In order for a line tool (contact area between the contact wheel and the workpiece can be approximated to a line) to follow a 3D surface profile, the finishing robot must have a greater dexterity as compared with a dedicated 5-axis CNC machine tool. It is quite natural that the operator uses two hands to manipulate the part to obtain desired contact force and compliance between the part and the tool. In principle, two robot arms could imitate the operator's two arms to achieve the required dexterity and rigidity.
However, it poses tremendous difficulties in controlling the two mechanical arms to accomplish contact tasks. Instead, a 6-axis robot arm is preferred for its ease of motion control.
A robot for 3D blending operations has to overcome extreme reactive forces as compared to conventional industrial robots for welding, pick-and- place, glue dispensing and painting. The finishing robot not only has to hold the turbine vane in position, but also to press the airfoil surface against the grinding wheel in the normal direction and with controlled contact force, in order to achieve the desired material removal. Thus a finishing robot must have a high loading capacity, stiffness, rigidity and desired dynamic performance when used for airfoil blending operation. After an in- depth evaluation, the six-degree-of-freedom Yamaha Z-II6 robot was chosen for the blending application, shown in Figure 1. Its 6, R and Z axes determine the position of the Tool-Centre-Point (TCP), i.e., {X, Y, Z) coordinates, while the a, fi, and y axes, which form a Roll-Pitch-Roll wrist configuration, determine the orientation of the tool frame. The robot can carry a payload as high as 40 Kg and has a repeatability of 0.1 mm in the worst case. The robot, driven by AC servomotors with absolute position sensing, is dust-proof by applying a positive internal pressure, which is advantageous in the dusty blending environment.
2.2 Self-Aligned End Effector
In order to cater to both concave and convex airfoils, a servo-driven Self- Aligned End-Effector (SAE) has been developed, as shown in Figure 2. It
has an active servo drive mechanism at one end (left) and a passive follower at the other end (right). The active end is mechanically coupled to the robot end-axis y which has a driving torque about 35 Nằm. The servo- driven SAE can rotate 360 degrees so that both concave and convex airfoils can contact the grinding wheel of the belt polishing machine in normal directions.
Figure 1 Yamaha six-axis finishing robot Z-II6.
Figure 2 Servo-driven self-aligned end-effector.
The passive follower ensures that the vane is held in place firmly, and in the meantime secures the axial alignment of the vane. In addition, there are three locators in the SAE to align the vane in a fixed direction. The innovative design minimises the gripping inaccuracies that would otherwise compound the airfoil distortions. The SAE has incorporated pneumatic sensors to detect any part jamming in SAE. An external laser through-beam sensor is integrated to the feeding table to check improper gripping.
2.3 Control Interface
As discussed in Chapter 2, there are two sub control systems, namely the Knowledge-Based Process Controller (KBPC) and the Data-Driven Supervisory Controller. The former, controlling all actuators and sensors, is implemented into the industrial robot controller with a powerful robot programming language dedicated to the blending operation. The latter is implemented into the host computer running the Windows NT operating system. Figure 3 shows the system communication and interface between the robot controller, interface PC, and host PC.
Digital I/O 24VDC 16 Inputs 16 Outputs
I — •
— •
Z-II6 Robot
H
Robot Controller T RS232C Interface Computer
Running Prg:Tbmain.exe
OS: MS-DOS
••
Etherne Host Computer
Running Prg:
AutoBlending.exe OS: Windows NT Digital I/O Board
Profile Measurement
Sensor and Instrument
Figure 3 System communication and interface.
The host computer is interfaced to the robot controller through digital I/Os for handshaking, but the data transactions between the two controllers become difficult. The chief reason is that the robot controller can only communicate with an interface computer, running the manufacturer's program "Tbmain.exe" in MS-DOS, through a RS-232 serial line in the
manufacturer's proprietary protocol. To bypass the problem, the host computer is connected to the interface PC via an Ethernet LAN. Data transactions between robot controller and host controller are relayed by the interface PC. The host computer runs the specially developed main program "AutoBlending.exe".
Via the interface computer, measurement data are sent from the robot controller to the host computer in the form of text files through a RS232C interface and an Ethernet network. The computed blending paths are sent back from the host computer to the robot controller by the same route. The executions of the robot programs and the host computer programs are synchronised by the digital inputs/outputs between them.