Loading...

Summer Research Fellowship Programme of India's Science Academies

Redesigning easy chair

Atanu Chowdhury

National Institute of Technology, Silchar, NIT Road, Silchar, Assam 788010

G. K. Ananthasuresh

Indian Institute of Science, C. V. Raman Road, Bengaluru, Karnataka 560012

Abstract

This work is concerned with redesigning and rectifying the errors of the mechanism and operation of a compliant assistive chair developed for the elderly so that they can sit onto it and stand from it easily without giving much effort in their weak bones and muscles. It accomplishes this task by giving the resistive torque while sitting and assistive torque while rising. The chair uses semicircular open section shells for intrinsic torque and cam-slots of different curvature for getting the desired torque-angle output. But the chair has some design shortcomings that will be rectified in this work. They are: (i) misalignment of the bearing rod and seat frame due to unsymmetrical operation of the chair; (ii) user oriented changing of the sitting and standing slots; (iii) unnecessary and unreliable joints and shafts. So, to rectify these errors, the sitting and rising operation of the chair is made symmetrical and the changing operation of the cam-slots for rising and sitting operation is made automatic so that the user does not have to pull a lever to change the cam-slots. To make the slot movement automatic, the slot holders are designed as mobile part and springs are used to enable it. To get the predicted observation of the automatic operation, first the finite element analysis of the slot changing mechanism is done, and then it is modelled by 3D printers and tested in a 2D plane. The wedge-spring mechanism is used for the automatic slot changing to happen. Then, the reliability is increased by redesigning the end-plate and mid-plate, using single shaft in the bearing rod and changing the coupling mechanism of the seat frame with mid plate.

Keywords: variable torque, adjustable stiffness, assistive chair, automatic slot movement, mobile slot holder, wedge-spring mechanism

Abbreviations

Abbreviations
 WGSWedge Guiding Slot 
 WRPWedge Resting Platform 
 Eq.Equation 
 FBDFree Body Diagram 

INTRODUCTION

Background/Rationale

An assistive chair was developed in the M2D2 laboratory in Mechanical Engineering at the Indian Institute of Science. It uses open-section cylindrical shells that bend and twist simultaneously to produce desired torque for as the seat-frame rotates from standing to sitting position. It has a pair of cams that guide the shell to customize the torque-angle profile for an user, onecam for sitting in and another for rising. The current design has a shortcoming that the user has to shift the cam. The motivation for this research work is to make a slot-changing mechanism of a compliant easy chair to be automatic so that it becomes easy for the elderly persons to use it. Whenever a person is sitting and rising from the chair, the sitting and standing slot must be in operation. Thus, it becomes a worrisome and effortful task to check and change the slots accordingly as needed. So, a mechanism is developed to ensure the automatic slot changing operation using wedge and springs as the main parts, and also by making a large part of the cam-slots to be mobile. A second shortcoming was the asymmetry of the supports of the seat frame on the bearing rod. It is also rectified in this research work by operating the cam-slots with bearings on either sides of the centre line of the chair, both during sitting and rising operations. The stability and reliability of this chair is also increased by redesigning many parts as discussed in this report.

Objectives

Following are the objectives of the undertaken research work:

  • Loading the shells from both sides.
  • Correcting misalignment of the bearing rod.
  • Making the bearing rod a single shaft.
  • Widening of the seat side mid plate to support bearing rod and seat frame with more stability and reliability.
  • Redesigning the seat frame - shells coupling mechanism.
  • Improved automatic cam shifting mechanism.
  • Removing unnecessary threads and increasing reliability.
  • Redesigning the cam-holding part as per the cam-shifting mechansim.

LITERATURE REVIEW

The focus of this work is a compliant assistive chair, which uses thin walled open-section shells that undergo combined bending and twisting to provide the tunable variable torque-angle profile of the seat-frame ​[1]​. ​A pair of semi-circular open-section shells that are rigidly connected to each other and fastened to the chair frame comprise the nonlinear rotational spring. A cam profile on a guide plate enables the shells to deform transversely even as they twist to provide customized torque–angle characteristic as per the weight and ability of the occupant​[1]​. There are variable torque output mechanisms [2][3][4], which could have been used in the easy chair but previous research works shows that the thin walled open-section shells along with designed cam-profiles have various advantage over other mechanisms like they do not require bearing-support to give a large rotation within linear elastic behaviour of the shells, with negligible shift in the axis of rotation; and most importantly, they can provide a wide range of torque-angle profiles including those with negative stiffness [1]. Previous work ensures that the design of the open section shells must have circular cut on the diagonally opposite longitudinal edges of the shell that allows partial wrapping, which reduces the magnitude of maximum stress and also generates uniform stress distribution [1]. An energy-based, graphical and computational design method is developed to obtain the guideway profile for a specified torque-profile. For a given target torque-angle profile, the area under the curve depicting the relationship between bending and twisting gives the magnitude of strain energy stored in the shell-spring, which is the target-energy profile to be achieved​ [1].

A physical prototype of the easy chair is constructed and tested by previous students in the M2D2 laboratory [1]but it has some limitations in its design that have not been taken into account. The one-sided loading of the shells creates asymmetricity in the seat frame and decreases the reliability and longevity, and the user oriented changing of the cam-slots is another worrisome task. Along with these, there are some of the design shortcomings and inconsistencies which are to be rectified in the present study to make it more reliable and user friendly. Further studies may be done to increase or widen the functionability of the chair to make it more useful for the elderly.

METHODOLOGY

Concept of Wedge-spring Mechanism

The main concept behind using the wedge-spring mechanism is that a moving wedge can displace a body accordingly as needed, simply by designing the contour of the contacting edges of wedge and the body that is to be displaced, and a spring can replace the body instantly to its initial position after the wedge leaves its total contact with the body. This mechanism also ensures that the body remains in its displaced position as long as the wedge is in contact with the body. The only requirement needed for this mechanism to work is that the body that is to be displaced should be mobile.

Wedge-spring Automatic Slot-changing Mechansim in 2D

Introduction

The wedge-spring mechanism is achieved by dividing the cam slots into two parts, mobile and fixed with the mobile part being much more longer than the fixed part. In this wedge-spring mechanism, the cam slots are displaced instantly with the help of a wedge and is again replaced in its initial position instantly with the help of springs when the wedge loses its total contact with the cam slots, provided the bearings remain on the fixed cam-slots during these transitions. This mechanism is developed keeping in mind that in the real model of easy chair, during the initial and final 5 deg. (approx.) movement of the bearings on the cam-slots, both the standing and sitting cam-slots have almost the same cam profile design, which are made as fixed cam-slots here. The labelled assembly of these mechanism is shown in Fig. 1.

front assembly - Copy.jpg
  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
  • 15
  • 16
  • 17
  • 18
  • 19
  • 20
  • 21
  • 22
  • 23
  • 24
  • 25
Labelled front view of the assembly (light yellow arrow showing the pathway of bearings on the cam-slots)
isometric assembly.jpg
    Isometric view of the assembly (when wedge is inside the protruding parts)

    In Figs. 1 and 2, the spring guides are attached to the mobile cam-slots and there is a fixed bar which is a constraint to the upward movement of the wedge. In the mechanism, the wedge is constrained to move only vertically, i.e. the upward and downward movement only, and the mobile cam-slots are allowed to move only horizontally, i.e. the sideways movement only. The WGS also pushes and pulls the wedge only in the vertical direction. The mechanism is designed in such a way that the bearings move along with the WGS. The compression spring is used which compresses between the mobile cam-slots and the fixed parts. The wedge is designed in such a way that after the full fitting of the wedge in between the protruding parts, the mobile cam-slots are displaced by only the amount such that the standing slots come accurately in line with the fixed cam-slots replacing the sitting slots. While designing of the wedge it is also ensured that when the wedge is in between the protruding parts and the upper surface of WGS is not connected to the wedge, then the wedge doesn't get out of the protruding parts due to the spring forces from sides. Also, the length of the wedge is designed to be same as the length of the part of fixed cam-slots traced by the bearing, so that the bearings remain on the fixed cam-slots when wedge is sliding along the protruding parts.

    Working

    Working of the above mechanism is explained in two parts - downward movement and upward movement of the wedge. In the figures given below, yellow arrow represents the WGS movement and blue line indicates the bearing movement on the cam-slots. So, the whole mechanism is explained as follows:-

    (A) Downward movement of the wedge:- During the downward movement, the sitting slot remains under operation. The whole scenario of the downward movement of the wedge is explained in various steps on the basis of different positions of the bearing on the cam-slots, as follows:-

    (1) Initial position:- Initially, when the person will be sitting on the chair, the wedge will not be in contact with the protruding part of the cam profile and the face having the edge CD of the WGS will be in contact with the wedge. At this stage, the bearings are on the 1st extreme position (as depicted in fig.1) i.e. on the fixed cam-slots. This stage is shown below in fig.3:-

    initial-DM-2D - Copy.jpg
    • 1
    Front view of the assembly at the initial position of bearings during downward movement

    (2) 1st Intermediate position:- As the WGS will move downward, it will not affect the position of the wedge until the face having edge AB pushes the wedge. Consequently, the position of the cam-slots will also not be affected. So, the bearings will also move down the fixed cam-slot and will reach on the moving cam-slot, indicated in fig.4 by 1st intermediate position, as shown below:-

    1st interediate-DM-2D - Copy.jpg
    • 1
    Front view of the assembly at 1st intermediate
    position of bearings during downward
    movement

    (3) 2nd Intermediate position:- As the WGS is moving downward without affecting the positions of both the wedge and cam-slots, the bearings will simply roll over the sitting cam-slots, as indicated in the fig.5 given below:-

    2nd intermediate-DM-2D.jpg
    • 1
    Front view of the assembly at 2nd intermediate
    position of bearings during downward
    movement

    (4) 3rd Intermediate position:- Now, when the face having the edge AB just touches the wedge, it will start pushing the wedge downwards and consequently the mobile cam-slots will be pushed in sideways. At that instant, the bearings will just left the mobile cam-slots and will be on the edge of the fixed cam-slots, so that the movement of the cam-slots does not affect the movement of bearings, as shown below in fig.6:-

    3rd intermediate-DM-2D - Copy.jpg
    • 1
    Front view of the assembly at 3rd intermediate
    position of bearings during downward
    movement

    (5) Final position:- At this stage, the WGS totally pushed the wedge inside the protruding parts and the cam-slots are displaced sideways such that the standing slot comes in line with the fixed cam slots replacing the sitting slots. The face having the edge AB remains attached to the wedge and the bearing is still on the fixed cam-slots, as shown below in fig.7:-

    final-DM-2D - Copy.jpg
    • 1
    Front view of the assembly at final
    position of bearings during downward
    movement

    (B) Upward movement of the wedge:- During the upward movement, the standing slot remains under operation. The whole scenario of the upward movement of the wedge is explained in various steps on the basis of different positions of the bearing on the cam-slots, as follows:-

    (1) Initial position:- When the person is about to start rising from the sitting position, the face having the edge AB of the WGS, which is still in touch with the wedge, is about to lose the contact and it will start moving upwards without disturbing the wedge and cam-slots, and the bearings which are on the 2nd extreme position (indicated in Fig.1) is about to start moving upwards also, as shown below in fig.8:-

    initial-UM-2D - - Copy.jpg
    • 1
    Front view of the assembly at initial
    position of bearings during upward
    movement

    (2) 1st Intermediate position:- As the WGS will move upward, it will not affect the position of the wedge until the face having edge CD pushes the wedge upwards. Consequently, the position of the cam-slots will also not be affected. So, the bearings will also move up the fixed cam-slot and will reach on the moving cam-slot (standing cam-slot), indicated in the fig.9 by 1st intermediate position, as shown below:-

    1st intermediate-UM-2D - Copy.jpg
    • 1
    Front view of the assembly at 1st intermediate
    position of bearings during upward
    movement 

    (3) 2nd Intermediate position:- As the WGS is moving upwards without affecting the positions of both the wedge and cam-slots, the bearings will simply roll over the standing cam-slots, as indicated in the fig.10 given below:-

    2nd intermediate-UM-2D - Copy.jpg
    • 1
    Front view of the assembly at 2nd intermediate
    position of bearings during upward
    movement 

    (4) 3rd Intermediate position:- Now, when the face having the edge CD of WGS just touches the wedge, it will start pushing the wedge upwards and consequently the mobile cam-slots will be coming to its initial position (i.e. moving sideways in inward direction) due to spring force. At that instant, the bearings will just left the mobile cam-slots and will be on the edge of the fixed cam-slots, so that the movement of the mobile cam-slots does not affect the movement of bearings, as shown below in fig.11:-

    3rd intermediate-UM-2D - Copy.jpg
    • 1
    Front view of the assembly at 3rd intermediate
    position of bearings during upward
    movement 

    (5) Final position:- At this stage, the WGS totally pushed the wedge outside the protruding parts and the cam-slots are displaced sideways (in inward direction) such that the sitting slot again comes in line with the fixed cam slots replacing the standing slots. The face having the edge CD remains attached to the wedge and the wedge movement gets restricted by colliding with the fixed constraint (labelled in fig.1), and the bearing is still on the fixed cam-slots, as shown below in fig.12:-

    final-UM-2D.jpg
    • 1
    Front view of the assembly at final
    position of bearings during upward
    movement 

    In this way the above mechanism works, which ensures that the cam-slots are automatically changed with the movement of WGS by pushing and pulling the wedge in and out of the protruding parts of the mobile cam-slots at intervals, as required.

    Design of the wedge

    During the upward movement of the wedge, there is a possibility that the wedge comes out of the protruding parts of the mobile cam-slots due to the spring forces. This situation arises in between the initial and 3rd intermediate position of bearings during the upward movement, when the face having edge CD or AB of the WGS is not in touch with the wedge. To ensure that it does not come out of the protruding parts, the wedge is designed accordingly as shown in fig.13, so that the wedge remains in equilibrium.

    wedge_sketch1.jpg
      FBD of the wedge contour

      In the FBD (fig.13) above, following are the parameters indicated:-

      α -> angle made by the faces NP and UV with the vertical

      ß -> angle made by the faces PQ and TU with the vertical

      F1 -> force exerted on the faces NP and UV

      F2 -> force exerted on the faces PQ and TU

      In the above sketch of FBD of the wedge, XY is the line passing through the mid point of the line AB, which means XY is passing through the mid point of the wedge and also the cam slots (as you will see later). EF and GH are the line of action of force F1 on the faces NP and UV respectively. Similarly, IJ and KL are line of action of force F2 on the faces PQ and TU respectively. The wedge is designed in such a way that the line of action of forces EF and IJ intersects at C, and GH and KL intersects at D, provided the points C and D are on the line XY. Also, the faces MN, QR, ST and VW are not vertical, instead tilted a little inwards so that they do not get in contact with the protruding parts of the mobile cam-slots (see fig.19) and thus experiencing no forces on these faces.

      Now, taking the upward and right direction forces as positive, and downward and left direction forces as negative, we have,

      net force acting on the wedge in the horizontal direction = F1cosα + F2cosß - F1cosα - F2cosß = 0 N ......Eq.(1)

      net force acting on the wedge in the vertical direction = F2sinß - F1sinα + F2sinß - F1sinα = 2(F2sinß - F1sinα) ......Eq.(2)

      So, eq.(1) shows that the wedge is in horizontal equilibrium but for vertical equilibrium eq.(2) must be equals to zero, which implies that it has to be prove that, F2sinß = F1sinα .

      Proof:- To prove that F2sinß = F1sinα, we will balance the forces on mobile cam-slot as it will be in equilibrium when the wedge remains inside the protruding parts of the mobile cam-slot. The FBD of the mobile cam-slot is shown below in fig.14 and also, the zoom in view of the protruding part is shown below in fig.15:-

      cam-slot sketch - Copy (2).jpg
        FBD of the mobile cam-slot
        ZOOM IN VIEW OF SLOT.jpg
          Zoom-in view of protruding part of the mobile cam-slot

          Following are the paramaters indicated in the FBD of mobile cam-slot :-

          α -> angle made by the face BC with the vertical

          ß -> angle made by the face CD with the vertical

          F1 -> force exerted on the face BC

          F2 -> force exerted on the face CD

          Fs1 -> force exerted by the upper right spring

          Fs2 -> force exerted by the lower right spring

          F5-> force exerted by the fixed part on the spring guides

          a -> perpendicular distance between the line of action of force Fs1 or Fs2 and the centre line XY

          b -> perpendicular distance between the line of action of force F5 from the point O

          XY is the line passsing through the middle of the cam-slot and protruding part, as well as through the middle of the wedge (as depicted earlier) because the protruding part is constructed at the middle of the mobile cam-slot, so when the wedge gets inside and fully fit in between the protruding parts (length of the wedge and protruding part being same), the line passing through the middle of the wedge coincides with that of the mobile cam-slot. EF and GH are the lines of action of forces F1 and F2 on the faces BC and CD respectively. These lines of action of forces are made to meet at point O on the XY line. Since, the spring guides are at the same distance from the centre line XY, thus the lines of action of both the forces Fs1 and Fs2 are at a distance 'a' from the centre line XY. The faces AB and DE are made vertical unlike the faces VW and TR of wedge, so that they don't remain in contact with each other.

          Now, taking the same sign convention as earlier,

          for horizontal equilibrium of the cam-slot, we have,

          F1cosα + F2cosß - Fs1 - Fs2=0, which implies that, F1cosα + F2cosß = Fs1 + Fs2 ......Eq.(3)

          for moment equilibrium at point O (taking anticlockwise as positive and clockwise as negative), we have,

          Fs1 * a - Fs2 * a - F5 * b = 0 (since both the springs are compressed by the same amount, thus Fs1=Fs2)

          which implies, F5 * b = 0 => F5 = 0 ......Eq.(4)

          for vertical equilibrium of the cam-slot, we have,

          F1sinα - F2sinß - F5 = 0 , since F5=0 , thus we get,

          => F1sinα = F2sinß .......Eq.(5) [prooved]

          Therefore, from Eq.5 and Eq.2 we get that net force acting on the wedge in vertical direction = 0 N

          So, we have proved that the net vertical and horizontal force on the wedge is 0 N. Hence, the wedge will remain in equilibrium in between the protruding parts even when WGS loses contact with the wedge between initial and 3rd intermediate position during the upward movement of WGS.

          Fig.16 and fig.17 below is showing the final model of the wedge. The final model of wedge has a protruding part for the WGS to handle it.

          WEDGE front.jpg
            Front view of the wedge
            WEDGE ISOMETRIC.jpg
            • 1
            Isometric view of the wedge

            Fig.18 and fig.19 below is showing the assembly of the wedge with mobile cam-slots. In the zoom in view of the assembly, one can notice that there is a gap in the middle between the wedge and protruding part of the mobile cam-slots so that they don't get fix with each other while pulling the wedge out of the space between the protruded parts. There is another thing to notice is that the slanted sides of the wedge at the top and bottom is not in contact with the protruded parts, as said earlier also, so there is no forces exerted on these faces.

            wedge slot assembly.jpg
            • 1
            • 2
            • 3
            • 4
            • 5
            Assembly of wedge and mobile cam slots
            wedge slot zoom in.jpg
            • 1
            • 2
            • 3
            Zoom in view of contacting surfaces in the assembly of wedge and mobile cam-slots 

            So, in this way the wedge is designed to make it work appropriately, without having any error in its functioning.

            3D printed prototype

            The 3D printed prototype of the above mechanism is shown below in fig.20 with labelled parts. CPE material (white in colour) is used to make the parts and the guides are made of MS bright steel. The base of the whole mechanism is also made of MS (Mild Steel). The parts are assembled with M3 and M6 bolts and nuts along with washers.

            IMG_20190710_154958.jpg
            • 1
            • 2
            • 3
            • 4
            • 5
            • 6
            • 7
            • 8
            • 9
            • 10
            • 11
            • 12
            3D printed prototype of the Wedge-Spring mechanism 

            The complete working of the wedge-spring mechanism as explained earlier is depicted in the fig.21–fig.27 below.

            IMG_20190710_160227.jpg
              Initial stage of the mechanism when downward movement of WGS starts
              IMG_20190710_160237.jpg
                Intermediate stage-1 of the mechanism during the downward movement of WGS
                IMG_20190710_160316.jpg
                  Intermediate stage-2 of the mechanism during downward movement of WGS 
                  IMG_20190710_160328.jpg
                    Intermediate stage of the mechanism when downward movement of WGS ends and upward movemnt starts
                    IMG_20190710_160337.jpg
                      Intermediate stage-1 of the mechanism during upward movement of WGS
                      IMG_20190710_160416.jpg
                        Intermediate stage-2 of the mechanism during upward movement of WGS
                        IMG_20190710_160428.jpg
                          Final stage of the mechanism when WGS stops moving upward

                          Space Model of the Actual Easy Chair

                          Based on the above wedge-spring mechanism and keeping in mind the limitations of the predesigned model of the easy chair, the space model of the actual easy chair has been redesigned along with many changes and transformations for its better stability, reliability and longevity. The semi circular open section shells are used in this redesigned model just like the earlier model. The model of different parts of the chair is shown below along with the details of their working and transformation that has been done:-

                          The fixed slot holders are supported by slot holder rods which are attached to the end plates at the end. These slot holder have space to hold the fixed cam-slots on it. Between the fixed cam-slots, there is a cut-out free space on the slot holder for the mobile slot holders to move sideways. A large gap is provided between the slot holders for better stability while in operation. The end plates are also widened so that it gets attached to the mounting plate (as you will see later) which will provide better reliability. The assembled isometric and front view of the fixed slot holders with fixed cam-slots are shown in fig.28 and fig.29 respectively.

                          isometric view of fixed slots and slot holder - Copy.jpg
                          • 1
                          • 2
                          • 3
                          • 4
                          • 5
                          • 6
                          • 7
                          • 8
                          • 9
                          • 10
                          labelled isometric view of the assembled fixed slot holders with fixed cam-slots
                          front view of fixed slot holder with fixed slots - Copy.jpg
                            Front view of the assembled fixed slot holder with fixed cam-slots

                            Fig.30 is the assembled side view of the fixed slot holder with fixed cam-slots which shows that during the initial and final 5 deg. movement of the bearings on the cam-slots, both the sitting and standing cam-slots have the same profile design (as indicated in sec. 3.2.1). Thus, these parts of the cam-slots are made as fixed parts.

                            side view of fixed slots and slot holder - Copy.jpg
                              Side view of the assembled fixed slot holder with fixed cam-slots

                              The mobile slot holders are designed and positioned as shown in fig.31 and fig.32. The mobile slot-cams are placed on the cut-out space present in the mobile slot holders. The protruding parts, for wedge to enter, is at a distance from mobile slot holders. It is connected to the mobile slot holders by a rectangular cross section steel structure so that when the protruding parts move sidewards, the mobile slot holders are also displaced sidewards.

                              isometric view of mobile slot holder with mobile slots - Copy.jpg
                              • 1
                              • 2
                              • 3
                              • 4
                              • 5
                              • 6
                              • 7
                              • 8
                              isometric view of the assembled orientation of mobile slot holders with mobile slot-cams
                              front view of mobile slot holder and mobile slots - Copy.jpg
                                front view of the assembled orientation of mobile slot holders with mobile slot-cams

                                Fig.33 and Fig.34 below shows the assembled views of the mobile and fixed slot holders with slot-cams. The blue part is the mobile slot holders with their mobile slot-cams. The yellow arrow shows the movement of the wedge and pink arrow indicates the simultaneous movement of the mobile slot holders, same as analysed earlier in the 2D wedge-spring mechanism.

                                top view of both fixed and mobile slot holder and slots - Copy.jpg
                                  Top view of the assembled fixed and mobile slot holders with their respective slot-cams.
                                  isometric view of both fixed and mobile slot holder and slots - Copy.jpg
                                    Isometric view of the assembled fixed and mobile slot holders with their respective slot-cams.

                                    As shown below in fig.35 and fig.36, the seat bar is rigidly attached to the mid-plate because other types of coupling, like with nuts and bolts, will simply make the system more hectic and bulky which is not desirable. The WGS is also rigidly welded to the seat frame, which will move along with the seat frame during sitting and rising operation and thus pushes and pulls the wedge in and out of the protruding parts of the mobile cam-slots respectively, same as that of the working of WGS in 2D spring wedge mechanism, as explained earlier.

                                    isometric view odf of fixed and mobile slots with mid plate cum seat bar - Copy (2).jpg
                                      Isometric view of the assembled seat frame and WGS rigidly attached to seat side mid plate

                                      The black arrow in fig.36 shown below shows the movement of WGS when the person will be sitting on the chair. Since the seat frame will move down, the WGS will also move as indicated below. This movement of WGS pushes the wedge inside the protruding parts, at the end of the sitting operation when lower edge of the WGS will be touching the wedge, so that the mobile cam-slots are displaced sideways and standing cam-slots comes in line with the fixed cam-slots (exactly like the 2D mechanism).

                                      zoom in view - Copy.jpg
                                      • 1
                                      • 2
                                      • 3
                                      Zoom in isometric view of the assembled seat frame and WGS rigidly attached to seat side mid plate

                                      As shown in fig.37, the bearings are attached to the bearing rod which in turn is rigidly welded to the seat frame and there is a platform for the wedge to rest upon, which is fixed to the slot holder rod. The platform is designed in such a way that it does not obstruct the combined movement of the seat frame, WGS and mid plate.

                                      isometric view of bearing and wegde holding platform with the assembly - Copy.jpg
                                      • 1
                                      • 2
                                      • 3
                                      Isometric view of the assembled bearings and WRP

                                      Fig.38 below, shows the model of the WRP and its various parts are labelled too. There are constraints to sideways, upward and downward (along the WRP) movements of the wedge. It is done by raising structures on the WRP as shown below:-

                                      wedge_resting_platform_isometric.jpg
                                      • 1
                                      • 2
                                      • 3
                                      • 4
                                      • 5
                                      Isometric view of the WRP

                                      Fig.39 shows the assembly of springs and mounting plates. Mounting plates are redesigned in such a way that they have protruding structure for the springs to compress upon. The springs are placed on the spring guides, and attached at one end to the mobile slot holders and at other end to the protruding parts of the mounting plate. The mounting plates have holes in the protruding parts for the spring guides (attached rigidly to the mobile slot holders) to enter and move freely when the mobile slot holders move sideways. Also, the end plates are redesigned in such a way that they are much more wider than before and touches the mounting plate. This will make the system more reliable than before as there will be some friction between the contacting surfaces which will provide more strength to the system.

                                      spring and mounting plates - Copy.jpg
                                      • 1
                                      • 2
                                      • 3
                                      • 4
                                      • 5
                                      • 6
                                      • 7
                                      • 8
                                      Top-front view of the assembled springs and mounting plates 

                                      Now, after assembling the remaining parts- seat, seat back and chair frame, the chair model is fully ready. Fig.40 and fig.41 shows the model of the actual redesigned easy chair.

                                      isometric view - Copy (2).jpg
                                        Isometric view of the assembled easy chair
                                        back side - Copy_1.jpg
                                          Backside view of the assembled easy chair

                                          Thus, the redesigned easy chair has lots of transformed parts to make it more reliable, user friendly and to increase its longevity. Keeping in mind the objectives (section 1.2) to be fulfilled, following are the transformation that has been undertaken in the above redesigned easy chair:-

                                          (1) The distance between the two slot holders is increased for better stability.

                                          (2) The shells are being loaded from both the sides during sitting as well as rising operation.

                                          (3) Diving the slot holders and slot-cams into parts- mobile and fixed.

                                          (4) Introduction of wedge-spring mechanism to ensure automatic slot changing at the end of sitting and rising operation.

                                          (5) The WGS, WRP, wedge and the mobile cam slots are designed accordingly to ensure the proper functioning of the wedge-spring mechanism.

                                          (6) The end plates are widened and are made to contact the surface of mounting plates.

                                          (7) The mounting plates are redesigned to have the protruding structure for the springs to compress.

                                          (8) Rigidly attaching (welded) the seat frame to the mid plate, instead of using the unnecessary methods of coupling.

                                          (9) The unreliable and unnecessary joints are removed.

                                          RESULTS AND DISCUSSION

                                          The transformations that has been done in this chair made the chair more stable, robust, reliable and user friendly. Increasing the gap between the two slot holders has resulted in more stable functioning. Introduction of automatic slot changing operation by using wedge-spring mechanism has made its operation more user friendly and less worrisome. Contacting the end plates to the mounting plates ensures friction to come into the picture which results in increase in its reliability. Removing the unnecessary joints while coupling the seat frame to the mid plate, and also in other parts of the chair, has resulted in more simple structure and functioning. Also, using the springs may result in little variation of torque, so the cam profiles should be made accordingly.

                                          CONCLUSION AND RECOMMENDATION

                                          Performing this research work it is concluded that:-

                                          (i) The more lesser the parts used to make a mechanism, the more simpler the mechanism will be and also it will be easy to find errors/defects if there happens any.

                                          (ii) Cam profiles should be designed keeping in mind about the torque that will come from springs.

                                          (iii) The dimension of the parts that are to made should be correct and there must be some clearance while joining and guiding parts.

                                          ACKNOWLEDGEMENTS

                                          I would like to extend my heartiest gratitude to respected Prof. G.K. Ananthasuresh, Department of Mechanical Engineering, IISc. Bangalore for his visionary guidance at each and every step of the research work. I have been extremely fortunate to have worked under his supervision.

                                          I would also like to convey my thanks to Mr. Yash Agrawal (Project Assistant) without whom it would have been difficult for me to complete the work within the stipulated time, for his inspirational guidance, timely help, valuable suggestions and discussions throughout my project.

                                          I am indebted to the Indian Academy of Sciences, Bangalore for giving me this opportunity to work at Indian Institute of Science, Bangalore, one of the top institutes for scientific research and higher education in India.

                                          I thank all the supporting staff of AuthorCafe for providing such an excellent space to write my report and collaborate with my guide.

                                          Last but not the least, I am deeply obliged to my parents, grandparents, teachers and friends for their love, care and everlasting support towards me.

                                          References

                                          • Shamanth Hampali, Pai S. Anoosha, G. K. Ananthasuresh, 2018, An Open-Section Shell Designed for Customized Bending and Twisting to Ease Sitting and Rising in a Chair, Lecture Notes in Mechanical Engineering,Machines, Mechanism and Robotics, pp. 427-439

                                          • Pei-Hsin Kuo, Ashish D. Deshpande, 2013, Novel Design of a Passive Variable Stiffness Joint Mechanism: Inspiration From Biomechanics of Hand Joints, Volume 2: Control, Monitoring, and Energy Harvesting of Vibratory Systems; Cooperative and Networked Control; Delay Systems; Dynamical Modeling and Diagnostics in Biomedical Systems; Estimation and Id of Energy Systems; Fault Detection; Flow and Thermal Systems; Haptics and Hand Motion; Human Assistive Systems and Wearable Robots; Instrumentation and Characterization in Bio-Systems; Intelligent Transportation Systems; Linear Systems and Robust Control; Marine Vehicles; Nonholonomic Systems

                                          • Yanlei SHI, 2018, Design and Analysis of a Active-passive Variable Stiffness Flexible Joint, Journal of Mechanical Engineering, vol. 54, no. 3, pp. 55

                                          • Sebastian Wolf, Oliver Eiberger, Gerd Hirzinger, 2011, The DLR FSJ: Energy based design of a variable stiffness joint, 2011 IEEE International Conference on Robotics and Automation

                                          More
                                          Written, reviewed, revised, proofed and published with