2.1.3. Embedded Cruise Control

The second manipulation mode presents some operation advantages for the pilot such as making movements at constant speed in front of perturbations in the wheels due to ground irregularities, upward or downward slopes, and minor obstacles; in this way, it is possible to make soft displacements avoiding accelerations that can affect the instruments and the robot. Additionally, it is possible to maintain a working rhythm independently of small discharges in the battery, as the control regulates the energy applied on the motors according to perturbations or voltage changes. To ensure a constant speed of the mobile during the scanning process, it was necessary to implement a speed control on each set of wheels, with a great robustness to cope with different terrains and obstacles, in addition to the effects of vibration and mechanical degradation. This design is based on the consideration of a second-order dynamic system:

A PID controller with robustness greater than 80% was designed using FRtool [22] with a mathematical model presented in the Equation equation and its estimated parameters. The settling time (ts) is another important parameter for the controller design, which has a trade-off with robustness. As our application does not require large accelerations, the speed does not have to change rapidly, so its settling time could be as slow as ts≈2 seconds. Finally, the maximum overshoot parameter (Mp), which is always desired to be as small as possible, was defined with a tolerance of less than 5%. Table 4 presents the estimated model parameters for the Equation (17), the design parameters for the control, and the obtained constants for the PID controller in its discrete implementation described in the following equation:

Design parameters for wheel speed controllers.

Figure 4 illustrates the control scheme, which was implemented as difference equations into the “control node” rosnode as shown in Figure 3. The gestures of Channels 1 and 2 are generated in the gamepad to generate the acceleration and direction signal respectively. A gain K transforms the operated signals from the gamepad in the radseg control set-points. These reference signals are saturated to guarantee the values of the angular speed limits. C(s) and G(s) are transfer function blocks that represent the PID controllers and the motors, respectively. The importance of speed control lies in giving the robot the ability to follow a constant speed reference and give it a certain degree of settlement in the presence of disturbances either from the ground or from its own construction characteristics.

Control scheme for the speed control system. CH1 and CH2 represent the gamepad gestures; ωR and ωL represent the controlled wheel speed for the right side and the left side, respectively.