Cnc machining nm09/2
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- 2.2.2 Closed loop control system
- 2.2.3 Transducers
- 2.2.4 Rotary transducers
- Tachometers
- 2.2.5 Linear transducers
- 2.3 Stored stroke limit
2.2 Motion control systems2.2.1 Open loop systemIn the open loop system, shown below, the tape is fed into a tape reader which decodes the information punched on the tape and stores it briefly until the machine is ready to use it. The tape reader then coverts the information into electrical pulses or signals. These signals are sent to the control unit, which energises the servo control units. The servo control units direct the servomotors to perform certain functions according to the information supplied. The amount each servomotor will move depends upon the number of electrical pulses it receives from the servo control unit. 
Figure 2.3 Open loop control Precision leadscrews, usually having 10 threads per inch (tpi), are used on NC machines. If the servomotor connected to the leadscrew receives 1000 electrical pulses, the machine slide will move 1 in. (25.4 mm). Therefore, one pulse will cause the machine slide to move 0.001 in. (0.0254 mm). The open loop system is fairly simple, however, since there is no means of checking whether the servomotor has performed its function correctly, that is, no means of feedback, this system is not generally used where an accuracy greater than 0.001 in. (0.025 mm) is required. 2.2.2 Closed loop control systemThe closed loop system is similar to the open loop system with the exception that a feedback unit is introduced into the electrical circuit. This feedback unit, often called a transducer, converts the movement by the servomotor to an electrical signal. The control compares the signal received from the transducer to the signal that was sent to the servomotor and will instruct the servomotor to make whatever adjustments are necessary until both the signal sent from the control unit and the one received from the servo unit are equal. 
Figure 2.4 Closed loop feedback control Closed loop NC systems are very accurate because the command signal is recorded, and there is an automatic compensation error. If the machine slide is forced out of position due to cutting forces, the feedback unit indicates this movement and the machine control unit (MCU) automatically makes the necessary adjustments to bring the machine slide back to position. ‘A transducer is a device that converts one form of energy into another form of energy in such a way that the output is a known function of the input’.
AnalogAnalog transducers, such as potentiometers and synchros, produce an electrical voltage which varies as the input shaft is turned or rotated. This voltage is in proportion to the rotation of the input shaft which can be converted into very accurate machine table positions. 
Figure 2.5 Analog transducer system DigitalDigital feedback units, attached to the leadscrew of a machine tool change the rotary motion of the machine screws to individual or discrete electrical pulses. This series of pulses can be counted to indicate exactly how much the leadscrew shaft has turned, which indicates the amount the machine table has moved. 
Figure 2.6 Digital transducer system Transducers used in closed loop system may be divided into two broad groups.
2.2.4 Rotary transducersResolversResolvers generate their signal directly by having windings at 90 0 on the rotor and two windings at 90 0 in the stator. An AC voltage is applied to a stator winding, and as a rotor winding passes it a voltage is generated on that winding. This voltage is treated in such a way as to provide an electrical pulse that can be used for positional information without the necessity of photo electric devices or brushes. Rotary transducers are usually fitted at the opposite end of the ballscrew to the motor as shown in the preceding sketches, or they are fitted to the end of the motor. This means they actually determine the motion of the ballscrew or the motor and not necessarily the machine slideway itself. TachometersTachometers are used to sense velocity and are small DC generators coupled to the leadscrew or motor shaft and supply a voltage that increases in direct proportion to its speed of rotation. This voltage represents the actual speed and can be used in comparison to the voltage supplied for the desired speed as a method of reducing the velocity error to a minimum. 2.2.5 Linear transducersThere are also linear devices, the most well known being the inductosyn. These transducers work by having two grating combs mounted closely over one another, one fixed, one moving with the slide. The gratings are pitched differently, giving an effect like an electronic version of a vernier scale. Signals are usually generated electrically, but can also be generated by photo electric devices. Linear transducers are similar in construction to the digital readout scales fitted to manual machines. These linear scales are fitted to the actual slideway so they determine the motion of the slideway. 2.3 Stored stroke limitLimit switches are used to control motion at the extreme position of each slide and may take the form of physical (hard wired) switches, or a stored (soft wired) position within the control. Depending on the machine, there may be two or three limit switches per slide with the outermost switch (usually hard wired) being the emergency limit position. Once the machine is in normal operation this switch is not activated. If, due to some error this switch is activated, an alarm condition will exist with power isolated from the servo motors and only resetable through the emergency over travel release sequence. This will prevent in many cases any mechanical damage to the axis drive assembly. Just inside the emergency limit position switch is the stroke end limit position (usually soft wired). If this position is exceeded during manual (JOG) operation an alarm will exist and can usually be cleared simply by moving the slide away in the opposite direction. On some machines (particularly lathes) there can be a third limit position -this is a soft wired variable limit and the position will be set as desired to suit the size of the workpiece. Turret indexing is executed at this position. The variable limit can be set to the same position as the stroke end limit position. Variable limits can also be set on some machines to create an area that is forbidden to the tool. This 'forbidden area' or 'barrier' is activated only when an appropriate code is programmed, or entered into the MCU from the keyboard. The function of this feature is to reduce the possibility of tool collisions. As there are variations between machines, it will be worthwhile conferring with your teacher to find out how the machine you are operating is configured.
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