Dimension and restriction of the assist device

Next, we need to calculate the actual size of the assist device to manufacture the first prototype. Finding the problem and make experiments on first prototype, and modify it on better size. First, we consider the restrictions in every components depended on the connection or method of fabrication, and then we find the restrictions from human body.

Finally we will discuss the restriction in preventing falling mechanism. Combing these restrictions mentioned in this section and the model above, we can make a new assist device directly if we need to make some changes on the sizes of any components.

First we would discuss the restriction on waist. Based on the above discussion of the hip joint, we wanted to make axes (flexion/extension and adduction/abduction) cross at the center of hip joint in human body. Because of that, we need to make two axes at the same height on the waist part, fitting in with the human’s hip joint. And then the motor and the fix bar can be mounted on the waist part. The distance of both center needs to be bigger than the motor’s radius so that the fixed bar and motor would not collide. The height of the waist part depends on the distance of motor center and the fixed bar. The height of the waist part also depends on the size of the motor. We use sheet-metal to make our waist part, so the thickness is restricted by sheet-metal fabrication. The restriction of the human body on waist part is the length to which need to fit in with the size of human’s waist.

As for the thigh bar, the width depends on the knee part for preventing falling mechanism. The thigh bar is the thickest component in our device, because the thigh bar would support some weight of human’s thigh. And the length of thigh bar depends on the length between human’s hip joint to knee joint. To make sure that the thigh bar covers human’s thigh tightly, the thigh bar is worn on the middle segment of human’s thigh since

The length of the knee part is fitted in with the height of human’s knee and the sliding groove. Because of preventing falling mechanism, the width of the knee part depends on the width of thigh bar and the sliding groove. The thickness of the knee part is based on the thigh bar. And then the wear on the knee is set at the bottom of the knee part in order to make relative rotation between the thigh bar and knee part easier.

Then, we add the restrictions of the components and combine with the equations (2.7) to (2.12). First, we talk about the waist part. Because of the sheet-metal fabrication, we set the thickness of waist part as 3mm for fabricating conveniently. The waist part need to be surrounded the wearer’s waist. As the result, we set 165mm and 210mm as the lengths of the front and side of the waist part respectively. Based on sheet-metal fabrication, the waist part and thus the heights of front view and side view of the waist part are same. The size of height depends on the size of motor. Figure 2.12 shows the detail of Maxon motor EC-60 flat. Since we know the size of the motor, we can get the restriction information that

r

₅ is smaller than 34mm.

Next, we talk about the thigh bar, we choose 10mm to be the thickness of thigh bar, and then we know that the rigid shaft coupling have 20mm in diameter. We can know our width or hip bar need to be bigger than 10mm. Finally 450 mm is selected to be the length of the thigh bar based on the wearer’s thigh.

As for the size of knee part, the wear portion is set at the bottom of the knee part to let the wear more convenient, so that the length of the knee part can be decided by the wearer’s knee height. And the width of the knee part depends on the width of thigh bar and the sliding groove in the preventing falling mechanism. In the other hand, we separate the knee part into three parts for convenience. The thicknesses of the three piece parts are 5mm, 10mm, 5mm. The knee part can be assembled via the three pieces. Finally, we talk about the size of the sliding groove. We choose 16mm as the width of the sliding groove

since that the slider is a cylinder whose diameter is 16mm. And the length of the sliding groove depends on the path of the slider. The path is chosen based on the limit of the rotation of hip joint (clockwise 10° to counterclockwise 30°).

The length of the fixed bar is

r

₅ which depends on the preventing falling mechanism.

The width of the fixed bar is chosen via the diameter of the fixed cylinder. And then we combine all the restrictions mentioned above to get some restriction terms.

5

34( )

At last we need to find the value of

r

₃, its length which influences the length of sliding groove. Actually, if the height of knee part is constant, the length of the sliding groove is as longer as better. That is, we can reduce the weight of knee part, making the assist device lighter. When the upward value of

r

₃ is bigger, the preventing falling mechanism would be better. However, the bigger upward value of 𝑟₃ would cause the bigger downward value of

r

₃. It means there must need a bigger knee part to fit the sliding groove. As a result, we set the maximum upward value 20 mm for preventing

falling system. Then, we need choice a value of

r

_{3 when}



is counter clockwise and



is clockwise. First, we want to talk about fall-preventing mechanism when

0, r

3

0

  

. We discuss equation (2.17) and combine with equation (2.2) and (2.3). Here

1

, , , ,

2 3 4 5

r r r r r

are the length, must be positive in equation (2.2). Combining with Figure

2.11 (d), x component in equation (2.2) of left side is depended on

r r r

₁

, ,

_{2 3} and

r

₁ is the biggest within input lengths (

r r r

₁

, ,

_{2 3}). It also show in Figure 2.11 (d). So the left side in equation (2.2) is negative and the equation has

r r

₄

,

₅ at the right side.

r

₄ is bigger than

r

1 because of fall-preventing mechanism. If

r

₄ is smaller than

r

₁, the value of

r

₃

would not have obvious value and the slider can’t work in preventing falling we discuss in section 2.3. The right side of equation (2.2) also need to be negative, but

r

₄ is bigger than

r

₁,

r

₅ need to be positive to fit the equation. Combing with all situation above,

r

₅

and



₅ need to fit with the equation. We can know that



₅

 I IV ,

and it has similar situation on equation (2.3) with equation (2.18) when

  0, r

₃

 0

in Figure 2.11 (f). We can also get



₅

 I II ,

, so



₅ is in the first quadrant. Combing all discussion above, we

need to choose a value of

r

₃ in another gait. Here we choose the maximum downward value on the gait when hip joint rotates 20° counter clockwise and knee joint rotates 60°.

Then we have six equations and

r r

₄

, ,

₅



_4(0)

, 

_4(0)

, 

_4(0)

, 

₅ six unknown. We can get the actual size of our mechanism. We would choose length size as integer for fabricating conveniently.

在文檔中 下半身輔具及防跌倒機構之設計 (頁 50-54)