**5. Defining grasps**

Artificial tendons provide movement in the hand. These tendons induce rotations about the joints of the digits. Assignments are shown in **Table 2**.

**Figure 15** shows the joint angles of each phalanx with respect to each previous joint. Using the relationship between tendon displacement and joint angles we can define a grasp type.


#### **Table 2.**

*Tendon arrangement for finger actuation.*

**Figure 15.** *Artificial hand finger tendon network and the associated joint angles.*

*Biomechanical Design Principles Underpinning Anthropomorphic Manipulators DOI: http://dx.doi.org/10.5772/intechopen.105434*

For simple tendon networks composed of 3 pulleys in a digit, the tendon extension, *ei*, is a linear function of the change of the joint angles Δ*θi*1,Δ*θi*2*and*Δ*θi*3. Therefore, tendon extension can be expressed as

$$
\sigma\_i = \sum\_j r\_{ij} \Delta \theta\_{ij} \tag{8}
$$

Where, *rij*, is a measure of the pulley radius at the *j-*th joint (*j* = 1, 2, 3 for *i* where *i* ¼ **1**, **2**, … , **5** and *j* = 1 for *i* = 6).

The extensions shown above can be used to determine grasp strength. Each tendon tendons is connected to individual servomotors to actuate digit movement. The artificial hand has six servomotors, one for each finger and two for the thumb. A grasp type in this context is defined as

$$\boldsymbol{\phi} = \begin{bmatrix} \boldsymbol{\rho}\_1 \boldsymbol{\rho}\_2 \boldsymbol{\rho}\_3 \boldsymbol{\rho}\_4 \boldsymbol{\rho}\_5 \boldsymbol{\rho}\_6 \boldsymbol{c} \end{bmatrix}^T \tag{9}$$

Where,

$$x = \{1, \text{for} forcedsurergra spping} \text{ or } \text{0,} forformlosureprasping}$$

when *φ<sup>i</sup>* (∀i) equals the pre-set maximum servomotor angle,*φmax <sup>i</sup>* the digit is in complete flexion. Otherwise, it is in complete extension (that is *<sup>φ</sup><sup>i</sup>* <sup>¼</sup> <sup>0</sup> *orφmin <sup>i</sup>* ).

A physical representation of Eq. (9) is shown in the working principle of finger actuation displayed below in **Figure 16**.

In continuation of the rule established above **Figure 17** below demonstrates the four common grasp types and their definitions. The grasps shown are:

**Figure 16.** *Digit extension and flexion due to the servo motor angle.*

#### **Figure 17.**

*Grasp types performed by the artificial hand and their definitions.*


In the case of objects where one of its dimensions are much larger than the other a tip pinch grasp or a power grasp is appropriate (**Figure 18**).

*Biomechanical Design Principles Underpinning Anthropomorphic Manipulators DOI: http://dx.doi.org/10.5772/intechopen.105434*

**Figure 18.** *Tip pinch and power grasp examples and definitions.*
