Platform having at least one transducer for reading from and/or recording on a data carrier
BACKGROUND OF THE INVENTION 
     Cross Reference to Related Applications 
     The present invention relates to transducer carriers or platforms of the type which are the subject matter of commonly-assigned copending application Ser. No. 037,725, filed May 10, 1979 (Docket Cii/HB 2177) by Jacques Desserre, the subject matter of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a platform for at least one transducer for reading from and/or recording on a data carrier. It is particularly applicable to magnetic reading and/or recording transducers for magnetic peripherals of data processing systems, in particular disc memories. 
     DESCRIPTION OF THE PRIOR ART 
     The use of magnetic disc memories in data processing systems is becoming increasingly popular by reason of their storage capacity and the relatively short time required by their reading and/or recording transducers (also called read and/or write transducers) to access data contained anywhere on the discs from the moment when an access order is received from the processing system. 
     Magnetic discs carry data on concentric, circular recording tracks whose radial width does not exceed a few hundreths of a millimeter and which generally cover the major proportion of both faces of the discs. In operation, the discs, which are parallel, co-axial, and generally of the same diameter, are driven in rotation at a constant speed by an electric motor. Current practice is generally to associate one or more transducers with each face of the disc, the number of transducers being very much smaller than the number of tracks. (If the number of tracks is several hundred, the number of transducers does not exceed approximately ten or twenty and is often no more than one). When it is desired to achieve a substantial reduction in the time taken by each of the transducers to access any item of data contained on the said face, a plurality of transducers is used rather than just one. 
     Transducers have electrical input and/or input leads connected to the electronic read and/or write circuits of the disc memory with which the transducers are associated. When data recorded on a face of a magnetic disc passes in front of the transducer or transducers associated with this face, the transducer or transducers emit electrical signals along the electrical leads, which signals are transmitted to the electronic read and/or write circuits. Conversely, if the electronic circuits feed electrical signals to the transducer or transducers via the electrical leads, data will be recorded on the face of the associated magnetic disc. 
     The transducers are usually magnetic transducers and have a magnetic circuit provided with an air gap, around which a winding is arranged, the latter having electrical input and/or output leads. The air gap of each transducer is arranged opposite that face of the disc with which the transducer is associated, at a distance of a few microns or even a few tenths of a micron and enables data to be read and/or recorded. It can thus be said that the air gap constitutes the means by which the transducer picks up and/or records the data contained on the face of the disc. 
     Current practice is for the transducers associated with one and the same face of a disc to be arranged on a single platform, which has to be moved radially above the said face to enable the transducers to access any data contained on the said face. Such platforms are called &#34;moving platforms&#34;. 
     Another known form of platforms is the so-called &#34;fixed&#34; platforms which, during reading and/or recording operations, remain stationary at all times above clearly defined tracks on the face of the disc with which they associate. In this arrangement, each of the transducers contained within a platform remains associated with one and the same track in the course of time. 
     Whether a platform is fixed or moving, a platform incorporating at least one transducer and associated with one face of a disc is secured to a rigid movable arm. In the case of the moving platform the arm is mounted on a carriage which moves externally of the discs, whereas in the case of a fixed platform it is mounted directly on the framework of the disc memory to which the disc belongs. 
     Whatever the type (fixed or moving), platforms incorporating one or more transducers are made up of two principal parts, namely, on the one hand, a part called the &#34;main body&#34; of the platform which incorporates the transducers and, on the other hand, a suspension device of which one end is secured to the main body and the other end is secured to the rigid detachable arm. Generally, but not exclusively, the main body of a platform is in the shape of a relatively shallow right-angled parallelepiped of which one major face, termed the &#34;lower&#34; face, contains the means which the transducers have for picking up and/or recording data. The major face termed the &#34;upper&#34; face contains the ends of the electrical input and/or output leads of the transducer or transducers and the means which enable these leads to be connected to the electrical read and/or write circuits of the disc memory. 
     Presently, increasingly frequent use is being made of the platforms known to those skilled in the art as &#34;Winchester&#34; platforms. Such platforms are manufactured inter alia by Applied Magnetic Corporation (A.M.C.) under the type numbers 3304 and 3306. 
     A platform of the &#34;Winchester&#34; is produced as follows: 
     In the lower face of its main body are formed one or more gulleys, also termed &#34;grooves&#34; or &#34;channels&#34;, whose depth is greater than or equal to 30 microns and may be as much as a few tenths of a millimeter. As a result, there are on the said lower face a plurality of projecting portions which are termed skids and which are, broadly speaking, ski-shaped. 
     The rotation of the discs causes a cushion of compressed air to be formed between the lower face of the main body of the platform and the face of the disc associated with the platform. This cushion of air prevents the main body from touching the disc which might damage it. It is, therefore, said that the main body and, by extension, the platform, flies above the face of the disc which is associated with it. 
     The cushion of air so developed exerts a pressure against the surface of the skids in a direction normal thereto. This pressure is directed from the lower face towards the upper face of the main body of the platform. Dynamic equilibrium for the platform when in flight is achieved by setting up, in opposition to the force F p  generated by the pressure of the cushion of air against the surface of the skids, a loading force F c  which is directed in the opposite direction. The so-called loading force is applied to the upper face of the main body by means of a pivot and has a modulus equal to that of the pressure force. The pressure force is proportional to the area of the flight surface. Thus, it should be apparent that by reducing the area of the flight surface the grooves formed in the lower face of the main body enable the pressure force, and thus the loading force, to be reduced. 
     The loading force is, for example, generated by a pre-stressed spring secured to the rigid arm carrying the platform. It is relatively small and of the order of 10 to 20 grams. Because of this, a platform of the &#34;Winchester&#34; type is known as a lightly loaded platform. 
     Generally, a platform of the &#34;Winchester&#34; type flies above the face of the disc with which it is associated in such a way that the surface of the skids is slightly inclined to the face of the disc. This allows the skids on the platform to produce sufficient lift to enable the platform to fly stably above the face of the disc. The total surface area of the skids is defined as being the flight surface of the platform. The front of the flight surface is defined as being that part of the surface which is the first in time to have pass before it the face of the disc with which it is associated. Similarly, the rear of the flight surface is defined as being that part of the surface which is the last in time to have this same face pass before it. 
     In cases where the transducer or transducers of the platform are magnetic, its or their air gaps is or are, as the case may be, arranged in the skids at the rear of the flight surface. The main body of the platform is inclined to the surface of the disc in such a way that the rear of the platform lies at a distance from the surface of the disc, termed the &#34;altitude of flight&#34;. This &#34;altitude of flight&#34; is of the order of a few tenths of a micron. This distance is less than the distance which separates the same face from the front of the flight surface, which is of the order of a few microns. 
     Because of the fact that &#34;Winchester&#34; platforms are lightly loaded, this enables low altitudes of flight to be achieved. Such platforms have distinct advantages. For example, by reason of their low altitude of flight, read signals from the transducer or transducers may be obtained whose amplitude is relatively high. This provides reliable reading. Also, because of their light loading, the main body of the platform is prevented from striking against the disc and thus damaging both the disc and the main body. 
     However, despite the aforenoted advantages, by reason of their light loading, the flight of the platforms may be relatively unstable. To this end, a platform of the &#34;Winchester&#34; type is subject to an oscillatory movement about a central position which causes it to approach and recede from the disc. The platform will thus oscillate about an axis which is parallel to the surface of the disc and which is situated outside and to the rear of the main body of the platform. This axis of rotation is called the &#34;virtual axis of rotation&#34; of the platform. 
     The principal causes of the oscillatory movement are as follows: 
     It is known that the surface of the disc is not absolutely flat. On a microscopic scale, it can in fact be seen to contain a plurality of dips and elevations. This being the case, when the platform is situated above an elevation on a disc, the thickness of the cushion of air is reduced. As a result the pressure force F p  increases and this in turn results in an increase in the loading force F c  because the spring which applies the pressure force F c  is stressed to a greater degree. As a result, the platform moves closer to the disc. In the opposite case, where the platform is situated above a dip on the disc, the thickness of the cushion of air increases and the pressure force F c  decreases. This entails a similar decrease in the loading force F c  because the spring is stressed to a lesser extent. As a result the platform rears up and moves away from the disc. 
     It can be shown that during its oscillatory movement about the virtual axis of rotation otherwise known as the axis of oscillation the platform may be affected by external upsetting forces caused, for example, by the presence of dust between the flight surface of the platform and the face of the disc or by a collision between the platform and the said face, which collision normally takes place at the point where the air gaps are situated. Under these conditions a mechanical force F M  is set up which causes the platform to rock about the pivot by means of which the loading force F c  is applied. 
     Under these conditions there is a probability that the platform will touch the disc. If there is a comparatively large number of collisions between the platform and the face of the disc with which it is associated when the platform is flying above this face of the disc, the platform will deteriorate fairly rapidly as also will the face of the disc. 
     SUMMARY OF THE INVENTION 
     The present invention enables these disadvantages to be overcome by making provision for the sum of the moments of all the forces applied to the platform about its virtual axis of rotation or axis of oscillation to be zero, which ensures that it is in dynamic equilibrium in flight. Because of this, the invention enables the pivot to be dispensed with and, in addition, on the one hand, enables the stability of the platform in flight to be increased and on the other hand enables the risk of collision between the platform and the disc to be substantially reduced. 
     The platform, according to the invention, incorporates at least one transducer for reading from and/or recording on a data carrier and comprises a main body containing the transducer. A suspension device is connected on the one hand to the main body and on the other hand to an arm carrying the platform. The lower face of the main body contains the means which the transducer has for picking up and/or recording data, which means are arranged facing the carrier. The arrangement is such that the sum of the moments of the forces applied to the platform about its virtual axis of rotation is zero. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become apparent from the following description, which is given by way of non-limiting example, and with reference to the accompanying drawings: 
     In the drawing: 
     FIG. 1 shows a prior art platform having at least one transducer, the platform being shown in flight about a magnetic disc, and illustrated in three separate views; 
     FIG. 1a is a side view; 
     FIG. 1b is an end-on view; and 
     FIG. 1c is a view from below (seen from that face of the magnetic disc with which the said platform is associated). 
     FIGS. 2a and 2b diagrammatically illustrate the oscillatory movement of a prior art platform about its virtual axis of rotation, and the disadvantages which result from this oscillatory movement as far as the flight of the platform is concerned. 
     FIG. 3 is a diagram illustrating the principle of a platform according to the invention; 
     FIGS. 4, 5 and 6 show preferred embodiments of a platform according to the invention; 
     FIGS. 4a and 5a are three-quarter perspective views showing the platform in flight above the disc, and; 
     FIGS. 4b and 5b show the same platform, viewed from above, in flight above the disc. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to better understand the principles of operation and construction of a platform constructed according to the invention and having at least one transducer, a few facts relating to the principles of construction and operation of prior art platforms will be reviewed with reference to FIGS. 1 and 2. 
     FIG. 1 illustrates a prior art platform P1 a . This platform comprises, on the one hand, a main body CPL a  of substantially parallelepiped shape which is shown in the form of a catamaran having two skids PA 1  and PA 2  between which is situated a groove R and on the other hand, a suspension device SUSP a . 
     Each of the two skids PA 1  and PA 2  contains a transducer. The skid PA 1  contains transducer TR 1  and the skid PA 2  contains transducer TR 2 . Transducer TR 1  has an air gap E 1  for reading and/or recording and transducer TR 2  has an air gap E 2  for reading and/or recording. The two skids PA 1  and PA 2  have, respectively, in the one case a main flight surface SV 1  and a bevel CH 1  and in the other case a main flight surface SV 2  amd a bevel CH 2 . If s 1  and s 2  are the surfaces of bevels CH 1  and CH 2 , respectively, the total flight surface of skid PA 1  is equal to SV 1  +s 1  and the total flight surface of skid PA 2  is equal to SV 2  +s 2 . The flight surface of the platform is thus equal to SV 1  +s 1  +SV 2  +s 2  =SV. 
     As can be seen in FIG. 1a, which shows the platform PL a  in flight above a magnetic record carrier such as a magnetic disc DISC which is moving at a constant speed V in the direction of an arrow E, the platform is slightly inclined to the surface of the disc so that the air gaps E 1  and E 2  of transducers TR 1  and TR 2  are situated at an altitude of flight a 1  above the disc which is less than the altitude of flight a 2  of bevels CH 1  and CH 2 , as most clearly shown in FIG. 1a. The flight surface SV forms an angle α with the surface of the disc. This angle is very small, being of the order of a few milliradians. It can also be seen that bevel CH 1  is situated at the front of the flight surface of the main body PL a , which will be designated AVSV. Similarly, it can be seen that the air gaps E 1  and E 2  are situated at the rear of the flight surface, which will be designated ARSV. 
     It is assumed in this description that the platform is of the &#34;moving&#34; type. 
     One of the ends (which is not shown) of the suspension device SUSP a  is fixed by any suitable means to a rigid movable arm (also not shown) which carries the platform PL 3 , and which is secured to a carriage (also not shown) situated clear of the disc which enables the platform to be moved radially above the circular recording tracks on the magnetic disc DISC. 
     In FIG. 1a and 1b the suspension SUSP a  of the platform is shown fixed to the floor of a gulley or channel G formed in the upper face of the body CPL a  but it is clear that the suspension device SUSP a  could equally well be fixed to the lower face of the main body, preferably in the groove R as is described in a French Pat. application No. 78.15765, entitled &#34;Platform Incorporating At Least One Transducer For Reading And/Or Recording On A Data Carrier&#34;, which was filed in France on May 26, 1978, and which corresponds to U.S. application Ser. No. 037,725, assigned to the assignee of the present invention. 
     It is current practice for the loading force F c  which, when the platform is in flight, to balance the force F p  exerted by the cushion of air against the flight surface SV, which latter force is proportional to the area of the said surface. The loading force provided by a spring s is applied by means of a pivot PIV secured to the upper face of the main body of the platform. 
     As described above, when the platform PL 1  flies above the disc DISC, the platform PL a  is subject to an oscillatory movement about a virtual axis of rotation or axis of oscillation AXV which is parallel to the surface of the disc and situated to the rear of the platform, that is to say, on the same side as the rear ARSV of the flight surface. This is best illustrated in FIG. 2a. In the same figure, it can be seen that while in flight the platform oscillates about a central position POS m , between an upper position POS SUP and a lower position POS INF. When the platform is in position POS SUP, its altitude of flight is equal to a sup  whereas when it is in the lower positions its altitude of flight is equal to a inf  and thus a sup  &gt;a inf . 
     As can be seen in FIG. 2b, the platform, under the prompting of external forces caused, for example by impact between the platform and the disc or by the presence of dust between the flight surface of the platform and the disc itself, may rock about the pivot PIV under the prompting of a mechanical force F M+  which forces the platform forwards, or a mechanical force F M-  which forces the platform backwards. The forces F M+  and F M-  may be due to the said impact or the said dust. This may give rise to relatively violent collisions between the front AVSV or the rear AVSV of the flight surface of the platform and the face of the disc DISC. Such impacts result, on the one hand, in damage to the platform itself and, on the other hand, in damage to the face of the disc. 
     In accordance with the invention, means are provided to apply a combination of forces to the platform PL i  such that the sum of the moments of all these forces about the virtual axis of rotation or axis of oscillation AXV i  of the platform is zero and the platform is stable as shown in FIG. 3. Thus, if the moment of the pressure force F p  about the axis AXV i  is called M p , and if the sum Σ M ci  of the moments which balance the moment M p  is called the loading moment M c , then the invention is satisfied by the equation: 
     
         (M.sub.p +M.sub.c).sub.AXV.sbsb.i =0                       (1) 
    
     FIG. 4 shows a platform PL i1  according to the invention whose main body CPL i1 , which is of the cataraman type, has two skids PA i1  and PA i2  between which is formed a groove R i1 . Skid PA i1  contains a transducer TR i1  and skid PA i2  contains a transducer TR i2 . Transducers TR i1  and TR i2  have air gaps E i1  and E i2 , respectively, which are situated at the rear ARSV of the flight surface of the main body CPL i1 . 
     The device SUSP i1  for suspending the platform is preferably attached to the lower face of the main body CPL i1  in the groover R i1  (as described in the above-mentioned patent application). SUSP i1  comprises a first rigid strut PO i1  containing a constriction RET i1  formed by means of two circular indentations EC i1  and EC i2   whose centers CENT i1  and CENT i2  allow the virtual axis of rotation or axis of oscillation AXV, best shown in FIG. 4b, of the platform PL i1  according to the invention to be defined. It can be seen that this axis coincides with the axis of symmetry of the constriction in the direction normal to the major dimension of the strut PO i1  (its length). 
     In other words, it can be seen that the constriction RET i1  enables the virtual axis of rotation AXV i1  of the platform to be given a physical form. 
     The first rigid strut PO i1  of the suspension device SUSP i1  is attached by attachment means M i1  to a second rigid strut PO i2  which in turn is attached to the arm (not shown) carrying the platform. When the platform PL i1  is flying above the disc, the rigid strut PO i1  will only flex at the point where its cross-section is at its smallest, that is to say, at the point where the constriction RET i1  is situated (where the strut is most flexible). When the platform PL i1  is in flight, it is assumed that the upstream strut PO i2  remains fixed and behaves as a solid support. 
     The bending moment of strut PO i1  (about axis AXV i1 ) due to the resilience of the constrictions RET i1 , tends to depress the platform towards the disc and to balance the effect of the pressure force F p . This moment is a function of the cross-section of the constriction and of the co-efficient of elasticity of the metal forming the strut. 
     If this moment is called M 1   c , and if the weight of the platform PL i  is assumed to be negligible compared with the pressure force F p , the following equation is obtained which represents dynamic equilibrium for the platform in flight: ##EQU1## M p  is the moment of the pressure force F p  about AXV i1 . 
     d shown in FIG. 4b represents the distance separating the virtual axis of rotation AXV i1  from the center of gravity GR i1  of the platform PL i1 . If M 1   c  is divided by the distance d, this gives the equivalent loading force F 1   ce  which balances the pressure force F p . In other words, the effect of the bending moment M 1   c  of the constriction is equivalent to the effect of an equivalent loading force F 1   ce . 
     FIG. 5 shows a platform PL i2  according to the invention whose main body CPL i2  is of the trimaram type and has three skids PA i3 , PA i4 , PA i5  between which are formed two grooves R i3  and R i4 . The skids PA i3  to PA i5  each contain one of the transducers TR i3  to TR i5 . The transducers TR i3  to TR i5  have respective air-gaps E i3  to E i5 . 
     Advantageously, the device SUSP i2  for suspending the platform PL i2  according to the invention is fixed to the main body PL i2  of the platform on the lower face of the body in the grooves R i3  and R i4 , as described in the above-mentioned French patent application. 
     The suspension device SUSP i2  includes two struts PO i3  and PO i4  each containing a constriction RET i3  or RET i4 , the common axis of symmetry of the constrictions defining the virtual axis of rotation ASV i2  of the platform PL i2  according to the invention, which is thus given a physical form. Each of the struts PO i3  and PO i4  is rigid. The suspension device SUSP i2  also includes a second rigid strut PO i5  which is attached by attachment means M i2  to the two struts PO i3  and PO i4 . 
     In flight, since strut PO i5  acts as a solid support, struts PO i3  and PO i4  will bend only at their points of minimum cross-section, that is to say, at the constrictions RET i3  and RET i4 . 
     If we let M c3  and M c4  be the respective bending moments about axis AXV i2  of the struts, these moments being of the same size and in the same direction, then if Si M 2   c  =M c3  +M c4  ≃2×M c1 , dynamic equilibrium for the platform PL i2  in flight will be defined by the equation: ##EQU2## 
     If M 2   c  is divided by d, which is the distance between axis AXV i2  and the center of gravity GR i2  of the platform, this gives the equivalent loading force F 2   ce  which balances the pressure force F p . 
     FIG. 6 is a view from above of a platform PL i3  showing an alternative embodiment of the invention of the cataraman type which has a groove R i5  situated between two skids PA i6  and PA i7  each containing a transducer TR i6  or TR 17 . Each of the transducers TR i6  and TR i7  has an air gap, namely, air gaps E i6  and E i7 , respectively. 
     The device SUSP i3  for suspending the platform includes a first strut PO i6  which is fixed in the groove R i5  on the lower face of the platform. This strut PO i6  is divided into two rigid struts PO i7  and PO i8 , each of which contains a constriction RET i7  or RET i8 . The constrictions RET i7  and RET i8  define and give physical form to the axis of rotation AXV i3  of the platform PL i3 . 
     If M c7  and M c8  represent the bending moments of constrictions RET i7  and RET i8 , with M 3   c  =M c7  +M c8  ≃2 M c7 , then ##EQU3## 
     Then, if M 3   c  is divided by d, which is the distance between AXV i3  and the center of gravity GR i3  of the platform PL i3 , this gives the equivalent loading force F 3   CE  which enables the pressure force F p  to be balanced. 
     It can be seen that in all the embodiments illustrated by FIGS. 4, 5 and 6, the virtual axis of rotation axis of rotation of the platforms according to the invention are given physical form by forming constrictions in the struts which make up the device for suspending the platform, the moment M p  of the pressure forces F p  about the virtual axis of rotation being balanced by the sum of the bending moments of these constrictions. 
     In all cases, an equivalent loading force can be defined which is obtained by dividing the sum of the bending moments of the constrictions by the distance d between the virtual axis of rotation or axis of oscillation and the center of gravity of the platform. 
     It is clear that without departing from the scope of the invention the device for suspending the platform according to the invention may, for example, be situated on the upper face of the platform. 
     It will be apparent that various modifications will suggest themselves to those skilled in the art. Accordingly, the foregoing description is not intended to limit the invention and resort should be made to the appended claims which define the true spirit and full scope of the invention.