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    I. S. Fischer

    Assistant Professor.

    Mem.ASME

    New Jersey Institute of Technology,

    Newark ,  NJ  07102

    R. N. Paul

    Mem.ASME

    W .  R. Graces Co.,

    Co lumbia , MD 21044

    Kinematic Displacement Analysis

    of a Double-Cardan-Joint Driveline

    The  input-output displacement relations of two Cardan joints arranged in series on a

    driveline has be en investigated in detail, including the effects of unequa l joint

    angles,

      the

     phase angle between the two Cardan joints and also such  manufacturing

    tolerance errors as nonright angle moving link lengths and offset joint axe s, A com

    bined Newton-Raphson method and D avidon-Fletcher-Powell optimization

    algorithm using dual-number coordinate-transformation  matrices

      was

     e mployed to

    perform the analysis. An experiment w as conducted to validate the results of the

    analysis. The apparatus consisted of a double-Cardan-joint driveline whose rota

    tions we re measured by optical shaft encoders that were sampled by a computer

    data-acquisition  system. The equipment was arranged  so that the phase angle be

    tween

     th e

      joints and the offset  angles betwe en the shafts at each of

      the

      two joints

    could be readily varied. Th e relative phase angle, the difference between the

    phase angle of  the two  joints and the angle between  the planes defined by th e input

    and intermediate and the intermediate and output shafts, was found to be the

    significant factor. If  th e  offset  angles at both Cardan  joints are equal the double-

    Cardan-joint drive line functions  as a constant-velocity coupling when the magnitude

    of

      the

     relative phase angle  is zero. If the offset angles at the two Cardan joints

      are

    unequal, a condition prevailing in the important front-wheel-drive automobile

    steering-column

     application,

      then fluctuation in output ve locity for  a constant input

    velocity is minimized although not eliminated for zero relative phase angle.

    Introduction

    The do uble-Cardan-joint driveline has been extensively used

    as a driveline in machinery, particularly the Hotchkiss

    automotive drive. The geometry of an application is specified

    by the offset angles (the angle between the input and in

    termediate shafts at the driving Cardan joint and the angle

    between the intermediate and ou tput shaft and the driven Car

    dan joint) and by the shaft offset (the minimum distance be

    tween the input and output shaft axes). In most applications

    the mechanism is arranged so that the offset angles at the two

    Cardan joints are equal and all the shafts are in the same

    plane. Under those conditions the double-Cardan-joint

    driveline will operate as a constant-velocity coupling. Analyses

    of this arrangement are given in Doughtie and James (1954),

    in Tyson (1954), and also by Austin, D enavit, and Hartenbe rg

    (1965) who added consideration of manufacturing tolerance

    effects. If the shafts are in different planes and the offset

    angles are equal, then the Carda n joints can be phased relative

    to one another to produce a constant velocity coupling. This

    case was  discussed by M abie and O cvirk (1975) and by Wagner

    (1979).

    Recently, the double-Cardan-joint driveline has been used

    as a steering shaft in front-wheel-drive automobiles. As

    reported in Cowles and Chiu (1984), this application does not

    allow equal offset angles. The yokes of the two Card an joints

    on the intermediate shaft are located in different planes, a pro-

    Contributed by the M echanisms Comm ittee for publication in the

     JOURNAL OF

    MECHANICAL DESIGN.

      Manuscript received September 1989.

    cedure called joint phasing, in order to best approach

    constant-velocity output. Saari (1954) approached this prob

    lem by combining the input-output displacements for two Car

    dan joints, Phillip (1960) used a spherical-trigonometry

    analysis, Maziotti (1964) developed a vector procedure and

    Cooney (1966) has given an expression but not its derivation.

    This study has modeled the double-Cardan-joint driveline

    with dual-number coordinate-transformation matrices and

    has applied numerical techniques to create a procedure for

    evaluating the effects of joint phasing and manufacturing

    tolerance errors on the input-output displacement

    relationship.

    In addition, an experiment was conducted to validate the

    results of the numerical analysis.

    Analysis

    The ideal double-Ca rdan-joint driveline is a six link spatial

    mechanism w ith revolute joints. Although the G riibler equa

    tion predicts zero degrees of freedom, the mechanism operates

    because of its special prop ortions . If those special proportions

    are lost because of manufacturing tolerance errors such as

    nonright angle moving link lengths and offset joint ax es, then

    it is necessary to consider one joint, in this study selected to be

    the output joint, as cylindrical to accommodate the sliding

    that o ccurs. This sliding m otion is similar to that exhibited by

    the single Cardan joint as described in Fischer and Freuden-

    Journal of Mechanical Design SEPTEMBER 1991, V o l .  113 / 263

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    Fig.  1 Double-cardan joint driveline

    stein (1984). Therefore, this study will model the double-

    Cardan- jo in t d r ivel ine as an RRRRRC mechanism.

    A dou ble-C ardan -joint driveline is i l lustrated in Fig. 1. It

    has six joints nu mbe red 1 throu gh 6. At joint i—

     1

      there is a

    coordinate frame fixed on the l ink connecting joints   i — 1  and

    /. Its position is specified by a z-axis aligned along the axis of

    joint /—1 and an x-axis aligned along the l ine of shortest

    distance between the axes of joints / — 1  and / . The t ransforma

    tion between the coordinate frame on the l ink connecting

    joints  i — 1  and /' and the coo rdinate frame on the l ink connec

    ting joints  / and /+   1  can be expressed as a dual-number matrix

    where the dual-num ber identity matrix [7 | is defined as

    M =

    /

    0

    0

    0

    1

    0

    0~

    0

    ; _

    + €

    0 0 0

    0 0 0

    0 0 0

    (4)

    IA,] =

    C(0 , ) - 5 ( 0 , )  0

    c(a

    i

    _

    l

    )s(d

    i

    )  c ( a , _ , ) c ( 0 , ) - . y ( a , _ i )

    sia^Jsid,)  • s ( a ,_ i ) c (0

    /

    ) c(&,-_,) (1)

    E quatio n (1) can be expand ed into prima ry and dual com

    ponents

    [A,]-

    c{d,) -3(6,) 0

    c ( a

    /

    _ , ) s ( 0

    ;

    ) c ( a , _ , ) c ( 0 , )  -s(a,_

    l

    )

    s(a

    i

    _

    l

    )s(e

    i

    ) s(a

    i

    _

    l

    )c(B

    i

    )  c ( a , _ , )

    + e

    -d

    iS

    (d,)

    df i

      (a,-_, )c (

    6,

    ) -  a,-_ s(a,-_,

     )s (0,

    )

    d,s (a,-_ i )c (6;)  a,_, c (a ,_ , )s (6,)

    -d,c(e

    t

    ) 0

    -djC^a^sidi)  -a , -_ i .s(a i_ i )c(0 i ) -« ,•_ ,cC a,- . , )

    -d

    i

    s(a

    l

    _

    l

    )s(6

    i

    ) +a

    i

    _

    l

    c(ot

    i

    _

    l

    )c(6

    i

    )  - ^ . ^ ( a , . , )

    (2)

    The six l inks comprising the double-Cardan-joint driveline

    are connected serially to form a closed loop. This infers that

    the six transforms associated with the mechanism are related

    by the condition of loop closure:

    [A

    {

    \[A

    2

    ] . . . [A

    6

    ] = [f\  (3)

    E qua tion (3) states tha t transforming from the frame fixed on

    one link in the mechanism throu gh the frames fixed on each of

    the other l inks will result in a return to th e original co ordina te

    frame. This equation governs the kinematics of the double-

    Cardan-joint driveline.

    The solution to the general form of equation (3) will be

    found by nume rical me thod s. Duffy (1980) l isted the solutions

    to var ious s ix- l ink mechanisms such as RCRRRR, RCRRRP,

    RRRCRR, and RRRCRP, bu t the d isp lacement analysis o f

    the RRRRRC by Ju and Duffy (1985) had not yet been

    published when it was decided to model the double-Cardan-

    joint driveline as an RRRRRC and obtain the displacements

    by a numerical scheme.

    D u a l - N u m b e r F o r m u l a t i o n o f N u m e r i c a l M e t h o d s

    The numerical m ethods used to determine the d isp lacements

    of the double-Cardan-joint driveline are based on the

    algorithm introduced by Hall , Root, and Sandgren (1977) that

    combines the Davidon-Fletcher-Powel l (DF P) op t imizat ion

    rout ine with the N ewton-Ra phson (N R) method as applied to

    kinematics by Uicker, Denavit , and Hartenberg (1964). These

    procedures were developed for use wi th 4 x 4 homogeneous

    transformation matrices, but for this study were adapted to

    3 x 3 d u a l -n u m b er m at ri ces .

    The algorithm for the numerical solution is shown in flow

    chart form in Fig. 2. The two methods of solution are

    available so that the most advantageous could be used de

    pending upon the requirements at each position of the input

    variable. The D F P me thod is slower but will almost always

    find the displacements resulting in loop closure even if the in

    it ial guess of the displacements is poor, a condition that is

    troublesom e for the N R me thod . If the init ial guess of the

    d isp lacements has been made reasonably w el l, the N R method

    is much faster. Th us the DF P m ethod is used for the initial

    position of the input-link rotation when it is difficult to guess

    the displacements at the output and particularly the in-

    264 / Vo l . 113 , SEPTEMBER 1991

    Transactions of the ASME

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    ( S T A R T )

    V£5

    (ST O P)

    Fig. 2 Simplified flowchart of algorithm

    termediate jo in ts . The N R m ethod is used to determine the

    displacements associated with subsequent steps in the input-

    link rotation because if those steps are small , then the

    displacements determined for the previous input position are

    generally adequate as an init ial guess. The displacements

    resulting in closure are usually found within a few iterations.

    Occasionally a solution cannot be found using the N R m ethod

    and the algorithm will then use the DF P meth od to attempt to

    locate displacement values resulting in loop closure.

    Details of the Newton-Raphson Method

    A first-order e xpansion of eq uation (3) is

    {A(\[A

    2

      + Q

    r

    A

    2

    8d

    2

    ][A

    3

      + Q

    r

    A

    3

    86

    3

    ]

    [A

    4

      + Q

    r

    A

    4

    56

    A

    ][A

    5

      + Q

    r

    A

    5

    86

    5

    ][A

    6

      + Q

    r

    A

    6

    56

    6

      +

    Q

    p

    A

    6

    8D

    6

    ]  = I  (5)

    where A „  are each evaluated at 0„ and D„ which are guesses of

    the displacement variables,  dd

    n

      an d  dD „  are the errors be

    tween the guesses and exact values of the displacements, and

    Q

    r

      and  Q

    p

      are the partial-derivative operators corresponding

    to equation (1) such tha t

    [Q,]

     =

    and

    [Qph

    0 -c(a)+eas(a) -s(a) -eac(a)

    c(a)-eas(a) 0 0

    s(ot)+eac(ct) 0 0

    (6)

    0

    •ec(a)  —  es(a)

    e c ( a )  0 0

    es(a) 0 0

    (7)

    E qua tion (5) can be expan ded and the higher-power term s

    neglected, resulting in a form which is com putationally advan

    tageous:

    H

    2

    8d

    2

      H

    3

    5d

    3

    +H

    4

    5d

    4

    +H

    5

    88

    5

      H

    6

    88

    6

      H

    1

    8D

    6

    =B

    T

    -I  (8)

    where

    H,=   {A,A

    2

      . . . A

    i

    l

    )Q

    x

    (A

    l

    A

    2

      . . .  A,^)

    7

    and

    (9)

    (10)

    x

      A

    2

      A

    3

      A

    4

      A

    5

      A

    6

    =B

    E quatio n (10) can be written in a mor e compac t form as

    P= B

    T

    ~J   (11)

    Writing out each term of equation (11) produces two 3 x 3

    dual-number m atr ices:

    1

      1,1

    1.2

    3,2

    1,3

    3,3 J

    B\,\~I

      2,1

    B

    3,1

    B

    1,2

    B

    K

    B

    2

    ,

    2

    4 , 3

    /  B

    3,2

    B

    3,3 '

    (12)

    This matrix equation comprises nine dual equations, only

    three of which are independent. Using the three terms in the

    upper-right triangle of each matrix and separating them into a

    set of real equations and a set of dual equations produces:

    F  =  Ft

    - ' r ea l i T - °rea l -> ,

    real

    1,3

    = B

    eal

    3,1

    and

    W

      =

      R

    • ' r e a l j 3 r e al

    dual

    1,2

    =  B,

    3,2

    2 , 1

    P

      =

      R

    r

    dualj 3

      £

    ' d u a l 3 j

    ^ d u a l 2 3 - ° d u a l 3 2

    (13)

    (14)

    The real set , equation (13), however, does not include the

    constraint that the diagonal terms must each be equal to unity

    at closure. This constrain t can be included either as three addi

    tional equations as described by Uicker, Denavit , and

    Hartenberg (1964) or through modification of the current

    equations set as discussed in S andor a nd E rdm an (1984).

    Adopting the second method modifies equation (13) to

    F  = R

    ^ r e a l j 2 r e a l'

    reaij 3

    ^

    r e a l

    2 , 3

    = B„

    2 ,1

    "3,1

    +

      B

    n

    +

      B

    r

    ,

    + B

    Ti

    + B„

    2 ,2

    '

    real

      3

    ^ r e a l

    2 ,2

    ' real-i

    (15)

    The left-hand side of equations (14) and (15) contain the

    linear combination of the respective elements of the  H

    matrices and the error terms as can be seen from the left-hand

    side of equation (8).

    E quation s (14) and (15) can be combined as parti t ions of a

    single matrix of real-number equations. The resultant equa

    tion will have the form:

    " -Mreal "

    . - M d u a l .

    A

    =

    ' r ea l

    . ^dual .

    (16)

    where

    M =

    'H,

    H,

    H,

    H,

    H.

    ' r . 1 , 2

    H,

    H

    A

    H-

    r,2,3

    H,

    r ,2 ,3

    H*

    H.

    H<

    H,

    H,

    Hi,

    , l , 2 ' r . 1 ,2

    H-,

    V, 2,3 ^ r . 2 , 3

    H, H, H

    A

    H<

    H

    K

    Hn

    d,l,2

      3

    d,l,2

      4

    r f , l , 2

      5

    r f , l , 2

      d,l,2 >d,l,2

    H

    ld,l,3

      H

    3d,l,l

      H

    *d,l,3

      H

    5d,l,3

      H

    6 r f, l , 3 ^ ' d . U

    H, H,

    rf,2,3

      3

    d , 2 , 3

    H, H,

    d , 2 , 3

    H„

    H

    r

    d,2,3

    (17)

    J o u r n a l o f M ec h a n i c a l Des ig n SEPT EM BER 1 9 9 1 , Vo l . 1 1 3 / 2 65

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    •Orpal

    2

      j + -"reaij j

    eal-) i ^ ^r ea li i ' -" real

    ° r e a l

    3 1

      +  ^ r e a ]  ̂ + -^real

    2,2

    3,3

    3,2

    +

      B„

    B

    e a U

    B

    A

    L ^ d u a l

    3 2

    (18)

    and

    b e

     

    50

    4

    86

    5

    86

    6

    5D

    6

    (19)

    Rearrangement of equation (16) to solve for the error terms

    produces the required equation for the N ewfon-R aphson itera

    tion technique:

    A = M~>  V  (20)

    The guesses of the displacement variables  d„ a nd  D

    n

      are re

    vised by the error terms, A, and this new set of joint variable

    positions is used in the next i teration to compute a new set of

    error terms. Convergence is obtained when each of the error

    terms obtained from equation (20) is less than some specified

    resolution limit.

    After obtaining a solution for the given position of the in

    dependent variable  Q

    {

    , a new position is selected and the en tire

    process repeated. The solution set from the previous position

    is used as the starting values for the new iteration. The step

    size between successive positions must be small enough to

    allow the N ewto n-Ra phson te chnique to find closure. This

    step size depends upon the specified mechanism and varies

    with its position. It is usually safe to use 20 degrees as a max

    imum step size.

    Equations for the Davidon-Fletcher-Powell Optimization

    Method.

      If perfect guesses of the displacement variables are

    substituted into equation (10), then matrix B will be the identi

    ty matrix as required by equation (3), the condition of loop

    closure. For imperfect guesses, matrix  B  will differ from the

    identity matrix, implying lack of closure. The difference be

    tween matrix  B  and the identity matrix can be embodied in an

    objective function such as

    ^ = E E l t ^ e a ^ - M ^ ' + ^ d u a , ] ^

    2

    )

    (21)

    /= i y= i

    which vanishes at closure but is greater than ze ro when closure

    does not occur. An optimization method can solve equation

    (3) by minimizing the objective function given by equation

    (21).

    The DF P algorithm req uires a set of gradients for each

    variable in addition to the objective function. The gradients

    provide the slope of the objective function w ith respect to each

    variable and are found by differentiation of equation (21) to

    be

    j = l  k = \

      ax

    i

    3 3

    +

    y'=l  k=\

    d[ B

    d

    duaUjfc

    dXi

    (22)

    E quation s (21) and (22) are used by the DF P meth od to find

    a solution. The DF P algorithm is described in detail by Fox

    (16) and basically consists of selecting the direction of steepest

    descent of the objective function from the variable vector,

    proceeding in that direction by an amount determined by the

    closeness of the objective function to the target value (in this

    case, zero) and then repeating thre e steps until the convergence

    criteria is satisfied.

    Ex per im enta l Inv es t ig a t io n

    A series of experiments were conducted to validate the

    results of the numerical analysis. An experiment apparatus in-

    ANGLE BRACKET

    OUTPLH SHAFT H00ULE

    BASEPLATE

    P HASE COW L I NG

    1HPW  SHAFT MODULE

    D O l M E - C A f t O A N - J O J N T S H A F T

    Fig. 3 Experimental apparatus

    266 / Vo l . 113, SEP TE M BER 1991 Transactions of the ASME

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    350-

    325

    300

    275

    250

    225

    200-J

    175

    150

    125 -j

    100

    75

    50 H

    25

    OH

    -25

    - 5 0 H

    l h e l o - 2

    l h c t o - 3

    t h e l o - 4

    l h e t o - 5

    l h e t o

    :

    6

    d-6, inches

    T — i — i — i — \ — i — i — i — i — i — i  i i i i i i i r

    0 20 40 60 80 100 120 140 160 180 200 220 240 260 2B0 300 320 340 360

    T HO A -I POSITION, degrees

    Fig.

     4 Computed joint motions offset angles both 35 deg., offset

     0.867

    in.,  phase angle SO deg.

    - 9

    -10

    T

    20

    40

    1 — i — i — i — i — i — i — i — i — i — i — i — i — i — r

    80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

    THETA-1 POSITION, degrees

    Fig.

      5 Computed I/O relationship offset angles both 35 deg., offset

    0.867 in., phase angle 50 deg.

    corporating a double-Cardan-joint driveline was designed on a

    modular basis so that the offset angle at each Cardan joint,

    the angle between the input and output shafts, the offset

    distance between the input and output shafts and the phase

    angle between the two Cardan joints could be varied. It is i l

    lustrated in Fig. 3. Althoug h provided for in the analysis, the

    experiment apparatus did not permit variation of l ink angular

    dimensions and joint-axis offsets within the two Cardan

    joints. Both the input and output shafts were equipped with

    optical shaft encoders to measure rotation. These were inter

    faced to a micro com puter for data acq uisit ion.

    Resu l t s a nd Co nclus io ns

    The numerical solution is capable of producing data on the

    motion of each joint in the double-Cardan-joint driveline. A

    plot of the joint motions for a typical double-Cardan-joint

    driveline are shown in F ig. 4. i?-type joints num bered 2

    throug h 5 display single-cycle variation per revolution of the

    , input shaft . M otion of prismatic joint D

    6

      is zero for this exam

    ple.  The m otion of the outpu t shaft is displayed as angle  6

    6

    and varies with two cycles per input-shaft revolution. This

    nonuniform motion can best be visualized by the input-output

    (IO)  relations hip, the lead and lag of the output-shaft rota

    tion with respect to the input-shaft rotat ion. F igure 5 i l

    lustrates the  IO  re lationship for the joint motions in Fig. 3.

    If tolerance errors result in offset joint axes, then sliding

    will occur at the prism atic joint. F igure 6 shows the sliding

    which was computed by the numerical method for a double-

    Journal of Mechanical Design SEPT EM BER 1 9 9 1 , Vo l . 1 1 3 / 267

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      5 . 1 5

    5 . 2

     H

    5 . 2 5

    5 . 3

    5 . 3 5

    5.4   i

    5 . 4 5

    5 . 5

    5 . 5 5

    5 . 6

    5 . 6 5

    5 . 7

    5 . 7 5

    0.1 inch errors in all links

    zero errors in all links

    1 1 I 1 1 I I I I I 1 I 1 1 I I 1 1

    0 20 40 60 80 100 120 HO 160 180 200 220 240 260 280 300 320 340 360

    IHE IA -1 POSITION, degrees

    Fig. 6 Computed D

    6

     motion for a mechanism with dimensional errors

    in the links offset angles both 35 deg ., offset 0.867 in., phase angle 50

    i~~[  i i  i i i i i  i  i i i  i i  r

    7 9 II 13 15 17 19 21 2 3 25 27 29 31 33 35 37

    JOINT ANCLE5. degrees

    Fig. 7 Effect of relative phase angle on I/O displacement relationship

    Cardan-joint driveline similar to that used to produce F ig. 5,

    but with joint-axis offsets of 0.1 inches. Fo r com parison, the

    motion of the same driveline but with zero joint-axis-offset

    tolerance errors is also shown in F ig. 6.

    Relative Phase Angle.  If the input and output shafts do

    not intersect, but are offset by a certain distance, then the

    plane formed by the output and intermediate shafts and the

    plane formed by the input and intermediate shafts are dis

    placed from one another by the "angle of twist." The rela

    tionship between the two planes each containing one of the

    Cardan-joint yokes which are mounted on the intermediate

    shaft (angle  4>  in Fig. 1) is called the "phase angle." The

    "relative phase angle," RPA, is defined as

    RPA  = (phase angle) - (angle of twist)  (23)

    The importance of the relative phase angle is observed in

    F ig. 7. For a driveline with equal joint angles, the amplitude of

    fluctuation in output displacement increases significantly with

    relative phase angle to a maximum amplitude occurring at a

    relative phase angle of 90 degrees. In the figure it is observed

    that for joint angles of 45 degrees, about the largest experi

    enced in practical applications, the fluctuation can range from

    zero degrees for R PA =  0 degrees to 20 degrees when RP A = 90

    degrees.

    Offset of Shaft  Axes. As seen in F ig. 8, for a given RPA,

    268 / Vo l . 113, SEP TE M BER 1991

    Transactions of the ASME

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    a l p h a - 6 = 0 . 0 0 0 i n

    a l p h a - 6 = 0 .8 6 7 in

    a l ph a -6 = 2 .1 69 i n

    1 1 1 1 I 1 1 1 1 I F 1 1 1 1 1 1 I I

    0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

    RELATIVE PHASE ANGLE, degrees

    Fig.  8 Effect of offset on I/O relationship

    10

    9

    8

    7

    6

    5 -

    4 -

    3

    2 -

    1 -

    0 -

    A —

    A

      sma l ler

    * — • sm a lle r

    # - • - £ s m a l l e r

    9-— 9  sma l ler

    /

    « ,

    .-'*''

    S s*

    ,'S'  .-•-

    / ̂   ...- .

    j o i n t

    j o i n t

    j o i n t

    j o i n t

    /

    /

    '

    ong le = 5  degrees  #

    a n g l e =  15 degrees  /

    ong le = 25  degrees  /

    a n g l e =  35 degrees  , •

    ' /  .•

    -t/

    y  -••'  ^

    *'  •••-• JS

    '  . . - • *  ^ ^ ^

    ••-'*' ' ^ J f ^ ^

    .•

    ~  1 1 I I I I I 1 I I I I I I

    6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

    DIFFERENCE BETWEEN JOINT AN GLES, degrees

    Fig. 9 Effect of unequal joint angles on I/O relationship

    variation of the offset of the input and output shafts has

    negligible effect on the IO  displacement relationship.

    Unequal Joint Angles.  When both joint angles in the

    driveline are equal, then arranging the mechanism so that the

    relative phase angle is zero will cause fluctuation in the input-

    output relationship to vanish. Thus for RPA  = 0,  when the

    phase angle equals the angle of twist, the double-Card an-joint

    driveline will function as a constant-velocity coupling. This

    agrees with the results reported by Wagner (1979).

    However, it has been observed in this study that unequal

    joint angles results in a situation where the joints cannot be

    phased to arrange for constant-velocity motion. It is apparent

    that the amplitude of input-output fluctuation is minimized

    for zero relative phase angle. This is seen in F ig. 9 which pro

    vides a measure of the input-output fluctuation amplitude for

    various combinations of joint angles at zero RPA . L arger

    amplitudes would be obtained for non-zero RPA's. Several

    characteristics can be observed in F ig. 8:

    (1 )  Increasing the difference between the two joint angles

    results in larger amplitudes.

    (2) L arger joint angles, with the difference between them

    held constant, produce greater amplitudes.

    These facts point to the conclusion that the need to

    minimize joint angle difference becomes ever more importa nt

    with larger joint angles.

    Journal of Mechanical Design

    SEP TE M BER 1991 , Vo l . 113 / 269

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    Experimental Results.  E xperiments were conducted for a

    variety of arrangements. The results for several phase angles

    in a typical arrangement of the input and output shafts are

    shown in F ig. 10. The irregularities in the curves are observed

    to have a magnitude of 0.36 degrees, equal to the resolution of

    the encoders. To evaluate the error, the experiment for which

    the joint angles were 10 and 30 degrees, the phase angle 60

    degrees and the input-output shafts offset 0.867 in. was

    repeated four times to arrive at a standard deviation in the

    measure output of 0.0323 degrees. In that the measured

    amplitude of the fluctuation was about 4 degrees, the error

    had a m agnitude of less than

      1

     percent of the peak signal. The

    small magnitudes of these errors indicated that the quality of

    the measurements was good.

    A comparison of experimental and computed results for a

    typical arrangement of input and output shafts with several

    phase angles is shown in F igs. 11 and 12. It is observed that

    there is close correlation between the experimental and

    analytical results.

    6

    °

    S

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    9/9

    10

    4 J

    >

    -o

    N

    O

    I

    l—

    L n

    O

    CL

    LU

    -<

    i

    6

    4

    2

    0

    -2

    -4

    -6

    -10

    . / ' ~

    v

    \  /'

    ,-v

    S

      -

    / /  \ \ / /

    /  e \ \ 7 •

    .

     / /  \ \ i i

    /  '' \ \ / /

    /

      •''•  V •/ /

    :

      \^/^

      i

    //  V /

    —•

    v

      ^

    e x p e r i m e n t a l , p h a se a n g l e  = 30 deg

    - c o m pu ted , pho s e a n g l e

      = 30 deg

    e x p e r i m e n t a l , p ha s e a n g l e

      = 60 deg

    c o m pu ted , pha s e a n g l e  = 60 deg

    1

      1

      i

      1 1 1 1 1 1 1 1 1 1 1

    • k

    N

    \  \

    \  \

    \ \

    \  \

    \ \

    \ ^

    V- .

    1  1 1 1 1

    0  20

      40 60 80

      100 120  140 160 180 200 220 240 260 280 300 320 340  360

    T HE T A -1 POSIT ION, deg ree s

    Fig.

      12 Comparison of experimental and computed data phase angles

    -  20.1  an d 0 deg. Offset an gles both 30 d eg., offset 0.867 in.

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      and

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    Journal

     of

     Mechanical Design SEPT E M BE R 1 991 , Vo l . 1 1 3 / 271