# Design and Calibration of a Sensory System of an Adaptive Gripper
## 2 Materials and Methods
### Sensory System Design
#### 2.2.2 Force Sensor Design  
The force sensor has the role of measuring the gripping force, in real time, up to the value necessary to grip a part, including adjusting this force to avoid the tendency of the part to slip in the gripper. In [18; 19], systems for adapting the gripper configuration to the shape of the piece being gripped are described, supporting the adoption of the solution proposed in this paper. The force sensors in the equipment of some grippers, including adaptive grippers, are described in [20; 21; 22; 23]. In this case, a Wheatstone bridge resistive force sensor was used, which was placed in the most appropriate part of the mechanical structure of the gripping module for maximum efficiency. For this, the material from which the components of the gripping module mechanism were made, namely duralumin, was first determined, and the gripping force applied to the gripping module jaw was established at a force of F = 1 kg. The MEF analysis of the mechanical structure of a gripping module was then used, and the area of maximum deformation was identified: the area framed in red in Figure 5a. The respective redesigned element is shown in Figure 5b, highlighting the size of the part of the element containing the force sensor. In Figure 5c, this part of the mechanism  
Figure 4: CAD design of the slip sensor assembly (**a**), left side view (**b**) and right side view of the slip sensor assembly (**c**).  
of the gripping module is shown, in which the notations represent the following: 1--the support piece of the force sensor, 2--the mounting parts in the mechanism element, 3--the Wheatstone half-bridge formed by two piezoresistive sensors, and 4--cutout to increase the sensitivity to deformation. Figure 5d shows the CAD shape of the force sensor area integrated into the respective element of the gripping module, with the parts marked with 1.  
The use of a resistive Wheatstone bridge force sensor in this application offers several benefits over other types of force sensors, such as piezoelectric or capacitive ones, namely, the following:  
Low cost: Resistive Wheatstone bridge sensors are generally cheaper than piezoelectric or capacitive sensors, which makes the solution more economical, especially in industrial applications where multiple sensor modules are required;  
Robustness: Resistive sensors are generally more robust and more resistant to shock and vibration than piezoelectric sensors, which are more fragile. This is an important feature for a gripper operating in a demanding industrial environment;  
Linearity: Resistive Wheatstone bridge sensors, properly calibrated, provide a fairly good linear relationship between the applied force and the output signal. This linearity simplifies signal processing and the implementation of control algorithms;  
Simplicity: Integrating a resistive sensor into a circuit is relatively straightforward, requiring a simple signal amplifier (such as the HX711 used in this application). This reduces system complexity and overall cost.  
Figure 5: FEM analysis of the gripping module (**a**), position and size of the force sensor support piece (**b**), component parts of the force sensor support piece (**c**), and CAD shape of the adaptive gripper highlighting the force sensor support pieces marked 1 (**d**).