**3. Simulation of hand stress**

#### **3.1. Comfortyping**

model of three finger segments with realistic bones, nails and soft tissue. In the dynamic simulation, the stiffness of the grip surface was varied and demonstrated in final results the decrease of the pressure distribution. The work [10] developed a FEM fingertip and simulated the pressure distribution on the fingertip when opening can a tab. The results revealed that over a large contact area pressure, distribution was reduced to the fingertip. With the thumb model the static pressure distribution for use of clip connections are simulated by [11]. These influences such as pressure level, material, geometry and position change of the clips are examined. This influence examination gives design proposals for clips [12]. Other digital hand models focus on realistic simulation. The work [13] presented a hand model for the simulation of non-linear contact deformations, in relation to realistic

20 Anatomy, Posture, Prevalence, Pain, Treatment and Interventions of Musculoskeletal Disorders

In the context of the present work there exist sensor gloves to measure forces when gripping and using products. Some works are focused exclusively on the development of new sensor gloves. The research of [14] shows that the sensor gloves are exploring the relations between forces and other variables such as muscle tension [15], handling and feel [16] and the per-

Since 1991 are sensor gloves designed to measure forces in gripping and operation of handheld products. To measure the force distribution on the palm most sensor gloves have piezo (-resistive) sensors such as in Refs. [18, 19] or [20]. Due to the simple programming and low cost compared to capacitive sensors these are preferred. The force sensors are varying in the number and in the position on the hand and in their form. For the investigation of pressure distribution on the palm [21] develops a sensor system of six force sensors based on FSR sensors (FSR—Force Sensing Resistor). These resistive sensors are based on the measurement of the resistance change of semiconductor materials such as silicon. In applying three different shaped handles are pressed on the palm and then calculated from the force-measuring the

overlapping of the skin (**Figure 2**).

**2.2. Support-systems to reduce the hand stress**

ceived gripping force [17].

**Figure 2.** Example of hand models.

pressure distribution.

The term "comforting" is an artistic word and has emerged from the composition of comfort and prototyping. Comfortyping is intended to function as a stand-alone simulation program, in which a digital hand model is included in the simulation environment. After importing a hand-held product as well as after selection of influencing variables, they should be calculated on the basis of their design proposals. A similar goal for assessing the ergonomics of the man-machine interface also exists as ergotyping of [27]. In contrast to this, the focus should be placed on the hand-arm system with Comfortyping. For example, it should be possible to deliver spline suggestions to the user, which can then be imported into the CAD system. For Comfortyping is a database with different influencing variables such as grip types, gripping forces as well as hand-type and handle-dependent material properties is required. The program should offer a choice of three hand types with little, much as well as medium subcutaneous fat content. The work of [28] differ fleshy, tendinous and normal skin types. In addition, it should be possible to automatically scale the hand models by percentile and gender. A snap function should allow the hand model to take the product automatically.

A first approach for Comfortyping was developed with Recurdyn (see **Figure 1**). Here the grip of an iron bender was taken and defined the reduction of pressure peaks as well as a homogeneous pressure distribution as a target function. The MBS/FEM program Recurdyn has an Autodesign function and can iteratively optimize geometries. In the example, a shape change handle was constructed with six cylinders and pressed onto the palm of the hand with 200 N. In addition, the handle was gripped with 50 N gripping force. The geometry and material properties from the hand (skin type dependent) and the force and movement conditions from the iron bender (product-dependent) were selected from the database. For the target definition, the reduction and homogenization of the pressure distribution was selected manually (**Figure 4**).

**4. Measurement of hand stress**

**Figure 5.** Results of Comfortyping.

(FSR 402, FSR 400) adapted to the preparatory work of [21].

The development of the JSI-System (Job-Strain-Index System) was made for a male person of the 50th percentile between 20 and 30 years (see **Figure 6**). As glove, a model (TouchGrip of UPIXX) in microfibre fabric was used. This remains firm in the fixed position of the sensors on the hand. For the sensors to measure the wrist angle ulnar and radial, a bending sensor (Interlink FSR 408) is mounted on the thumb side of the hand, at the level of the wrist. For the wrist angle, palmar and dorsal a potentiometer was used. To this on the back of the glove a sheet metal was sewn to attach the potentiometer. In the forearm a velcro strip was applied and on the velcro a sheet metal and a rod were attached. To measure the force distribution on the palm (thenar, hypothenar, palmar) and on the fingertips, were force sensors Interlink

Analysis for Objective Evaluation the Stress of the Hand http://dx.doi.org/10.5772/intechopen.71474 23

The wiring was led through a slot outwardly toward the back of the hand, so this could cause no hindrance on the palm. All signals of the sensors were lead to the microcontroller (Arduino MEGA 2560). The Arduino has been built with an LCD-Display in a plastic housing. Thus, it is possible to wear the evaluation system on a belt. For calibration, the raw data are transferred on the computer in Excel (PLX-DAQ: Parallax Data Acquisition Tool). Approximation

**4.1. JSI-system**

**Figure 4.** Comfortyping in RecurDyn.

#### **3.2. Results**

The results show before the optimization on the hand regions O and Q high pressure loads. The radius of all steps is R14. The pressure loads were redistributed to the hand center P with the optimization. For this purpose, the program changed the radius of the steps of the handle until a desired pressure distribution is obtained. To do this, the radius in the hand center changes to R22. This information is output as a spline into the CAD model of the handle. A pressure evaluation with different subjects confirmed the optimized grip shape as comfortable.

For comfort evaluation, the shape change handle was designed. The developed shape change handle consists of six spreader jaws, which can be moved by a threaded pin rotation. The threaded pins have right-hand and left-handed threads. The so-called entraining jaws spread the counter-jaw during an outward movement. To conceal the edges of the jaws, a rubber covering was applied. These six jaws have taken together the width of a male palm of the 50th percentile of about 95 cm. The whole mechanism was tested by FEM for strength and is made of ABS. Various tools can be connected to the shape change handle (**Figure 5**).

**Figure 5.** Results of Comfortyping.
