Programmable Automation Technologies. Daniel Kandray
with a machine-tending robot to load or unload a workpiece. When it does this, it must cease machining operations, open any guards or gates to allow easy access to the workpiece, release the automated fixture, and instruct the robot to proceed. Not only does the controller have to execute all of these impressive tasks accurately and in the correct order, but also the tasks must be carried out very quickly. This is particularly true for the motion control of the axes, which is perhaps the most important and complicated function the controller provides. Thus, the motion control is in a category among controller functions are listed. The other functions discussed above are categorized as either auxiliary control or part of the operator interface. This is shown in Figure 3-10.
Figure 3-10 CNC controller functions
Although the performance capabilities of a CNC machine are impressive, the machine cannot perform a single task without first being properly set up. In the machining industry the term “setup” refers to preparation of the machine or machines to process a specific workpiece in a specified manner. Setup typically involves acquisition and installation of the correct cutting tools and fixturing. In CNC machining, setup also entails the correct part program be loaded into memory and specifies the program reference zero (PRZ). The PRZ is the location of the x, y, and z zero positions of the coordinate system that the program references when providing relative positioning of the tool and workpiece. Setup-related tasks are performed through the operator interface of the machine. The PRZ is discussed in more detail in subsequent sections.
The operator interface typically (and minimally) consists of a CRT or LCD screen and keypad. Small nonindustrial machines used by hobbyists or by operators in the educational environment use a PC that runs operator interface software. Regardless of the type of operator interface, the functions it provides are the same, and they typically include manual motion control of the axes (for establishing the PRZ), manual control of spindle rotation speed and direction, tooling specifications and setup, starting and stopping the machine, program file management, and input and editing of programs—as well as other functions. An operator interface for a CNC lathe is shown in Figure 3-11.
Figure 3-11 CNC operator interface
Auxiliary control is essentially discrete control over machine functions that support the machining or processing operation—functions that are not directly related to motion (continuous) control of the axes. This is an important distinction because it affects the coding used in the program of instructions (see Chapter 4). Auxiliary control is considered a form of discrete control in the sense that the status of the function (spindle motor) changes at discrete or distinct moments in time. Additionally, the status is binary (1 or 0). As an example, consider the status of the spindle motor during a machining operation. Its status is either on or off. This can be represented in binary terms by either a 1 (on) or 0 (off). This is in stark contrast to the motion control of the axes. The status or position of an axis is continuously changing. Hence, motion control is a form of continuous control.
Figure 3-12 shows the motion control system for a single axis of a vertical mill. Based on a program of instructions, the CNC controller sends the desired position and sets the speed at which it travels to the motor drive unit. The motor drive unit then analyzes this signal and turns on and rotates the servomotor to obtain the desired position at the desired speed. The encoder informs, or feeds back, the position of the axis continuously to the motor drive unit, while the tachometer reports the rotational speed of the lead screw. The motor drive unit continuously compares the desired position and speed to the actual position and speed and adjusts the servomotor accordingly. This type of continuous control is termed a closed loop feedback control system. Most CNC machines used in industry employ this type of control. However, for smaller CNC machines used by hobbyists and in academia, an open loop control system is often used.
Figure 3-12 Single-axis closed loop motion control system
An open loop control system utilizes a stepper motor instead of a servomotor to control axis movement. Also, the system does not use a feedback device such as an encoder or tachometer. Hence, it cannot provide feedback to the motor controller regarding the actual axis position and speed. Thus, the signal sent to the stepper motor is the only means by which the speed and position of the axis is controlled. The signal is sent, and the controller assumes the axis has achieved the desired position at the desired speed, but this assumption cannot be verified. Even though open loop control systems are much simpler and less expensive than closed loop feedback control systems, they are generally only used where resistance to axis motion is low. High resistive forces could prevent the axis from achieving the desired position. A single-axis open loop control system is depicted in Figure 3-13.
Figure 3-13 Single-axis open loop motion control system
In a typical machining operation, simultaneous control of more than one axis is required. For a CNC lathe or a CNC plasma cutter, two axes will be controlled simultaneously. For a mill, at least three and up to five axes have to be controlled. Therefore, the motion control system has to perform complicated geometric computations to coordinate the motion of each axis to achieve the desired relative position of the tool to the workpiece. Additionally, the path taken to achieve that position may be of particular importance.
Consider Figure 3-14. Figure 3-14(a) shows the desired end product. Two holes are to be drilled at positions A and B. First, the tool is moved to position A and the hole drilled. Subsequently, the tool moves to position B to drill the second hole, as shown in Figure 3-14(b). The path taken to get to position B is of little importance. What is important is that tool position B be located accurately. This type of control, where the tool is moved from point A to point B without consideration of the path taken is called point-to-point control. To achieve this motion two single-axis motion control systems can be placed together, one to represent the x-axis and one to represent the y-axis. A signal is sent to the x-axis motor drive unit to move to position xB and another is sent to the y-axis motor drive unit to move to position yb. Each axis will move to the desired position without regard to the motion of the other axis. This is the essence of point-to-point control: to obtain the desired position without regard to the path taken. Figure 3-14(c) shows a graph of how the path may look on the (x,y)-coordinate system, depending on the values of xB and yB. This type of control is suitable for drilling operations, except when the path taken between the two positions is important, as is true in milling and turning (lathe) operations.
Figure 3-15(a) shows a block with a diagonal slot milled in it from point A to B. In order to perform this operation, the tool will be positioned above point A, then moved down into the workpiece to the desired depth. From this position it is moved to point B following a straight (linear) path, as shown in Figure 3-15(b). In this case, the path taken from point A to point B is important. Additionally, the speed at which the tool moves along this path is also very important because of speed’s effect on the surface finish of the final product. The resulting tool path is shown graphically in Figure 3-15(c). Consequently, the CNC controller must perform complicated calculations, and do so very quickly, to ensure that the desired path is followed at the desired speed. This is called continuous path control, which the controller must “interpolate” to achieve. This idea is now explained.