To control the servomotors, the device has been equipped with the Servo controller SSC-32 produced by Lynxmotion. It is a small pre-assembled servo controller with some important features. It has high resolution (1 ^s) for accurate positioning, and extremely smooth moves. The range is from 500 to 2500 ^s for a range of about 180°.
The SSC-32 allows us to connect and control simultaneously 32 different servomotors. Therefore, with this board, it is possible to manage up to 5 modules in the same time. By means of the serial connection, it is possible to send to the controller the RC PWM signal from the PC/Laptop. The RC PWM (pulse width modulation) is a position communication protocol based on a signal that is a periodic pulse with a width between 500 and 2500 ^s. The operation principle behind this position
Fig. 6.8 Configuration of connection channels
protocol is that 1500 ^s commands the servo to go to the centre position. A 500 ^s pulse commands the motor to attempt reaching its leftmost position and 2500 ^s its rightmost position. Any pulse measuring in between these values is decoded as a position in between leftmost and rightmost. Therefore, the resolution of the servo rotation is equal to 1 ^s = 0.09°.
Each servomotor is equipped with a connection cable, which is connected to the controller by means of a specific channel. Figure 6.8 shows the distribution of the channels for each servomotor of the three developed modules. Thanks to the servo controller, it is possible to connect the tactile device to the PC/Laptop by means of a Serial/USB cable and control the position of each servomotor by using serial signals. These signals specify the channel, the position to achieve and the rotation speed that the servomotor has to perform to obtain the wanted position.
In order to control the tactile device we have developed a control process based on Matworks Matlab platform. We have chosen this software because it allows us to perform all the mathematical process needed to analyse the surface to render, as well as to calculate the kinematics. In addition, it is able to send serial signals to the servo controller. The process is developed in three main phases:
- • Phase 1: Surface definition and import;
- • Phase 2: Managing of the control points by means of three different modalities;
- • Phase 3: Calculation of the servomotors positions and sending of serial signals.