CN105619449A - Zero-free length spring gravity compensation method based on force feedback equipment - Google Patents

Abstract

A zero-free length spring gravity compensation method based on force feedback equipment comprises the steps that 1, one end of an upper arm gravity compensation spring is connected to a spring fixed installation surface of a base speed reducing mechanism driven wheel, and the other end of the upper arm gravity compensation spring is connected to a steel wire rope for being connected to an upper arm speed reducing mechanism driven wheel and the junction point of the reverse extension line of an upper arm and the edge of the driven wheel through a fixed pulley; 2, one end of a forearm gravity compensation spring is connected to the spring fixed installation surface of the base speed reducing mechanism driven wheel, and the other end of the forearm gravity compensation spring is connected to a steel wire rope for being connected to the junction point of the edge of a forearm speed reducing mechanism driven wheel through a fixed pulley. According to the method, the inertia of an upper arm mechanism or a forearm mechanism of the force feedback equipment is lowered, the steel wire ropes are prevented from interfering with the upper arm mechanism or the forearm mechanism at a certain position, the influence of the radius length and the installation positions of the fixed pulleys on gravity compensation is fully considered, and the spring gravity compensation method can realize complete gravity compensation for the force feedback equipment.

Description

A kind of zero drift spring gravity compensation method based on force feedback equipment

Technical field

The present invention relates to zero drift spring gravity compensation method of a kind of force feedback equipment.

Background technology

Force feedback equipment often uses spring-compensating gravity, has an advantage in that: spring-mass is light, will not increase too much weight and inertia on force feedback equipment, thus the operating characteristics of power of influence feedback device. In force feedback equipment, spring is used to carry out gravitational compensation method and have multiple: simple spring gravity compensates, zero drift spring gravity compensates and cam spring gravity compensation and some other spring gravity compensation way.

So-called simple spring gravity compensates, and refers to not by other auxiliary equipment, for instance: steamboat, connecting rod, irregular cam etc., only use spring that force feedback equipment is carried out gravity compensation. in the method, spring one end is connected on the motion arm of the force feedback equipment of required compensation, the other end is fixed on the fixed support of force feedback equipment, compensate to obtain spring gravity completely, spring rate K must be variable, therefore, such spring is all nonlinear spring, causes difficult design in practical engineering application. in common engineer applied, in order to reduce the complexity of spring design, conventional Hookean spring replaces nonlinear spring, the meansigma methods of the gravity compensation amount that the stiffness coefficient of this Hookean spring is required when being in different angles frequently with the connecting rod of required compensation gravity is to ask for the mean rigidity coefficient of spring, and with this mean rigidity coefficient stiffness coefficient as Hookean spring, thus realizing the gravity compensation of connecting rod. but, owing to the force feedback equipment motion arm of spring and required compensation is joined directly together, easily cause spring, with connecting rod, mechanical interference occurs in the space of part. impact in order to avoid above-mentioned existence, spring is arranged on reducing gear driven pulley by AhmadMashayekhi cleverly, rather than be joined directly together with force feedback equipment motion arm, the simple spring gravity compensation way devising a kind of novel structure (refers to: VirSense:anovelhapticdevicewithfixed-basemotorsandagravi tycompensationsystem, IndustrialRobot:AnInternationalJournal, 2014,41 (1): 37��49.). but the method for designing of AhmadMashayekhi is artificial provide spring connecting point position, when designing spring gravity and compensating, do not set up optimum spring gravity and compensate mathematical model, do not consider the optimum drift in the spring optimum junction point position of gravity compensation, spring and optimum mean rigidity coefficient. for the problems referred to above, Li Chunquan etc. propose a kind of based on the force feedback equipment optimum spring gravity compensation method improving simple particle algorithm, spring is arranged on the line gear reduction driven pulley of force feedback equipment large arm and forearm, take into full account the junction point position of spring, drift and the stiffness coefficient impact on gravity compensation, establish the gravity compensation model of nonlinear restriction relation, introduce " stretching drift ratio ", using junction point position and " stretch drift than " as optimized amount, average torque error is as the fitness function optimized, use and improve letter single-particle optimizing algorithm iteration optimization, the arm mechanism making force feedback equipment is obtained in that the spring gravity of optimum compensates.

In simple spring gravity compensates, although said method can carry out gravity compensation, but can not realize force feedback equipment spring gravity completely and compensate, this is owing to spring free length is not zero, if needing to realize complete gravity compensation, designed spring rate is inevitable non-linear. but, in Practical Project, adopted Hookean spring replaces the nonlinear spring rates coefficient of reality. therefore, utilizing simple linear spring is force feedback equipment to be carried out complete gravity compensation. in order to use Hookean spring to realize adequately compensating for of force feedback equipment, RongfangFan employs Hookean spring, fixed pulley and steel wire rope, fixed pulley is arranged on PHANToMPremium1.5 motion arm, steel wire rope one end is after to be fixed on motion arm tangent with fixed pulley, it is connected on the fixed support of force feedback equipment, the gravity compensation mode devising a kind of zero drift spring (refers to: ImprovementofDynamicTransparencyofHapticDevicebyUsingSpr ingBalance [C] .Proceedingsofthe2012IEEEInternationalConferenceonRoboti csandBiomimetics, 2012:1075��1080.), when the fixing shaft length of fixed pulley radius and fixed pulley is equal to zero, the method can obtain gravity compensation completely. in actual design, owing to the radius of pulley and the fixed position length of pulley are always it is difficult to ensure that be zero all simultaneously, in addition the connection between pulley and steel wire rope is constantly present friction, and these complete gravity compensation modes having resulted in zero drift spring also can produce to compensate deviation. comparing simple spring gravity to compensate, zero drift spring gravity compensation way needs extra steel wire rope and the fixed pulley of increasing, fixing with the former complexity relative is installed. additionally, with simple spring gravity compensate the same, pulley and steel wire rope also can and force feedback motion arm interfering between formation mechanism on some position, affect the work space of connecting rod. and in the method for above-mentioned RongfangFan, be only that single operation arm in PHANToMPremium1.5 is carried out gravity compensation. actually, in zero drift spring gravity compensation way in RongfangFan proposed method, fixed pulley and steel wire rope are separately mounted on the end to end large arm of force feedback equipment and forearm, owing to large arm and forearm relative position in operation constantly changes, now, the gravity compensation of forearm can be lost efficacy by the method. furthermore, connecting rod on forearm, pulley and steel wire rope and large arm are motions mutually, therefore, may result in the spring in large arm and steel wire rope interferes with forearm in some locations, also likely to be present some position so that the spring in large arm is fully relaxed, do not reach the effect of compensation. therefore, adopting this type of zero drift spring gravity compensation method that Hookean spring, fixed pulley and steel wire rope are arranged on series connection force feedback equipment motion arm is also existing defects.

Cam spring gravity compensation mode also is able to realize the gravity of force feedback equipment is compensated. Omega force feedback equipment adopts spring, steel wire rope the cam that constitutes in conjunction with single irregular circumference that force feedback equipment carries out gravity compensation (patent No.: US8,188,843B2). RongfangFan also using and is similar to gravitational compensation method that Omega force feedback equipment cam spring combines in a kind of principle for carrying out gravity compensation (refer to: ImprovementofDynamicTransparencyofHapticDevicebyUsingSpr ingBalance [C] .Proceedingsofthe2012IEEEInternationalConferenceonRoboti csandBiomimetics, 2012:1075��1080.) in PHANTOMPremium1.5. Cam spring gravity compensation mode also is able to completely the motion arm of force feedback equipment be carried out gravity compensation, and this is the great advantage of which. But, this compensation way there is also following problem: first, wants additional designs and machining cam, and to be installed to by cam on the power transmission shaft of connecting rod of existing force feedback equipment, which increase cost and the complexity of design and installation when carrying out gravity compensation; Secondly, owing to the radius of cam is the function using angular displacement of the cam as independent variable, when force feedback equipment is when rapid movement, the steel wire rope connecting cam and spring will not be always maintained in the same plane, this has resulted in spring and steel wire rope produces drawing gap, introduces delay volume, and between this and gear, backlash is similar, easily make spring and steel wire rope landing, also can reduce the stability of force feedback system.

Above-mentioned mentioned spring-compensating gravity mode, is mainly used in force feedback equipment and carries out gravity compensation. Additionally, there is also other spring gravity compensation ways many, in existing document and patent, these methods are used in force feedback equipment. Although the mode that those spring gravity compensate has advantage, but they or design are complicated, not easily realize and be retrofitted on existing force feedback equipment, or designed by some specific mechanisms, not there is the universal meaning of force feedback equipment gravity compensation design.

In sum, by analyzing the feature of all kinds of force feedback equipment spring gravity compensation method, we disclose a kind of zero drift spring gravity compensation method based on force feedback equipment, the spring of force feedback equipment gravity compensation and fixed pulley are separately mounted on the spring fixed installation face on the base mechanism driven pulley of force feedback equipment, spring one end connecting steel wire ropes one end, this steel wire rope other end is connected to the reducing gear place of force feedback equipment; This spring other end connects other steel wire rope one end, and this steel wire rope other end is fixed on the spring fixed installation face on base mechanism driven pulley. The advantage of the method is in that: spring and fixed pulley are arranged on the spring fixed installation face on base mechanism driven pulley, rather than it is arranged on big arm mechanism or the little arm mechanism of force feedback equipment, alleviate the inertia of the big arm mechanism of force feedback equipment or little arm mechanism, it is to avoid steel wire rope interferes on some position with big arm mechanism or little arm mechanism. And to this zero drift spring gravity compensation method founding mathematical models, in theory fully demonstrate the method and be capable of gravity compensation completely.

Summary of the invention

The present invention is directed to the problem existing for existing spring gravity compensation technique, disclose zero drift spring gravity compensation method of a kind of force feedback equipment, for force feedback equipment is carried out gravity compensation. The spring of force feedback equipment gravity compensation and fixed pulley are separately mounted on the spring fixed installation face on force feedback equipment base mechanism driven pulley, gravity compensation spring one end is connected to the spring fixed installation face on base reducing gear driven pulley, the other end is connected by steel wire rope and is connected to through fixed pulley on large arm or forearm reduction driven wheel, for realizing large arm or little arm mechanism zero drift spring gravity are compensated.

The present invention is achieved by the following technical solutions.

Of the present invention it is characterized in that method step is:

(1) the gravity compensation spring one end of large arm is connected to the spring fixed installation face on base reducing gear driven pulley, the other end is connected by steel wire rope and is connected on large arm reducing gear driven pulley and large arm reverse extending line and interface point place, driven pulley edge through fixed pulley, for realizing big arm mechanism zero drift spring gravity is compensated;

(2) the gravity compensation spring one end of forearm is connected to the spring fixed installation face on base reducing gear driven pulley, the other end is connected by steel wire rope and is connected to interface point place, forearm reducing gear driven pulley edge through fixed pulley, for realizing little arm mechanism zero drift spring gravity is compensated.

The present invention has taken into full account radius length and the installation site impact on gravity compensation of fixed pulley, this zero drift spring gravity compensation way is established mathematical model, demonstrates this spring gravity compensation way theoretically and force feedback equipment can be realized gravity compensation completely.

The invention have the advantage that and spring and fixed pulley are arranged on the spring fixed installation face on force feedback equipment base mechanism driven pulley, rather than it is arranged on big arm mechanism or the little arm mechanism of force feedback equipment, this just alleviates the inertia of the big arm mechanism of force feedback equipment or little arm mechanism, it is to avoid steel wire rope interferes on some position with big arm mechanism or little arm mechanism. In addition, this inventive method also takes into full account radius length and the installation site impact on gravity compensation of fixed pulley, this zero drift spring gravity compensation way is established mathematical model, it was demonstrated that force feedback equipment can be realized gravity compensation completely by this spring gravity compensation method.

Accompanying drawing explanation

Fig. 1 is that spring gravity of the present invention compensates bindiny mechanism figure.

In figure: 1 is base, 2 take turns for base reduction driven, and 3 is base deceleration drivewheel, 4 is direct current generator, and 5 is base shaft, and 6 is big arm mechanism, 7 take turns for large arm reduction driven, and 9 is large arm gravity compensation spring, and 10 is fixed pulley, 13 is little arm mechanism, and 14 take turns for forearm reduction driven, and 15 is forearm deceleration drivewheel, 16 is forearm gravity compensation spring, and 17 be fixed pulley 18 is direct current generator, and 19 is forearm rotating shaft, 20 take turns rotating shaft for forearm reduction driven, and 21 is universal joint, and 22 fixedly mount face for spring.

Fig. 2 is that turn clockwise 180 �� of spring gravity of Fig. 1 compensate bindiny mechanism figure.

In figure: 1 is base, 2 is base reduction driven wheel, and 5 be base shaft 6 is big arm mechanism, 7 take turns for large arm reduction driven, 8 is large arm deceleration drivewheel, and 10 is fixed pulley, and 11 is direct current generator, 12 be large arm rotating shaft 13 is little arm mechanism, 14 is forearm reduction driven wheel, and 16 be forearm gravity compensation spring, and 17 be fixed pulley 19 is forearm rotating shaft, 21 is universal joint, and 22 fixedly mount face for spring

Fig. 3 is the present invention big arm mechanism zero drift spring gravity compensation principle figure.

Fig. 4 is the present invention zero drift fixed pulley schematic diagram calculation.

Detailed description of the invention

The present invention will be described further in conjunction with accompanying drawing.

Illustrate as follows in conjunction with accompanying drawing 1-4: as shown in Figures 1 and 2, this force feedback equipment is a serial linkage with 6 rotary joints, it is similar to the hands arm of people, key component is: base 1, base reduction driven wheel 2, base deceleration drivewheel 3, direct current generator 4, base shaft 5, big arm mechanism 6, large arm reduction driven wheel 7, large arm deceleration drivewheel 8, large arm gravity compensation spring 9, fixed pulley 10, direct current generator 11, large arm rotating shaft 12, little arm mechanism 13, forearm reduction driven wheel 14, forearm deceleration drivewheel 15, forearm gravity compensation spring 16, fixed pulley 17, direct current generator 18, forearm rotating shaft 19, the wheel rotating shaft 20 of forearm reduction driven, universal joint 21, spring fixed installation face 22 composition such as part such as grade.

As shown in Figures 1 and 2, the parts such as big arm mechanism 6, large arm reduction driven wheel 7, large arm deceleration drivewheel 8, large arm gravity compensation spring 9, fixed pulley 10, direct current generator 11, large arm rotating shaft 12, little arm mechanism 13, forearm reduction driven wheel 14, forearm deceleration drivewheel 15, forearm gravity compensation spring 16, fixed pulley 17, direct current generator 18, forearm rotating shaft 19, the wheel rotating shaft 20 of forearm reduction driven, universal joint 21, spring fixed installation face 22 are arranged on base reduction driven wheel 2 each through support, rotate around base shaft 5 together with taking turns 2 with base reduction driven.

As shown in Figures 1 and 2, base deceleration drivewheel 3 is nested on direct current generator 4, when base direct current generator 4 drives base deceleration drivewheel 3 so that both coaxial rotation. Base reducing gear drivewheel 3 drives base reduction driven wheel 2 by steel wire rope. Seat reduction driven wheel 2 rotates around base shaft 5, and pass through wire rope gearing, big arm mechanism 6, little arm mechanism 13 and universal joint 21 is driven all to rotate around base shaft 5, so that force feedback equipment can produce feedback force, base reduction driven is taken turns 2 rotational angles and is calculated acquisition by the photoelectric encoder on direct current generator 4.

As shown in Figures 1 and 2, large arm deceleration drivewheel 8 is nested in the rotating shaft of direct current generator 11, when direct current generator 11 drives large arm deceleration drivewheel 8 so that both coaxial rotation. Large arm deceleration drivewheel 8 drives large arm reduction driven wheel 7 by steel wire rope. The large arm reduction driven wheel 7 big arm mechanism 6 of drive, both rotates around large arm rotating shaft 12, so that big arm mechanism 6 can produce feedback force, big arm mechanism 6 rotational angle is calculated by the photoelectric encoder on direct current generator 11 and obtains. Large arm gravity compensation spring 9 one end is connected to the spring fixed installation face 22 on base reduction driven wheel 2, and the other end is connected by steel wire rope and is connected to large arm reduction driven through fixed pulley 10 and takes turns 7 ends, compensates for realizing the spring gravity to big arm mechanism 1.

As shown in Figures 1 and 2, forearm deceleration drivewheel 15 is nested in the rotating shaft of direct current generator 18, when direct current generator 18 drives forearm deceleration drivewheel 15 so that both coaxial rotation. Forearm deceleration drivewheel 15 drives forearm reduction driven wheel 14 by steel wire rope. Forearm reduction driven wheel 14 is taken turns rotating shaft 20 around forearm reduction driven and is rotated, and pass through wire rope gearing, little arm mechanism 13 is driven to rotate around forearm rotating shaft 19, thus realizing making little arm mechanism 13 can produce feedback force, little arm mechanism 13 rotational angle is measured by the photoelectric encoder on direct current generator 18 and is obtained. Universal joint 21 is arranged on little arm mechanism 13 end, the center of gravity of universal joint 21 concentrates on the end of little arm mechanism 13, universal joint 21 is made up of three passive rotary joints, and the axis of these three rotary joints is mutually perpendicular to, and the anglec of rotation measures gained respectively through respective angular potentiometer. Forearm gravity compensation spring 16 one end is connected to takes turns the spring fixed installation face 22 on 2 at base reduction driven, the other end is connected by steel wire rope and is connected to forearm reduction driven through fixed pulley 17 and takes turns 14 ends, for realizing little arm mechanism 13 and universal joint 21 gravity are compensated.

For the ease of analyzing, schematic diagram in the zero drift spring gravity compensation method Fig. 3 in Fig. 1 and Fig. 2, big arm mechanism 6 adopted is described. Assuming that big arm mechanism 6 connecting rod OO₁Represent, connecting rod OO₁Gravity torque is T=Bcos ��. Large arm rotating shaft 12 represents with O, and the stiffness coefficient of large arm gravity compensation spring 9 is K, and the arm of force OI length of O institute force of labor square is h by the tensile force of spring, then the pulling force of spring is F_s, moment is T_s=F_sH. Connecting rod OO₁Corner �� O₁OL=��. It is a, �� ION=�� FQP=�� that large arm reduction driven takes turns the length of 7 radiuses₁, �� QON=��, OG length is equal to rcos �� equal to rsin ��, QG length. One end of Hookean spring is fixed on M place, and the other end level that is connected with steel wire rope is tangent and meet at fixed pulley R point place. The other end of steel wire rope be fixed on large arm reducing gear driven pulley Q point and extend to fixed pulley intersect with fixed pulley P point, Q point is at OO₁Reverse extending line and driven pulley annulus external diameter intersection. Fixed pulley 10 is such as Fig. 4, and for lower section disk in Fig. 4, its center of circle is O₃, half path length is r ', O₀Point is the fixing strong point of fixed pulley, and the distance to the fixed pulley center of circle is l. In figure, W point is the intersection point after QP and the tangent extension of fixed pulley with FM. �� QWF=�� PO₃E=��.

Calculate in order to convenient, the sheave segment in Fig. 3 is repainted in the diagram. Shown in left figure in Fig. 4, now connecting rod OO₁With OO₀During coincidence, gravity torque T=Bsin �� is equal to 0. In order to ensure that zero drift spring carries out gravity compensation, now, spring elongation length should be equal to drift. So, Hookean spring combines with fixed pulley and steel wire rope and just constitutes zero drift spring gravity compensation way. Can obtain length between steel wire rope QR from the left figure of Fig. 4 is that QP length addsBetween arc length:

${\mathrm{QR}}_0=\sqrt{{(a-r-l)}^2-{\left(r^{\′}\right)}^2}+(\frac{\mathrm{\π}}{2}+arcsin\frac{r^{\′}}{a-r-l})r^{\′}---\left(3\right)$

Connecting rod OO₁With OO₀Time misaligned, as shown in the right figure of Fig. 4: �� QWF=�� PO₃E=��, then when Q point is positioned at any point position, O₃The length of E is that r ' cos ��, DF length is equal to l-r ' cos �� equal to ER length, so QD length is a-rsin ��-l+r ' cos ��, DP length is rcos ��-r ' sin ��, wherein, in �� QDP, the length of QP is:

$QP=\sqrt{{(a-r\sin \mathrm{\θ}-l+r^{\′}\cos \mathrm{\β})}^2+{(r\cos \mathrm{\θ}-r^{\′}\sin \mathrm{\β})}^2}---\left(4\right)$

Then the QR length of steel wire rope is:

$QR=\sqrt{{(a-rsin\mathrm{\θ}-l+r^{\′}cos\mathrm{\β})}^2+{(rcos\mathrm{\θ}-r^{\′}sin\mathrm{\β})}^2}+r^{\′}\mathrm{\β}---\left(5\right)$

So, the tensile elongation of spring is:

��_QR=QR-QR₀(6)

Then the pulling force of spring is:

F_s=K ��_QR(7)

The moment of spring is:

T_s=K ��_QRh(8)

Wherein,

H=rcos (��-��₁)(9)

Utilize QD length, DP length and the QP length obtained, can obtain:

${\mathrm{sin\θ}}_1=\frac{r\cos \mathrm{\θ}-r^{\′}\sin \mathrm{\β}}{\sqrt{{(a-r\sin \mathrm{\θ}-l+r^{\′}\cos \mathrm{\β})}^2+{(r\cos \mathrm{\θ}-r^{\′}\sin \mathrm{\β})}^2}}---\left(10\right)$

${\mathrm{cos\θ}}_1=\frac{a-rsin\mathrm{\θ}-l+r^{\′}cos\mathrm{\β}}{\sqrt{{(a-r\sin \mathrm{\θ}-l+r^{\′}cos\mathrm{\β})}^2+{(rcos\mathrm{\θ}-r^{\′}sin\mathrm{\β})}^2}}---\left(11\right)$

So that spring elongation moment can be fully compensated the gravity torque T=Bcos �� of connecting rod, then by spring produced compensation gravity moment should be equal to T=Bcos ��, so there being following equation to set up:

Bcos ��=K ��_QRH=Kr ��_QRcos(��-��₁)(12)

By formula (3), (5), (10) and (11) substitution (12) can be obtained the stiffness coefficient K of spring and be equal to:

$K=\frac{B}{{\mathrm{r\φ}}^{\′}\[a^{\′}-u^{\′}tan\mathrm{\θ})}---\left(13\right)$

Wherein, �� '=(QP+r ' ��-QR₀)/QP, a '=a-l+r ' cos ��, u '=r ' sin ��.

For formula (13), if l=r '=0, the center of circle O of fixed pulley in a=r then Fig. 3₃With O₀Just it is completely superposed. In order to ensure that zero drift spring carries out gravity compensation, there will necessarily be: as connecting rod OO₁With OO₀During coincidence, namely gravity torque T=Bsin �� is equal to 0, and now, the tensile elongation of spring is equal to drift, i.e. Q point and O₀Two coincidences of point, that is to say QO₀Length is equal to 0. So, Hookean spring combines with fixed pulley and steel wire rope and just constitutes zero drift spring gravity compensation way completely, and the stiffness coefficient K of spring is constant:

$K=\frac{B}{ra}---\left(14\right)$

The big arm mechanism of force feedback equipment and little arm mechanism are required for carrying out gravity compensation, but both compensation principles are identical. Therefore, here big arm mechanism spring-compensating principle being carried out theory analysis, little arm mechanism spring-compensating principle is similar with it, only need to change the parameter of little arm mechanism into.

By above-mentioned mathematical derivation, it was demonstrated that it is the complete gravity compensation that can realize force feedback equipment completely in theory that novel zero-initial-length spring compensates.

In actual design process, if keeping a radius r equal to reducing gear, the bearing length l and radius r ' of fixed pulley are very little, completely can so that the stiffness coefficient K of spring is approximately constant, and this can realize completely in actual production and processing. Therefore, the method simply and easily realizes.

In addition, the spring of the method and steel wire rope are all fixed on the spring fixed installation face on the base mechanism driven pulley of force feedback equipment, rather than on the connecting rod of operator, this will not increase gravity when force feedback equipment operates and inertia, without going into interference between bar with bar, it is to avoid the shortcoming of common zero drift spring.

Claims (1)

1., based on a zero drift spring gravity compensation method of force feedback equipment, it is characterized in that step is:

(1) the gravity compensation spring one end of large arm is connected to the spring fixed installation face on base reducing gear driven pulley, the other end is connected by steel wire rope and is connected on large arm reducing gear driven pulley and large arm reverse extending line and interface point place, driven pulley edge through fixed pulley, for realizing big arm mechanism zero drift spring gravity is compensated;

(2) the gravity compensation spring one end of forearm is connected to the spring fixed installation face on base reducing gear driven pulley, the other end is connected by steel wire rope and is connected to interface point place, forearm reducing gear driven pulley edge through fixed pulley, for realizing little arm mechanism zero drift spring gravity is compensated.