# Impact of a Robot Manipulation on the Dimensional Measurements in an SPC-Based Robot Cell
## 5 Discussion
Increasing the degree of the robot's manipulation means more manipulation or more extended robot movements, and above all, an increase in the number of gripping sequences during the measurement process. For example, scenario 1 does not involve any gripping sequence, which means the misalignment error is practically zero. With the added short gripping sequence in scenario 2, a small amount of misalignment error is already present. As evident from Figures 9 and 10,in the case of length measurements (Magnet code 30 and PKR codes 10, 100, and 140), the misalignment error increases with an increase in the degree of robot manipulation, resulting in a decrease of the \(C_{p}\) value. The cause is also in the axially symmetrical shape of both objects, Magnet and PKR. Due to the axial symmetry, there is no pronounced edge that would easily ensure optimal rotation of the object in the gripper and, consequently, at the Equator's measuring point. Because of the robot's repeatability and accuracy errors, and due to the overall gripping situation, one of the fingers comes into contact with the object earlier than the other and not entirely in the intended place, which, combined with the loose fit, result in a small, but unwanted, rotation. A tighter fit would be a potential solution. However, we must be aware of the limitations of tightness. A tight fit requires a very accurate and tight object positioning in the fixture. In the case of a too tight fixation of the object in the pallet (or fixture), the robot with the gripper will not be able to pick the object out of a particular place (the object would slip from the grip). A similar situation applies to the placement of the object. An excessive gripper force would also cause damage or deformation to the subject. The most significant impact on angular misalignment originates from grasping and tightness in fit to fixture. Further, robot positional and orientational errors increase object-to-fixture interaction forces and, this way, cause increased misalignment. It is evident that a higher number of grasps of the measured object with a robotic gripper causes a greater scatter of the measurements (scenarios 8 and 9). The same applies to the number of active joints of the robot when performing movements. A lower number of active joints (scenarios 3 and 4) during robot movements means minor scatter of measurement data than movements with a higher number of active joints. Nevertheless, minor changes in the object's orientation also result in less scatter. Practical suggestions originating from this work are minimization of grasping actions, minimization of active joints, and manipulation and measurement sequence optimization. For measurements with the Equator, special fixtures were developed to position objects with a robot in the Equator's working area. Fixtures were designed to fit the object's inner diameter, while the spring provides an additional tight fit, as shown in Figure 5. The \(C_{p}\) values for scenarios 1 for all the measurement sets show that the fixtures are suitable for the given characteristics. Furthermore, it is clear that the whole measuring system (robot, fixture, Equator, and other peripherals) is capable in every scenario for all the characteristics of achieving the minimum calculated values of the \(C_{p}\) factor, which were, respectively, 7.3, 2.3, 2.6, 3.1, 7.4, 30.1, and 3.0, for the Magnet characteristics 20 and 30 and PKR characteristics 10, 100, 130, and 140. In the most common cases, the minimum allowable \(C_{p}\) value is set as \(C_{p,min}=1.33\). In the worst case, the calculated capability factor \(C_{p}\) of the measurement system is approximately twice as large (\(C_{p}=2.3\) for Magnet code 20).