CA1201643A - Servo amplification system - Google Patents

Abstract

Servo Amplification System Abstract A Servo Amplification System is created particularly for heavy construction equipment, but has a general utility that is much broader.
The system utilizes a hydraulic analog system with a separate sub-system far each dimension of motion. The operator moves the operative element, such as the backhoe bucket, of the analog replica which ordinarily would be situated in the cab of the backhoe or other piece of equipment. A small hydraulic cylinder operative in response to movement at each articulated connection of the backhoe operates a pilot valve which controls a pilot piston mechanically linked to the drive valve of the drive cylinder of the corresponding articulation in the actual backhoe. A feedback system comprising a mechanical link from the actual drive piston to a feedback cylinder and piston delivers hydraulic fluid back to the inlets of the pilot valve in such a way as to cancel the pilot orders from the initial control cylinder. The resulting action is virtually perfect analog simulation by the actual backhoe of the movements of the replica backhoe.

Description

3 ~ 1-Back~und o~ the ~ven~on Modern constructi.on equ;pment is generally hydraulic,~ly operated. The contrcils used by the operator con~stitute hydrdulic valves which dir~ctly connect to the hydrctulic cy.linders at the 5 ar~ic~tions, or other sites of relative motion between the stt~lctural mernbe~ which mount the operative e1ement of the equipment. It is com mon to have three, four, an~l more hydraulic drive cy.linders which operate the equipment, requiring the corresponding number of hand operated valves.
A skilled operator of a backl1oe or other piece oY equipment can operate it as though it ~lere an e~l;en~on Or his own body utiliz1ng the hydraulic valves. However, it may take a coup.le of` years before an operator achieves this level of slcill, and in the m eantim e an extre m ely experLsive piece of equip m ent .is being uncler utilized during 15 the training procecs.
AdditionallLy, during the l~arnil1~ perio~l, when the operator does not accurately move an~ stop the machine as he sho~ , it is diffic-~t on the equipment and puts a ~stl~in on most of the operati.n~ par~s.
Thi~s is especially evident in rent~l ur1its. A hackhoe in a rental year 20 will rc~quire quite frequent maJor maintenance.
Marty of these t~ining prohlems carc a r~s~t of the hydrau1ic control s~ste m avail~tble to the opet~tor. L ike ~laying a m llsical inst~ument, it takes a time before the operator can fneely move all of the controls concurrently in a smooth, synchr~ni7.ed faAshion which ma~imi~es the productiv.ity of the machine and minimize~s stt~lctu~l ~2~69L3 -2-damage. A system which could irtegr~te all of this motion into a sin~le analog control device, ir. which khe operator merely move.s the operative element such as a backhoe on a miniature scale, causing the comparable movement of the actual backhoe, would unquestionably speed the operator learning process, save equipment, and er~le nonpr~fes~ionals such as those renting units from a rental yard to utilize equip m ent m are effectively.
Summa~y af t}le ~vention The present invention fu~ills the above stated need by repli-cating in mit~iature the operative par~s which support the opcr~tive element of the equipment. Although the sy~3tem is applicah1.e to a wide range of construction e(luipment, earth moving machines and virtually anything where an operator controls a machine, and thu~
the syste m described and claim ed her~in is intended to cover all applicat;Lons, the descr.iption from this point on will per~Lin to a backhoe to eliminate the need for repetitive, broadening verbiage.
It is clear the principles and systemic ~ements of the backhoe device can he generalized to any hydraulica1ly operated machine having any nu m ber of dim en~Lons of m otion.
The replica backhoe contairs intern~l hydraulic lincs to avoid the need for locse external lines WiliCh would be subject to breakage.

"

3L20~ 3 -~-Each articLiLation in the operative portion of the backhoe arm is provided with a contr~l cylinder which .is operated by motion about the artJc~ tion and transmits infcrmation concerning the motion by way of hydrdL~Lc lines to a pilot valve. The pilot valve, which in 5 its~ is a novel element created b~ the inventor fc)r this particL~ar purpose, operates a pilot plqton which iq incorporated in the same unit which mechanir.~lly controls the v~lve for the dr.ive cylinder for the respective articul~tion on the actual. boom structure.
Each of the analog drive mechanisms for each articulatAion 10 described above also has a~ociated with it a fbedback xystem compri~ing a cyl;nder mountcd on or ~d~ccnt; the (h~ive cy.~irl(ler of the respective articulatiQn and mechanic.Llly driven by motion at the articulation to deliver hydra~ic fll.LiCI to the pilot inlets of the respective pl~ot valves. The feedback system delivers pilot fluid at 15 what amo~ts to 180 out of phase wit.h the control system ~o that, for exam~el rotation of the replica shovcl callses r otation of the actual shovel which is irn mediate.ly cance~led by the neg,cltive feedback from the feedback syst~m, as soon ~s l;he r~pli.ca contr~l v~-Llve stops moving. In this f~shion, ~ d.irect analo~, movement OCCll~; virt~lally ;~0 si.multaneously in the actual operative memher3 of the backhoe with the repLica ~ackhoe.

~Z~43 1~.

Brief De~cription of t2:~e Drawir~;
Figure 1 is a som ewhat diagra m m atic Dlus~ration of the r~plica backhoe;
Figure 2 is a front elevation view of the r~plica;
Fig~re 3 is a top elevation view of the rep~ica .showing the hand knobs in place;
Figure 4 is a schematic illus~rating the hydraulics of the system;
Figure 5 is a section through the pilot valve-pilot piston;
F`igure 6 i~ an elevcaWon vl.ew f~om li.ne h-6 of Figure 5;
Figures 7 through 9 are sections t~ken along the lines numerically.indicated in F.i~lre 5;
Figu~e 10 is a side elevation, parti..~lly cut away and partiaUy in phantom, of a fragment of the spindle which operates the pilot piston;
Fi~ure 11 is a pro;ection of the perimeters of the r~lieved sections of the pistorl of Fi~1re 10 as proiect~d into a p~nar configuratlon;
Fi~lres 12 through 15 are SeCt.'lC)r9 taken al.ong the indicated section line of Fi~lre lO;
~igure 16 is a partially cut away, partially phantom i~lustration of the hydra~lic feedback mechanism;

~l2~6~3 ~j Figure 17 i~s a section taken along line 17-17 of Figure 16;
Fi~lres 18 and 19 are sections taken through the indicated ~ines of Fi~ure 17;
Figure 20 is a section taken through the backhoe arm structure to ;llustrate the opera~ive contr~l mechani~sm therein;
Figures 21 tt~ough 26 are æctions taken along the respect.ive section ~ines indicated in Figur~ 20;
Figure 27 is a æction taken along line 27-27 of ~igure 26;
and Fig~lre 28 is a ~.ctlon through the portion of t~ ba.ckhoe arm including the two outermost articula1;ions.
Deta~d Dcscription OI the Pr~fe~l Embodiment The rep~ica backhoe arm is shown at 10 in Figure 1. The arm cor~ists of a replica ~shovel 12, a dipper stlck 14, a boom 16 mounted 1~i to a swivel 18 which is a~icu~ated abo~lt a ver~i.c~l axis on a base m e m ber generally indicated at ~0.
l`he re,~Lica 10 is mounted ln its entir~ty in the cab of the backhoe and ordinarily positioned such that the operator straddles the baæ 20 in op~ration, and has a full view of the actual backhoe. The actual backhoe arm is not shown in the drawings, as it is not needed in order to clearly unde~tand the operation of the sy~tem.

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9~Z~64~ -6-. .

Befc~ e turn~ng to the m echanical details of the device, the operation of the hydr~ulic system will be explained. This system i~s fully set f~th in Figure 4. Actu-Zlly, it is m~ accurate to say Figure 4 r~presents one suhsyste m, there being four identical 5 subsystems in the backhoe imFilementation which to~ether def~Zne the comF~ete system .
Between each of the parts of the backhoe arm which de~ne r~lative movement a (limen~sion of movemerlt is de~5ned. Fcr example, the bucket 12 sweeps a cir~le about ;ts axis to define an ang~ar 10 progre~sion about a horiæontal a:~is as it~ dimension of motlon. This is a single dimersion Or motion in that it may be defYned by a sin~le, non-vectorial coordinate. Thus, each of the ar~icul~tions 24, 26, 28 and 30 defines a æparate dimension of movement in which any and all positions may be exactly located with a single number. It will 15 become apparent from the description of the h~ydraulic system that each dimension Or motion is separately treated by its own hydraulic bsystem and, acting independently of the other dimensions, causes the analog movement to occur on an amplif~ed scale in the actual articulation OI the real ~ackhoe arm.
20Retum~ng to Figur~ 4, the control cy~inder 22 is a generalized control cylinder, as ar~ all the other c1ernent~ in liigure 4, which ~2~643 -7~

ar~, in fact, found fottr times in the actual p~ly.s~cal implementation.
For ~irnplicity, Fig~u~e 4 w.ill be described as though the control cylinder 22 was connected to and represented the ar~iculation 24 between the dipper stick and the bucket 12.
Assuming that the simulated bucket i~s dipped and this causes the control piston 32 of the cylinder 22 to move to the left, thls in turn causes a pressure in the p~ot inlet 34 of the pilot valve 36 fr~m the contr~l chamber 38 and simultaneotlsLy permits d~inage f~om the p~lot ir~et 40 back irto the secon(i p.ilot chamher 42. ~ction 0 of the pilot inlets 34 and 40, of cour~e, causes the pilot val.ve 36 to ~ift, aCtlE.tting the pilot piston ~16.
The pilot piston 116 is mechanica.lly linked to the dt~ve cylinder valve 48 which operates the re~spective actt~l hucket on the real backhoe arm. Assuming the valve 48 is moved to the rlght, flu~d will com mence to flow into the drive cylinder 50, which actually powers the real bucket, and move the p;ston 52 t~ the l.eft. Thi.st in tt~n, moves the mechaniccll feedbacl~ linka~e 511 to the left, ~ivin~
the feedbt.tck piston 5fi in the feedback cylincler S~ to the lef~, which has the eff`ect of fllli.llg pilot ir~let 40 and ~raining pilot inlet 34, which cancels the action of piston 32.

12~ 3 ~

The feedback mechanism 54, 56 and 58 is mechanicaUy imp,lemented by a sprlng loaded cable connected to the distal end of the rod of piston 52 in an ar~angement detailed below. An analysis of the hydraulics of the system revea1s that dispa~cement of the 5control cylinder 32, by rotating the bucket 12, will cause valve 48 to open until this d~placement is equaled by piston 56 in the feedback cy]inder, which neutralizes the effect of control. cvlinder 22. In normal operation ther~ i~s a]so a l~straint on the cont,T~ piston 32 caused by the hydraulic back pres~ whi.ch will occur as the inl.et 1034 fi~ls, and will not be re~ieved until the feedback cy.linder 58 supplies enough feedback fluid. Therefore, thcr~ is a virtual sim~taneous anal~gous motion between the replica arm and the real arm. Unless too much fcrce is appIied to the pLston 32, it will not anticipate the action of the drive cylinder 50 by more than a few milliseconds. For 15all intents and purpcses, the operator, opexat;.n~ on the rep~ica, is simultaneou~ly operat,ing on the real. world through the actual backhoe arm .
Reverse action o~ the bllcket ~l2, of cour~se, ha.s exact.].y the opposite action through the hydraulic syste m . This bi~lir~ctional c20analoc~ is duplicated at each of l~he artic~atior~s 24 th~ough 30 and acts through a bcmk of four side-by-~side p.ilot valves 36 which are ~2~ 3 _9_ mechanica~ly linked directly to the us~l operating knoi~s of the conventional hydraulic controLs, not shown. ~`rom this bank of vc~lves, hydrat~ic lines e~tend both the the t~^tse 22 of the replica, and, in the other direction, out to the fee~back cy~inders 58 which are 5 mounted on the drive cylinders 50.
Tuming now to the mechanical description, with a few exceptions the deta~ls of construction of the dipper stick do not form part of the novelty of the invention The exact com hination Or plates, panel~, pivot ~ins, anm~lar hydraulic fl~lid duct.s and other 10 hydt~u~c lluid passages, .are for the most pat~ standarcl engineering design. For thLs reason the arm of the hackhoe wDl be de~.cri.hed in som ewhat sum m ary fashion.
The base 20 def~nes a ~ ity of p.~ageways hO in a kind of manifold which communicate from the arm to the pilot valves.
5 These passageways are defined in a block structt~e which includes (a~s the same or separate piece) a portion having a bore 64 through it with a ~lity of ann-lar ~I.uid p-t~geways 66 s~parated by O~irt~s 6~.
A drum 70 f~ts ~nugly within the bor~ 64 and defines a plurality 20 of gener~ly radial bores 72, each of which comm-micates with one : l2(1 ~643 -lo-of` the pa~sageways 66, and extends interna~ly of the drUM and downwardly into the hlock por~ion 71J which, by virtue of the angular sLiding abiLity of the drum 70 in the bore h4, 3S f~eely articulated about a vert,ical axis.
Each art,;elll~t,;-~n, including the articuLation 30 between the base member and thR lower rot,ating hlock 74, operat,es a control piston and cylinder co m binat,ion such as the diagra m m aticaLly illu~
strated piston 32 and cylindel~ 22. For the ar~icuLation 30, a spur 76 e~ctends above the drum 70 and r~tates ther~wi.th, operat,inlr a 10 rack on a piston rod 78 which connects t,o pi~ston 80 ~liding in cy.linder 82. The piston has a rear piston rc~d 84 so that di~splace m ent, on both sides of the p.iston ls equal. The piston preferably has relief checkvalves such as v~lves 86 shown in Fi~ur~ 22 to prevent damage to the structure shoul.d a jam occur. Once ~ id pa~es through the 15 checl<valves, the control and actual c~-linders will he out of synchrorlization. To l~synehr~niz.e t,he ,~a;r, the actual bucket can be brought ag,a.inst an im movcahl.e ob~ct, and the cont;rol cy:l:i.nder pu.slled beyond the abiLity of the actùal cylinder's ab.ility to respond until s,ynchrorlzation is achieved.
The pinion, rack, and piston-c,ylinder arrangement ~or aLl four artic-~tiors are simiLar to that ~ st described for c~1culation 30, :1201643 and will not be redescribed for each artAiculat.ion, but wi~l be referred to in each instance as a "contrca cy.linder system", which will be understood to include the above components, plus~sea~s, plates, and other items that are apparent from the drawings and necessary for 5 the proper operation of the machine.
In the block 74 another relief cyIinder system 88 is provided for art.i c~ llat.i tn 28 . This cylinder co m m uNicates through passage ways 90 to the drum 70. This is the boom control system, and operation of the boom ca~ses the pinion to operate the rack and move the 1 0 piston.
Figure 25 .illustrates the pinion ~s~ft q2 on which the. pinion 94 is pinned. Passageways 96 (sho~n in Figure 25) comm~micate with two pairs of annuL~ pa~ssageways 9~3 and 100 which interface with transmittal port pairs 102 and 104 which commlmicate r~spectlvely 5 with the bucket control cylil1der sy~.*em 106 and the dipper stick cont;r cl syste m 1 n8.
The port p~irs 102 and 104 com muni.cate with pa~.~geway pca~3 110 and 112, seen in phantom in Fi~lre 20, which resl-ect~ively service the bucket cont~ol cylinder and dipper st-ick control cylinder. These 20 pa~ssageway~s ar~ def~ned in boom si.de plates 114, th~u~;h which pins 116 pa~s to engage F~nion ~sh~f~s ,q2 shown in Figure 25. Cover F~ates 118 cover the boolT side p]ates 11~1 to create a discrete set of channels deflned within the side ~ates 114.
Without going irto more detail, it can be seen that at each articulation a pinion drives a pi~ston within the appropriate contrcl 5 syste m as the articulation m oves, and control nuid f ro m these cylinders ~s delivered through the dr~lm 70 and manifold h~.ocks 62 to be distributed t,o the appropriate pilot valve.
Attention is now directed to thc pilot valve, illustrated in the Figune sequence f~om Fi~lre 5 thr~ugh E~`igure 1 r~, Generarl.y speahing, 10 all the body parts of the valve are rectangulcu w.ith the exception of the piston and cylinder. The v~lve has a h~se ~te 120, a cylinder wall 122, a cylinder cap 124 and an end wall 126, al. being held together by the bc~ts 128. The cylinder wall def~nes cyinder 130 in which rides the piston 132, which i~ pr~vented from axial motion by 15 two parallel piston rods 13~ which pa~ss through suitable sealed apertures in the cylinder cap 124 to terminate in a t.ie bracket 136.
A pair OI through bc~lt holes 138 pa~c~s throl~gh the c~linder ~ock and join fo~ b~ocks together.
Drairage is prov1clecl to the c~ylinder through rc.stricted orifice 20 nuts 140 ~inked with passageway ill2, wi.th consecutive cylinder blocks being linked by pas~through bc~e 111l~.

lZ~1643 Twning to the piston assem~ly, the pis~on 132 h~s end æals 146 and a r~ieved annular area between the se~ls def~r~ng a chamber 148. This chamber is continuous aroun(~ the piston and com m~licates w.ith a bore 150 in the cylinder ~ilock, indicated in phantom in Figure 5 8. This bore crdir~ily would be the hydraul;ic fluid supply line, and agairl wo~l pass through a~l cylinder blocks. A radial. bore 152 .in the ~ston communicates between the chamber 148 and an axial bore thr~ugh the pi~ston which seats spdndle 154. The spindle is mounted on bearings 156 ~t either end and locked in place w;.th nut 158.
The spindle has two relievecl portiors 1~0 and 162 shaped ~somewhat.like elongated lamb chop~s and illustrated ~9 they would appear if the perimeters were rolled into a ~l~t plane in Figure 11.
Due to these relieved pcrtions, which com ml~nicate between oppt~site ends of the cy]inder 130 and the centri~ port.ion of the piston bore, 15 rotatitxl of the spindle in one directi.on or the other will cau~se the nearest por~ion of the inclined s~u~ace lh4 of the respec~ive bore to index with the piston bore 152, permitt~i.ng~ the fLuitl, which is ~der pres3ure in the cylinder, to escape f~m the re.spective end of the cylinder through the bore 152, and the h~r~s 150 in the ~locks, to a 20 hydraulic reservoir. Glearly, rotation of the spindle in opposite ~Z~1643 direetions eauses the piston to m ove in opposite direetions. The tapered edges 164 of the re~teved portions causes the piston to smoothly slow down, coming to a stop as the orifice 152 pa.sses acr~ss the edge 164, out of the relieved zone.
It ean thus be seen that power contrc~ of the piston 132 ean be effected by rotating the spindle 154 in a controlled manner. This is accom~ished with a rack 166, cut in a cylindrical rack bar 168 having 0-ring seals 170 and end plugs 172 which are too small to seal the opening. The O~ings 170 provide the seal.
The ir~Let ports 34 and 36 shown in conJunction with the pilot portion of the system ln the deseripti(-n of F;gllre ll can also be .seen in the physical embodiment of the valve in Figure 5, Feedback ports are bored direetly into the base pLat,e 12û and indic~ted at 17)~ and 176. All ports communicate through relief baU checkvalves 178 into 15 the eylinder which drair~ throtlgh orifice 140, to permit thermal expansion. It shoukl th~ts be clectr from the above de~script1on that p~ot; presst~es and feedbaek pre~ s delivered t~ the valve at the respeetive inlet ports will eause the r~ol< bar lh8 to translate, thus rotating the spdndle and ca-~sing the pi.ston to move one direct,ion or 20 the other. As will be reea~led from the discu~ on of the ertire 1;~ 3 -15-hydraulic circuit and the feedback systern, the piston, throu~h ;ts coupl~ng ~te 136, drives the valve of the actual equipment hydraulic cylinder which, through a m echclnica] and hydraulic coupling, im mediately feecls back through the appropriate port 174 or 176, 5 cance~ling the initial pilot action, unl~ss the initial action is continued by the continual operation of the contr~ cylinder 132. In the Latter event, the pilot piston 132 will remain in a fi~ecl, displaced position as long as the con~l cylinder move~s at a fixed velocity. In other words, the static? offset pasition of the pi~;ton 1~2 corresponds to 10 the steady, pl~portional movement of the contrQl piston an(l drive piston in their respective cylinders.
Another feature of the pilot valve-piston system can be seen in Figures 10, 13 and 14. A passageway 180 def~necl ~xially inside the s~indLe communicates with the c~1inder thr~ugh opening 181 and 15 an outlet 1g2, which indexes with port 152 when the system is in its neutral through position. Thi~ pa~ageway suplllies EIuid to the contr~1 system in the event of contraction due to t~rmperat~n~e.
The feedbaclc struct-lre is shown in Fif~rre 16. This subsystem, which ordinarily wol~ld be on the order of ~six to e;ght inches long, 20 wou~d mount directly on the cy3inder hou~ing, c~ near the cylinder housing, out on the arm of thè act~l e~ ipment. The mechani~sm ~2~6~L3 _16-has a housing 1û4 with mounting bracket,s 86 of the equivalent to permit mounting of the unit as a retroflt item. A cable 188 Ik~s a t,erminal 190 which is bolted into the other side of the equipment artict~ation, or~iinarily the distal end of the piston rod or ac~cent structure. The extension of the cabl.e 188 is thus exactly the same as the extension of the drive piston.
The housing, which includes a ~ir of sLde plat,es 192 and 191J
and a peripher~l wall 1~6, also mounts a spool or reel 198 for the cable. Cable tension is m~intained by a leaf spri~ 200 ins~dc the 1 0 reel.
Mounted coaxially on the a~de 202 is a sprocket 204 for dri.ving a chain or toothed belt 206 which passes around a larger puUey or sprocket 210 at the other end of the holsing. The ratio between the two sprockets is such that several revol~ltions of the smaU sprocket will cau.se only a parti~l rQtation of the l~r~e sprocket.
A ca m 212 is t~rlven by the L~rge sprocket, and is m ount,ed coa~ially therewith on a post 214. A cam fo31Ower 216 is held firmly a6ainst the cam surface by ext~n~sion sr)rin~s 218.
The feedback cylinder 58 occurs on the other side of an end wal~ 220 and is driven by a piston rod 222 connected to the cam foUower 216. As can be ~seen in Figure 16, hydral,llic fluid pa~ssageways 12()~16~3 -17-224 and 226 termincat~ in pc~s into whi.ch return lines connect, com municating with the feedback ports 174 and 176 of the p:~ot apparatus.
Althou~h there are clearly more direct wcays of converting the 5 extension of the drive cy~inder into the operation of a feedhack h~,rdral~ic cylinder, ~several advantages are inherent in the use of the cam ar~sem~y illustrated. First, because ten~sion on the cable drives the cam into its lower profSle pc~sit1or~s, there can be no f`t.~rceing action on the feedback piston in cc~ of ~ m bing. ~Since ~ ca~le 10 cannot be pushed, force fl~m the ec;uipment c~annot c~ .se dama~e to the feedback unit in c~se anything is jam med. Und~le pre~r~sure b~ckulg up into the pas3ageway 22~ would not occur bet~se thi.s pa~;ageway comm~u~icat~s with the thre~shhold rclief valvt-~ 17~.
Another, and mc~e import~mt advantage of the c~m lies in the 1~ fact that the rack and pinion contr~l movement ~t the ~articul~ti.on~s of the rep]ic~ do not phy.sic~Uy dllplicatc the~ geomet;ry of movemt.~nt of most hydraulic cyli.nder~s. The pr~sence of the o~m provide~s an ideal mean~s of propc~rtionatillg what wol.llcl otherwise be a non-~inearity in the operation of the drive cylinde~r~ by the contro]. cylinders.
20 Another control feat~lre ~ies in the sprirlg,-lo~ded nature of the dr~lve cylindens 48 which will bring them back to the neu~u~al po~sition absent ~ZQ~ 43 a control force. This caus~s a blas toward the stopped mode fc~ the ent~re system.
Wh~e I have described the preferred embodiment of the invention, other embodiments may be devised and diff`erent uses may 5 be achieved without departing f~om the sp~rit and scope of the appended c~ims.

cO

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A Servo Amplification System comprising:
(a) a control cylinder having a double-acting control piston defining a first and second control chamber on opposite sides of said piston;
(b) a pilot valve and a pilot cylinder having a doubleacting piston and being operated by said pilot valve, said valve having a first and second inlet port connected to said first and second control chamber, respectively, to drive said pilot piston selectably in a first or second direction;
(c) a drive cylinder having a drive piston activated by a directional valve operated by said pilot cylinder;
(d) a feedback cylinder with a double-acting feedback piston operatively connected to said drive piston to be moved thereby, said feedback cylinder defining first and second feedback chambers on opposite sides of said feedback piston, said first and second feedback chambers communicating with said second and first inlet ports respectively, such that movement of said control piston in a first direction operates, through said pilot valve and cylinder, said drive cylinder which moves a distance proportional to the distance moved by said control cylinder.

2. Structure according to Claim 1 wherein said feedback piston is mounted on a rod operated by a cam which is rotated by said drive piston as same extends and retracts and said cam is shaped to proportionate the distance moved by said drive cylinder relative to the distance moved by said control cylinder.

3. Structure according to Claim 2 wherein said cam is driven by a coaxial pulley driven by a belt driven by a spool having a cable wound thereon and connected to said drive piston.

4. Structure according to Claim 2 wherein said feedback cylinder and piston are mounted in a housing having means to mount same on a drive cylinder.

5. Structure according to Claim 4 wherein said housing includes a cable-wound cylinder driving a cam and said feedback piston is mounted to a cam follower, and including means to fix the free end of said cable to the rod of said drive piston whereby said Servo Amplification System is mountable as a retrofit on existing equipment.

6. Structure according to Claim 1 wherein said Servo Amplification System includes a plurality of sub-Servo Amplification Systems each including a control cylinder and piston, a pilot valve, cylinder, and piston, a drive cylinder and piston, and a feedback cylinder and piston, and each subsystem controls a different dimension of movement.

7. Structure according to Claim 6 wherein said system is mounted on a unit of construction equipment having an actual operative element having a plurality of dimensions of motion powered by respective ones of said sub systems, and including a miniature replica operative element having a plurality of dimensions of motion corresponding to those of the actual operative element and having the control cylinders and pistons of said subsystems operatively connected to said miniature operative element responsive to the respective dimensions of motion thereof.

8. Structure according to Claim 7 wherein said unit is a backhoe and said actual operative element. is a backhoe bucket articulated on a dipper stick articulated on a boom articulated for rotational motion about a vertical axis on a base member.

9. Structure according to Claim 3 wherein said replica includes one of said sub-system control cylinders and pistons operatively mounted for each articulation and operated by the motion of the respective articulation to operate proportionately the articulations of said actual backhoe bucket.

10. Structure according to Claim 9 wherein the pistons of said control cylinders are each provided with bi-directional pressure relief valves.

11. Structure according to Claim 9 wherein said replica includes a plurality of hydraulic lines passing fluid from said control cylinders through to said base, and said hydraulic lines are defined by the replica dipper stick and boom.

12. Structure according Claim 9 wherein said control pistons are each mounted on a piston rod defining a rack at one end, said control cylinders are mounted at one side of said articulation and including a pinion fixed to the other side of the respective articulation coaxially with the respective axis of articulation.

13. Structure according to Claim 12 wherein each of the feedback pistons is driven by a cam shaped to linearly proportionate the motions of the actual and replica operative elements.