# Collaborative Robotics: Enhance Maintenance Procedures on Primary Flight Control Servo-Actuators
## 4 Automatic Adjustments on an EHSA  
The most complex task to command to the robot during the maintenance of an EHSA is an adjustment. Since each unit has a different load history and health status, each repair operation is different from another. In so doing, hard-coding an adjustment is impossible since it is not known, a priori, if this procedure is necessary or not and, if yes, in which direction, and how much, to regulate a specific component of the faulty unit. To do so, it has been decided to structure the robot program with a series of functions, recalled by the macro-sequencer, whose inputs depend on the signals coming from the IMS. The control algorithm has been designed to adapt itself according to the values acquired through the measurement system, and its working principle is schematized in Figure 9.  
To perform an adjustment, a specific movement, such as the rotation of the screwdriver (Rott), is commanded to the robot. However, the magnitude and the direction of such rotation is not preprogrammed, but calculated using a proportional control logic. So, the Central Computer would command a higher rotation if the actual measurement (Xt) is further away from its target (XtARGET), while more refined regulations are performed when the actual value is closer to the desired one. The gain adopted for each task has been defined through an experimental campaign to obtain a fast adjustment but, at the same time, avoid any overshoot. On the other hand, the rotation direction is defined by comparing the previous (Xt-1) and the actual (Xt) measures with the target value. If, as reported in Figure 5b for the trail rotation, the controlled variable moves further away from its target (\(\mid\)Xt-XTARGET \(\mid\) > \(\mid\)Xt-1-XTARGET \(\mid\) ), the direction is reversed. This is done by getting the sign of the previous rotation commanded to the robot (sign(Rott-1)), and by changing it only in the case the previous command led the actual value of X to be further away from the desired one. As an example, the adjustment procedure related to the Rotary Variable Differential Transformer (RVDT), used to measure the servo-actuator ram position, is analyzed. In Figure 10a, it is reported a view of the sensor and the three safety screws that have to be loosened to regulate the RVDT orientation according to the value of the difference between the voltages of its secondary coils (\(\Delta\)U). As already mentioned, since the tools mounted on the robot are prototypes, there could be a certain amount of backlash between them and the EHSA components. For example, the one between the RVDT plate and the wrench, estimated to be around \(5^{\circ}\), is schematized in Figure 10b. This must be considered when the RVDT rotation direction has to be reversed since there would be a delay between the movement of the robot arm and the rotation of the sensor, similar to the one reported in Figure 5b. Since the adjustment curve does not add any significant information, the time series is not reported. Figure 9: Flowchart reporting the control logic used to perform an automatic adjustment on the servo-actuator. A critical aspect of this adjustment is related to the relative position between the robot and the servo-actuator. The RVDT is located further away than the other areas of the unit, with respect to the work cart on which the UR10 is mounted. This requires the industrial manipulator to be almost fully extended, leading to a reduced accuracy of the positioning of the TCP, which could negatively affect the correct insertion of the screwdriver inside the screws. In order to validate the automatic adjustment procedure, an experimental campaign is ongoing and the first results, related to the regulation of the RVDT of three servo-actuators, are reported in Figure 11.  
The repeatability of the proposed approach has been verified by testing each unit three times. It can be noticed that, in all the tests carried out in the experimental campaign, the adjustment was always successful.