a high degree of reliability of the actuator itself, due to the inertia phenomena on the moving loads at the stoppage.
An object of the present invention is to furtherly improve the operation of the pneumatic actuator known from EP-A.- 0469253 in such a way as to ensure extremely precise positioning within very small tolerances, using extremely simple and highly efficient means which are not influenced at all by mechanical or pneumatic parameters of the same actuator and the entire control apparatus.
A further object of the present invention is to provide a pneumatic actuator, of the abovementioned kind, provided with a locking system able to withstand notably high loads, far greater than the maximum force which can be exerted by the actuator itself, without the latter being damaged or the load being displaced.
A further object of the present invention is to provide a pneumatic actuator in which the errors due to the presence of any mechanical clearance are entirely negligible or totally eliminated.
These and other object are obtained by means of a pneumatic or fluid pressure operated actuator according to the present invention, comprising a mechanical system actuated by a pneumatic turbine motor for micrometric moving of the piston of the actuator under the control of a processing unit managing the working program of the entire actuator; in this way fine and extremely precise positioning is obtained.
According to the invention, a linear pneumatic actuator has been provided comprising the characterising features of Claim 1.
The general features and a preferred embodiment of a linear pneumatic actuator according to the present invention, will be illustrated in greater detail hereinbelow, with reference to the accompanying drawings, in which.

Fig. 1: is a general diagram of the pneumatic actuator and the respective control apparatus;
Fig. 2: is an enlarged detail of the actuator shown in Figure 1;
Fig. 3: is a cross-sectional view along the line 3-3 of Figure 2;
Fig. 4: is a cross-sectional view along the line 4-4 of Figure 2.

With reference now to Figure 1, it can be noted that the pneumatic actuator assembly, denoted overall by 10, substantially comprises a hollow cylinder 11 inside which there slides a reciprocating piston 12 having a hollow rod 13 which axially extend into the cylinder and emerges from an end cup 14 of the actuator. The rod 13 is adapted for connection to a suitable load device; means are furtherly provided to prevent the piston 12 from rotating. A screw shaft 15 is coaxially extending through the piston 12 and the hollow rod 13 and engages with a nut 16 fixed inside the piston 12. Since the piston 12 is coupled to the screw shaft 15 through the nut 16, it will be evident that the screw shaft 15 is made to rotate in both directions by the reciprocating movement of the piston 12.
The screw shaft 15 is slidingly and rotatably supported inside the rod 13 and extends with a pilot shaft 15 wich is connected to a signal generator 23 and is rotatably supported by bearing means 17 or by any other suitable bearing means 17 or by any other suitable bearing system.
In Figure 1, reference 18 denotes an inlet port for the pressurised air, located on one side of the chamber of the cylinder 11, while 19 denotes the pressurised-air inlet port on the opposite side. The air inlet ports 18 and 19 of the cylinder can be connected to a pressurised-air source 20 by a solenoid valve unit 21 controlled by a programmable processing unit CPU.
The pilot shaft 15' extends at the rear and is connected by a coupling 22 to the signal generator or encoder 23 which thus sends to the CPU control signals indicative of the position of the piston 12 during its entire working travel, as well as the speed and direction of movement of the piston itself.
The screw shaft or more precisely its pilot part 15' is further connectable to a turbine motor 25 by a fluid pressure actuated coupling and braking device denoted overall by 24; the coupling device 24, in the embodiment shown, is designed to perform both the function of a safety brake for the piston 12 of the actuator and that of coupling for the screw shaft 15 with the impeller 26 of a control turbine motor 25 which may be fluid pressure actuated or locked as described hereinbelow and intended for the purposes specified.
The pneumatic turbine motor 25 comprises an impeller 26 which may be made operated to rotate in one direction or in the opposite direction, or may be locked by means of jets of compressed air supplied by tangential and opposite nozzles 27, 28 connected to the pressurised-air source 20 via respective valves of the unit 21 controlled by the CPU.
The coupling and braking device 24, its features and the connection to the turbine motor 25 will be illustrated in greater detail hereinbelow with reference to the remaining Figures 2, 3 and 4 of the accompanying drawings. In these figures, the same reference numbers as in Figure 1 have been used to indicate similar or equivalent parts.
As shown in Figure 2, the coupling device 24, designed to perform the function of both a safety brake for the actuator and for the connection of the screw shaft 15 to the turbine motor 25, comprises engageable and disengageable friction members 29 and 30 which can be engaged and disengaged each other by means of a relative axial movement. The friction member 29 is connected to the pilot shaft 15' and is able to rotate therewith, while the second friction member 30 is suitably supported on the pilot shaft extension 15' of the screw shaft and is suitably connected to the turbine motor 25, so as to be able to rotate and move axially between an advanced position, where it engages with the first friction member 29, and a retracted or disengaged position, as shown.
In particular, the second friction member 30 is mounted inside a cup element 31 integral with a hollow shaft 39 which extends at the rear coaxially with respect to the pilot shaft 15'. The assembly consisting of the hollow shaft 39, the cup element 31 and the second friction element 30 is normally maintained in an advanced position against the first friction element 29, by thrusting means for example in the form of cup springs 40 housed inside a cavity of a ring member 41 fitted inside the cylinder 11; the springs 40, through a thrust bearing 42 and a central gear 43 connected with the shaft 39, act to move the cup element 31 and keep normally engaged with one another the two friction elements 29 and 30 coupling and/or braking actions for the screw shaft 15 and the piston 12 of the actuator. The hollow shaft 39 at its rear end is therefore connected to a piston 44 inside the chamber 45 of the same turbine 25 or in an independent chamber supplied on one side with pressurised air for causing the backward movement of the friction element 30, overcoming the reaction of the springs 40.
As previously mentioned, the cup element 31 containing the second friction member 30 is rotatably supported by the pilot shaft 15' which constitutes an extension of the screw shaft, and is made to rotate by the pneumatic turbine motor 25 so as to cause the screw shaft 15 to rotate in both the directions and consequently move the piston 15 of the actuator during the last portion of its travel, in the vicinity of a stoppage point for the load or the piston itself.
For this purpose, the second friction member 30, or more precisely the hollow shaft 39 of the cup element 31, is rotatably and slidably connected to the impeller 26 of the turbine by means of a gear reducing system of unreversible type, in the manner described hereinbelow. More particularly, a dual reducing system is provided, comprising a first gear reducer formed by a cylindrical gear 43 connected to the tubular shaft 39, and a pair of diametrically opposite planetary gears 46 and 47 provided at one end of respective shafts 48 and 49 rotatably supported by the ring member 41. The arrangement of the cylindrical gears 43, 46 and 47 is illustrated in greater detail in the cross-sectional view of Figure 4.
At the other end, each of the shafts 48 and 49 has a helical- tooth gear 50, 51, respectively, each of which, as shown in the cross-sectional view of Figure 3, engages with a respective worm screw 52, 53 performed at each end of a cross shaft 54; the shaft 54 in turn is provided with a central helical gear 55 engaging with a main worm screw 56 on a tubular hub 57 of the impeller 26 rotatably and slidably supported with respect to the hollow shaft 39.
As shown, the assembly consisting of the helical gears 50, 51 of the two worm screws 52 and 53, the helical gear 55 and the worm screw 56, forms a second gear reducer of unreversible type which, together with the gears 43, 46 and 47, provide a high reduction ratio, of the order of some hundred times, such that for each rotation of the impeller 26 there is a corresponding infinitesimal or micrometric sliding displacement of the piston 12 of the actuator.
The working of the pneumatic actuator may be summarised as follows: in the absence of pressurised air at the two nozzles 27 and 28, the impeller 26 is at a standstill, and the friction member 30 with the coupling control piston 44 are moved forwards by the action of the thrust springs 40 which keep the friction member 30 and the friction member 29 constantly engaged. In this condition, the impeller 26 of the turbine motor is operationally connected to the screw shaft 15 of the piston 12 by the gear assembly; furthermore, owing to the unreversibility of the gear reducing assembly, the piston 12 is prevented from moving under the action of an external load. Let us now assume that, starting from the conditions shown in Figure 1, the piston 12 must be moved forwards on the left side and stopped in a predetermined position defined by the magnetic limit switch 12'. In these conditions and according to the operating program stored in the CPU, after disengagement of the friction members 29 and 30 by control piston 44, the CPU emits a control signal for the solenoid valves 21 allowing the pressurised air to be supplied on the right-hand side of the piston 12 via the inlet 19, while the inlet 18 is vented. The piston 12 is therefore made to move forwards rapidly towards the stoppage position defined by the limit switch 12'. During the forward movement, the piston 12 causes the rotation of the screw shaft 15 to actuate the encoder 23 which sends to the CPU a set of electrical signals indicative of the position of the piston 12.
When the piston 12 is approaching the stoppage position, the control unit of the CPU emits a control signal for the solenoid valves 21 to create a counter-pressure inside the actuator cylinder, namely connecting both the inlet ports 18 and 19 to the pressure source 20 so as to dampen the kinetic energy accumulated by the piston and the entire system of the moving load to which the actuator is connected; therefore the piston 12 slows down rapidly until it reaches a prescribed position. Then the CPU, which continues to receive the control signals from the encoder 23, causes the immediate activation of the coupling and braking device 24 by cutting off the supply of pressurised air at the nozzles 27 and 28 of the turbine motor 25. At this point, the space which the piston 12 must still travel in order to precisely reach the required stoppage position is extremely small, namely may be of the order of a few millimetres. Therefore the CPU maintains the engagement of the coupling 24 and activates the turbine motor 25, supplying only one of the nozzles 27 or 28, depending on the required direction of rotation, with short calculated blasts of pressurised air. In view of the high reduction ratio provided by the gear assembly described above, each rotation of the impeller 26 causes a micrometric feeding of the piston 12 which can thus be made to reach the desired position with great precision withouht being influenced by inertia phenomena. Once this position has been reached, the CPU deactivates the turbine motor 25, while maintaining the engagement of the coupling 24; the latter, together with the gear reducer which is made unreversible owing to the presence in the latter of helical gears and worn screws, act as locking device for the piston of the actuator. As a result of the high reduction ratio, therefore, an efficient stoppage and locking action can be achieved, which is able to withstand peak loads on the rod 13 of the actuator, equivalent to four or five times the maximum force exerted by the actuator itself.
From the above description and illustrations in the accompanying drawings, it will therefore be understood that the invention relates to a linear pneumatic actuator provided with additional pneumatic control means, inside the actuator itself and designed to move the piston by micrometric amounts, thereby enabling the latter to be positioned in an extremely precise manner within tolerances which cannot be achieved by other actuators of the known type, all of which by means of an extremely simple and extremely reliable solution.
It is understood that the above description and illustrations with reference to the accompanying drawings have been provided purely by way of an illustrative example of the principles of the invention; therefore, other modifications or variations may be made to the actuator or to parts thereof, without departing from the present invention.

Claims

A fluid pressure operated actuator comprising a cylinder (11) and a piston and nut unit (12, 16) reciprocable therein, a freely rotatable screw shaft (15) operatively connected to said piston and nut unit (12, 16), said screw shaft (15) axially extending with a pilot shaft portion (15') connected to an electric signal generator (23), and locking means to lock the rotation of said screw shaft (15), under the control of a programmable processing unit (CPU), characterised by comprising a fluid pressure actuated turbine motor (25) inside the cylinder and engageable-disengageable coupling means (24) to operatively connect said screw shaft (15) to said turbine motor (25), said coupling means (24) comprising first friction member (29) rotatably connected to the screw shaft (15), and second friction member (30) slidably supported and rotatably connected to the impeller (26) of the turbine motor (25), and control means (40, 44) to operate said coupling means (24) between engaged and disengaged conditions under the control of the processing unit (CPU) mentioned above.
A fluid pressure operated actuator according to Claim 1, characterised by comprising a gear speed reducing assembly (43, 46, 47; 50, 51, 56) between the impeller 26 of the turbine motor (25) and the coupling means (24).
A fluid pressure operated actuator according to Claim 2, characterised in that said gear speed assembly (43, 46, 47; 50, 51, 56) is of unreversible type.
A fluid pressure operated actuator according to Claim 1, characterised in that said coupling means (24) comprises engageable and disengageable friction members (29, 30), the first one (29) of said friction members being connected to said screw shaft (15) so as to rotate therewith, the second one (30) of said friction members being connected to a tubular shaft (39) slidingly supported and coaxially extending with respect to a pilot extension (15') of the abovementioned screw shaft (15) and fluid pressure actuated control means (44) connected to said tubular shaft (39), said control means (44) acting to engage and disengage the friction members (29, 30) of said coupling means (24) in opposition to the action of elastically yieldable thrusting means (40).
A fluid pressure operated actuator according to Claims 1 and 4, characterised in that the impeller (26) of the turbine motor (25) comprises a tubular hub (57) rotatably supported by the abovementioned tubular shaft (39).
A fluid pressure operated actuator according to the preceding claims, characterised in that said control means for the friction members comprise a piston (44) inside a chamber in communication with the chamber (45) of the impeller (26) of the turbine motor (25).
A fluid pressure operated actuator according to preceding claim 2, characterised in that said speed reducing assembly comprises at least one helical gear and wron screw coupling of the unreversible type.
Actuator according to any one of the preceding claims, characterised in that said turbine motor (25) comprises at least diametrally opposed nozzles (27, 28) for supplying pressurised air, jets to said turbine, and valve means (21) operatively controlled by the processing unit (CPU) to selectively supply pressurised air to a selected one or to both of the nozzles (27, 28) at the same time.