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<< cont. from part 1
4.5 Other forms of flow control valves
The type of control valve described in the earlier parts of this section is basically the split-body globe valve body with a plug or plugs. This is the most commonly used form. There are, however, other forms. FIG. 26(a) shows a 3-way globe. Other valve types are the gate (FIG. 26(b)), the ball (FIG. 26(c)), the butterfly (FIG. 26(d)) and the louvre (FIG. 26(e)). All excise control by restricting the fluid flow.
Ball valves use a ball with a through-hole which is rotated; they have excellent shut-off capability. Butterfly valves rotate a vane to restrict the air flow and, as a consequence, suffer from the problem of requiring significant force to move from the full-open position and so can 'stick' in that position.
FIG. 26 (a) S-way globe, (b) gate, (c) ball, (d) butterfly, (e) louvre
FIG. 27 Shut down if air pressure fails
4.6 Fail-safe design
Fail-safe design means that the design of a plant has to take account of what will happen if the power or air supply fails so that a safe shut-down occurs. Thus, in the case of a fuel valve, the valve should close if failure occurs, while for a cooling water valve the failure should leave the valve open. FIG. 27 shows a direct acting valve which shuts down the fluid flow if the air supply to the diaphragm fails.
5. Motors
Electric motors are frequently used as the final control element in position or speed-control systems. The basic principle on which motors are based is that a force is exerted on a conductor in a magnetic field when a current passes through it. For a conductor of length L carrying a current / in a magnetic field of flux density B at right angles to the conductor, the force F equals BIL. There are many different types of motor. In the following, discussion is restricted to those types of motor that are commonly used in control systems, this including d.c. motors and the stepper motor. A stepper motor is a form of motor that is used to give a fixed and consistent angular movement by rotating an object through a specified number of revolutions or fraction of a revolution.
5.1 D.C. motors
In the d.c. motor, coils of wire are mounted in slots on a cylinder of magnetic material called the armature. The armature is mounted on bearings and is free to rotate. It is mounted in the magnetic field produced by field poles. This magnetic field might be produced by permanent magnets or an electromagnet with its magnetism produced by a current passing through the, so-termed, ^/eW coils. Whether permanent magnet or electromagnet, these generally form the outer casing of the motor and are termed the stator. FIG. 28 shows the basic elements of d.c. motor with the magnetic field of the stator being produced by a current through coils of wire. In practice there will be more than one armature coil and more than one set of stator poles. The ends of the armature coil are connected to adjacent segments of a segmented ring called the commutator which rotates with the armature. Brushes in fixed positions make contact with the rotating commutator contacts. They carry direct current to the armature coil. As the armature rotates, the commutator reverses the current in each coil as it moves between the field poles. This is necessary if the forces acting on the coil are to remain acting in the same direction and so continue the rotation.
FIG. 28 Basic elements of a d.c. motor
For a d.c. motor with the field provided by a permanent magnet, the speed of rotation can be changed by changing the size of the current to the armature coil, the direction of rotation of the motor being changed by reversing the current in the armature coil. FIG. 29 shows how, for a permanent magnet motor, the torque developed varies with the rotational speed for different applied voltages. The starting torque is proportional to the applied voltage and the developed torque decreases with increasing speed.
D.c. motors with field coils are classified as series, shunt, compound and separately excited according to how the field windings and armature windings are connected.
1. Series-wound motor
With the series-wound motor the armature and field coils are in series (FIG. 30(a)). Such a motor exerts the highest starting torque and has the greatest no-load speed. However, with light loads there is a danger that a series-wound motor might run at too high a speed. Reversing the polarity of the supply to the coils has no effect on the direction of rotation of the motor, since both the current in the armature and the field coils are reversed.
2. Shunt-wound motor
With the shunt-wound motor (FIG. 30(b)) the armature and field coils are in parallel. It provides the lowest starting torque, a much lower no-load speed and has good speed regulation. It gives almost constant speed regardless of load and thus shunt wound motors are very widely used. To reverse the direction of rotation, either the armature or field current can be reversed.
3. Compound motor
The compound motor (FIG. 30(c)) has two field windings, one in series with the armature and one in parallel. Compound-wound motors aim to get the best features of the series and shunt-wound motors, namely a high starting torque and good speed regulation.
4. Separately excited motor
The separately excited motor (FIG. 30(d)) has separate control of the armature and field currents. The direction of rotation of the motor can be obtained by reversing either the armature or the field current.
FIG. 30 (a) Series, (b) shunt, (c) compound, (d) separately wound
Application --- Section 4.4.4 shows a closed-loop arrangement that could t)e used to control the speed of a motor shaft.
FIG. 31 indicates the general form of the torque-speed characteristics of the above motors. The separately excited motor has a torque-speed characteristic similar to the shunt wound motor. The speed of such d.c. motors can be changed by either changing the armature current or the field current. Generally it is the armature current that is varied. The choice of d.c. motor will depend on what it is to be used for.
Thus, for example, with a robot manipulator the robot wrist might use a series-wound motor because the speed decreases as the load increases. A shunt-wound motor might be used if a constant speed was required, regardless of the load.
FIG. 29 Permanent magnet motor characteristic
The speed of a permanent magnet motor can be controlled by varying the current through the armature coil, with a field coil motor by either varying the armature current or the field current though generally it is the armature current that is varied. Thus speed control can be obtained by controlling the voltage applied to the armature. Rather than just try to directly vary the input voltage, a more convenient method is to use pulse width modulation (PWM). This basically involves taking a constant D.C. supply voltage and using an electronic circuit to chop it so that the average value is varied (FIG. 32).
FIG. 31 Torque-speed characteristics of D.C. motors
FIG. 32 PWM: (a) principle of PWM circuit, (b) varying the average
armature voltage by chopping the constant D.C. voltage
FIG. 33 Brushless permanent magnet D.C. motor
5.2 Brushless permanent magnet D.C. motor
A problem with the D.C. motors described in the previous section, is that they require a commutator and brushes in order to periodically reverse the current through each armature coil. Brushes have to be periodically changed and the commutator resurfaced because the brushes make sliding contacts with the commutator and suffer wear. Brushless D.C. motors do not have this problem.
A current-carrying conductor in a magnetic field experiences a force and with the conventional D.C. motor the magnet is fixed and the current-carrying conductors consequently made to move. However, as a consequence of Newton's third law of motion, the magnet will experience an opposite and equal force to that acting on the current-carrying conductors and so, with the brushless permanent magnet D.C. motor, the current carrying conductors are fixed and the magnet moves. With just one current carrying coil, the resulting force on the magnet would just cause it to deflect. In order to keep the magnet moving, a sequence of current carrying coils have to be used and each in turn switched on.
FIG. 33 shows the basic form of such a motor. The rotor is a ferrite or ceramic permanent magnet. The current to the stator coils AA, BB and CC is electronically switched by transistors in sequence round them, the switching being controlled by the position of the rotor so that there are always forces acting on the magnet causing it to rotate in the same direction. Hall sensors (a magnetic field input to the sensor gives a voltage output) are generally used to sense the position of the rotor and initiate the switching by the transistors, the sensors being positioned around the stator. FIG. 34 shows the transistor switching circuits that might be used with the motor shown in FIG. 33.
To switch the coils in sequence we need to supply signals to switch the transistors on in the right sequence. This is provided by the outputs from the three sensors operating through a decoder circuit to give the appropriate base currents. Thus when the rotor is in the vertical position, i.e. 0°, there is an output from sensor c but none from a and b and this is used to switch on transistors A+ and B-. When the rotor is in the 60° position there are signals from the sensors b and c and transistors A+ and C- are switched on. Table 2 shows the entire switching sequence.
The entire circuit for controlling such a motor is available as a single integrated circuit.
Table 2 Switching sequence
5.3 Stepper motor
The stepper or stepping motor produces rotation through equal angles, the so-called steps, for each digital pulse supplied to its input. For example, if with such a motor 1 input pulse produces a rotation of 1.8 degrees then 20 input pulses will produce a rotation through 36.0°, 200 input pulses a rotation through one complete revolution of 360°. It can thus be used for accurate angular positioning. By using the motor to drive a continuous belt, the angular rotation of the motor is transformed into linear motion of the belt and so accurate linear positioning can be achieved. Such a motor is used with computer printers, x-y plotters, robots, machine tools and a wide variety of instruments for accurate positioning.
There are two basic forms of stepper motor, the permanent magnet type with a permanent magnet rotor and the variable reluctance type with a soft steel rotor. FIG. 35 shows the basic elements of the permanent magnet type with two pairs of stator poles.
FIG. 34 Transistor switching
FIG. 35 The basic principles of the permanent magnet stepper motor (2-phase) with 90° steps.
Each pole is activated by a current being passed through the appropriate field winding, the coils being such that opposite poles are produced on opposite coils. The current is supplied from a D.C. source to the windings through switches. With the currents switched through the coils such that the poles are as shown in FIG. 35, the rotor will move to line up with the next pair of poles and stop there. This would be, for FIG. 35, an angle of 45°. If the current is then switched so that the polarities are reversed, the rotor will move a step to line up with the next pair of poles, at angle 135° and stop there. The polarities associated with each step are:
There are thus, in this case, four possible rotor positions: 45°, 135°, 225° and 315°.
FIG. 36 Basic principles of a 3-phase variable reluctance stepper motor
Application ----- Section 4.4.2 shows the application of a stepper motor to the control of the position of a tool.
FIG. 36 shows the basic form of the variable reluctance type of stepper motor. With this form the rotor is made of soft steel and is not a permanent magnet. The rotor has a number of teeth, the number being less than the number of poles on the stator. When an opposite pair of windings on stator poles has current switched to them, a magnetic field is produced with lines of force which pass from the stator poles through the nearest set of teeth on the rotor. Since lines of force can be considered to be rather like elastic thread and always trying to shorten themselves, the rotor will move until the rotor teeth and stator poles line up. This is termed the position of minimum reluctance. Thus by switching the current to successive pairs of stator poles, the rotor can be made to rotate in steps. With the number of poles and rotor teeth shown in FIG. 36, the angle between each successive step will be 30°. The angle can be made smaller by increasing the number of teeth on the rotor.
There is another version of the stepper motor and that is a hybrid stepper. This combines features of both the permanent magnet and variable reluctance motors. They have a permanent magnet rotor encased in iron caps which are cut to have teeth. The rotor sets itself in the minimum reluctance position in response to a pair of stator coils being energized.
The following are some of the terms commonly used in specifying stepper motors:
1. Phase
This is the number of independent windings on the stator, e.g. a four-phase motor. The current required per phase and its resistance and inductance will be specified so that the controller switching output is specified. FIG. 35 is an example of a two-phase motor, such motors tending to be used in light-duty applications. FIG. 36 is an example of a three-phase motor. Four-phase motors tend to be used for higher power applications.
2. Step angle
This is the angle through which the rotor rotates for one switching change for the stator coils.
3. Holding torque
This is the maximum torque that can be applied to a powered motor without moving it from its rest position and causing spindle rotation.
4. Pull-in torque
This is the maximum torque against which a motor will start, for a given pulse rate, and reach synchronism without losing a step.
5. Pull-out torque
This is the maximum torque that can be applied to a motor, running at a given stepping rate, without losing synchronism.
6. Pull-in rate
This is the maximum switching rate or speed at which a loaded motor can start without losing a step.
7. Pull-out rate
This is the switching rate or speed at which a loaded motor will remain in synchronism as the switching rate is reduced.
8. Slew range
This is the range of switching rates between pull-in and pull-out within which the motor runs in synchronism but cannot start up or reverse.
FIG. 37 shows the general characteristics of a stepper motor.
FIG. 37 Stepper motor characteristics
To drive a stepper motor, so that it proceeds step-by-step to provide rotation, requires each pair of stator coils to be switched on and off in the required sequence when the input is a sequence of pulses (FIG. 38). Driver circuits are available to give the correct sequencing and FIG. 39 shows an example, the SAA 1027 for a four-phase unipolar stepper.
Motors are termed unipolar if they are wired so that the current can only flow in one direction through any particular motor terminal, bipolar if the current can flow in either direction through any particular motor terminal. The stepper motor will rotate through one step each time the trigger input goes from low to high. The motor runs clockwise when the rotation input is low and anticlockwise when high. When the set pin is made low the output resets. In a control system, these input pulses might be supplied by a microprocessor.
FIG. 39 Driver circuit SAA1027 for a 12V 4-phase stepper motor
Some applications require very small step angles. Though the step angle can be made small by increasing the number of rotor teeth and/or the number of phases, generally more than four phases and 50 to 100 teeth are not used. Instead a technique known as mini-stepping is used with each step being divided into a number of equal size sub-steps by using different currents to the coils so that the rotor moves to intermediate positions between normal step positions. For example, this method might be used so that a step of 1.8° is subdivided into 10 equal steps.
FIG. 38 Input and outputs for a drive system for a stepper motor
Application ------ A manufacturer's data for a stepper motor includes: 12V 4-phase, unipolar Step angle 7.5° Suitable driver SAA1027
FIG. 40 Example
Example:
A stepper motor is to be used to drive, through a belt and pulley system (FIG. 40), the carriage of a printer. The belt has to move a mass of 500 g which has to be brought up to a velocity of 0.2 m/s in a time of 0.1 s. Friction in the system means that movement of the carriage requires a constant force of 2 N. The pulleys have an effective diameter of 40 mm. Determine the required pull-in torque.
The force F required to accelerate the mass is: F=ma= 0.500 x (0.2/0.1) = 1.0 N. The total force that has to be overcome is the sum of the above force and that due to friction. Thus the total force that has to be overcome is 1.0+ 2 = 3 N.
This force acts at a radius of 0.020 m and so the torque that has to be overcome to start, i.e. the pull-in torque, is torque = force x radius = 3 x 0.020 = 0.06 N m
6. Case studies
The following are case studies designed to illustrate the use of correction elements discussed in this section.
6.1 A liquid level process control system
FIG. 41 one method of how a flow control valve can be used to control the level of a liquid in a container. Because there may be surface turbulence t»i a result of liquid entering the container or stirring of the liquid or perhaps boiling, such high frequency 'noise' in the system is often filtered out by the use of a stilling well, as shown in FIG. 41.
However, it must be recognized that the stilling well constitutes a U-tube in which low frequency oscillations of the liquid level can occur.
FIG. 41 Liquid level control
6.2 A robot control system
FIG. 42 shows how directional control valves can be used for a control system of a robot. When there is an input to solenoid A of valve 1, the piston moves to the right and causes the gripper to close. If solenoid B is energized, with A de-energized, the piston moves to the left and the gripper opens. When both solenoids are de-energized, no air passes to either side of the piston in the cylinder and the piston keeps its position without change. Likewise, inputs to the solenoids of valve 2 are used to extend or retract the arm. Inputs to the solenoids of valve 3 are used to move the arm up or down. Inputs to the solenoids of valve 4 are used to rotate the base in either a clockwise or anticlockwise direction.
FIG. 42 Robot controls
6.3 Milling machine control system
FIG. 43 shows how a stepper motor can be used to control the movement of the workpiece in an automatic milling machine. The stepping motor rotates by controlled steps and gives, via a lead screw and gears, controlled displacements of the worktable in the xx direction. A similar arrangement is used for displacement in the yy direction. The system is open-loop control with no feedback of the work table position.
The system relies on the accuracy with which the stepper motor can set the position of the work table.
FIG. 43 Automatic milling machine
Problems
Questions 1 to 19 have four answer options: A, B, C and D. Choose the correct answer from the answer options.
1. Decide whether each of these statements is True (T) or False (F). For a hydraulic cylinder: (i) The force that can be exerted by the piston is determined solely by the product of the pressure exerted on it and its cross-sectional area.
(ii) The speed with which the piston moves is determined solely by the product of the rate at which fluid enters the cylinder and the cross-sectional area of the piston.
A (i)T (ii)T B (i)T (ii)F C (i)F (ii)T D (i)F (ii)F
2. A pneumatic cylinder has a piston of cross-sectional area 0.02 m^3
The force exerted by the piston when the working pressure applied to the cylinder is 2 MPa will be:
A 100 MN
B 40MN
C 40 k-Ohm
D 20 k-Ohm
A hydraulic cylinder with a piston having a cross-sectional area of 0.01 m^2 is required to give a workpiece an average velocity of 20 mm/s. The rate at which hydraulic fluid should enter the cylinder is:
A 4 X 10^-6 m^3
B 2 X 10^-4 m^3
C 0.2 m^3/ s
D 2 m^3/s
FIG. 44 Problems 4 to 6
Questions 4 to 6 refer to FIG. 44 which shows a valve symbol.
4. Decide whether each of these statements is True (T) or False (F).
The valve has:
(i) 2 ports (ii) 4 positions
A (i)T (ii)T
B (i)T (ii)F
C (i)F(ii)T
D (i) F (ii) F
5. Decide whether each of these statements is True (T) or False (F). When the push button is pressed: (i) Hydraulic fluid from the supply is transmitted through port B. (ii) The hydraulic fluid in the line to port A is returned to the sump.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i) F (ii) F
6. Decide whether each of these statements is True (T) or False (F). When the press button is released: (i) Hydraulic fluid from the supply is transmitted through port A. (ii) The hydraulic fluid in the line to port B is returned to the sump.
A (i)T (ii)T B (i)T (ii)F C (i)F (ii)T D (i) F (ii) F Questions 7 to 10 refer to FIG. 45 which shows a pneumatic circuit involving two valves and a single acting cylinder.
FIG. 45 Problems 7 to 10
7. Decide whether each of these statements is True (T) or False (F). When push button 1 is pressed:
(i) The load is lifted, (ii) Port A is closed.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i) F (ii) F
8. Decide whether each of these statements is True (T) or False (F).
When push button 1, after being pressed, is released: (i) The load descends, (ii) Port A is closed.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i)F (ii)F
9. Decide whether each of these statements is True (T) or False (F). When push button 2 is pressed: (i) The load is lifted.
(ii) Port B is vented to the atmosphere.
A (i)T (ii)T B (i)T (ii)F C (i)F (ii)T D (i) F (ii) F
10. Decide whether each of these statements is True (T) or False (F). When push button: (i) 1 is pressed the load is lifted. (ii) 2 is pressed the load descends.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i) F (ii) F
11. Decide whether each of these statements is True (T) or False (F). FIG. 46 shows a two-way spool valve. For this valve, movement of the shuttle from left to right: (i) Closes port A. (ii) Connects port P to port B.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i)F (ii)F
12. A flow control valve has a diaphragm actuator. The air pressure signals from the controller to give 0 to 100% correction vary from 0.02 MPa to 0.1 MPa above the atmospheric pressure. The diaphragm area needed to 100% open the control valve if a force of 400 N has to be applied to the stem to fully open the valve is:
A 0.02 m^3
B 0.016 m^3
C 0.004 m^3
D 0.005 m^3
FIG. 46 Problem 11
13. Decide whether each of these statements is True (T) or False (F). A quick-opening flow control valve has a plug shaped so that: (i) A small change in the flow rate occurs for a large movement of the valve stem.
(ii) The change in the flow rate is proportional to the change in the displacement of the valve stem.
A (i)T (ii)T
B (i)T(ii)F
C (i)F (ii)T
D (i)F (ii)F
14. A flow control valve with a linear plug gives a minimum flow rate of 0 and a maximum flow rate of 10 m^3/s. It has a stem displacement at full travel of 20mm and so the flow rate when the stem displacement is 5 mm is:
A. 0 m^3/s
B. 2.5 m^3/s
C. 5.0 m^3/s
D. 7.5 m^3/s
15. A flow control valve with an equal percentage plug gives a flow rate of 0.1 m^3/s when the stem displacement is 0 and 1.0 m^3/s when it is at full travel. The stem displacement at full travel is 30 mm. The flow rate with a stem displacement of 15 mm is:
A 0.32 m^3/s
B 0.45 m^3/s
C 1.41 m^3/s
D 3.16 m^3/s
16. Decide whether each of these statements is True (T) or False (F). A flow control valve has a minimum flow rate which is 1.0% of the maximum controllable flow. Such a valve is said to have a: (i) Rangeability of 100. (ii) Turndown of 100. A (i)T (ii)T B (i)T (ii)F C (i)F (ii)T D (i) F (ii) F
17. Decide whether each of these statements is True (T) or False (F). A stepper motor is specified as having a step angle of 7.5°. This means that: (i) The shaft takes 1 s to rotate through 7.5^ (ii) Each pulse input to the motor rotates the motor shaft by 7.5''.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i)F (ii)F
18. Decide whether each of these statements is True (T) or False (F). For a series wound d.c. motor: (i) The direction of rotation can be reversed by reversing the direction of the supplied current. (ii) The speed of rotation of the motor can be controlled by changing the supplied current.
A (i)T(ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i) F (ii) F
19. Decide whether each of these statements is True (T) or False (F). With a shunt wound d.c. motor: (i) The direction of rotation can be changed by reversing the direction of the armature current. (ii) The direction of rotation can be changed by reversing the direction of the current to the field coils.
A (i)T (ii)T
B (i)T (ii)F
C (i)F (ii)T
D (i)F (ii)F
20. A force of 400 N is required to fully open a pneumatic flow control valve having a diaphragm actuator. What diaphragm area is required if the gauge pressure from the controller is 100 kPa?
21. An equal percentage flow control valve has a rangeability of 25. If the maximum flow rate is 50 m^3/s, what will be the flow rate when the valve is one-third open?
22. A stepper motor has a step angle of 7.5 degree. What digital input rate is required to produce a rotation of 10.5 rev/s?
23. A control valve is to be selected to control the rate of flow of water into a tank requiring a maximum flow of 0.012 m^3/s. The permissible pressure drop across the valve at maximum flow is 200 kPa. What valve size is required ? Use Table 1. The density of water is 1000 kg/ml
24. A control valve is to be selected to control the flow of steam to a process, the maximum flow rate required being 0.125 kg/s. The permissible pressure drop across the valve at maximum flow is 40 kPa. What valve size is required? Use Table 1. The specific volume of the steam is 0.6 m^3/s.
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