Chapter 2: Building Blocks of Automation PDF

CHAPTER 2 BUILDING BLOCKS OF AUTOMATION  CHAPTER 3 BUILDING BLOCKS OF AUTOMATION The objective of this chapter is to identify basic devices for implementing an automation project, whether or not an industrial robot will be used in the final impleme...

CHAPTER 2 BUILDING BLOCKS OF AUTOMATION  CHAPTER 3 BUILDING BLOCKS OF AUTOMATION The objective of this chapter is to identify basic devices for implementing an automation project, whether or not an industrial robot will be used in the final implementation. Successful productivity improvement projects usually isolate tiny bottlenecks or inefficiencies at the work station level, where people-not machine- are still playing the lead role. One by one, better methods are being found to sense, move, position, orient, fabricate and assemble products using a wide variety of ingenious basic components the building blocks of automated systems. Until a manufacturing work station has been analyzed thoroughly and fitted out with the basic components of automation, it usually is not ready for more foreign hardware such as industrial robots. Indeed, the industrial robots themselves are constructed of some of these same basic components of automation. The purpose of this chapter is to examine and classify the basic components of automation at the work station level. Its objective is to recognize the usefulness of these items and to see how they can be integrated into systems for enhancing manufacturing productivity. To make sense out of the diversity of automation components, some sort of crude classification is needed. The classification is rough because some of the most useful components find their way into several of the categories, depending upon how they are used. We can, however, talk about components as primarily belonging to one of the following four classes: 1. Sensors, 2. Analyzers, 3. Actuators, 4.Drives. The approximate relationship of these four broad categories is shown in Fig. 1. It should be pointed out that the operator in Fig. 1 is a human, not a robot. The industrial robot is a part of the automated system (upper half of the figure).  Fig. 1. The automation components. The industrial robot is actually an integrated system made up of all four the basic automation component categories: sensors, analyzers, actuators and drives. 1 Sensors Sensors are the first link between the typical automated system and the conventional process, as was indicated in Fig. 1. Sensors transfer information from the manufacturing process equipment, the piece part being manufactured, and from the human operator, if any. It may seem strange that the automated system senses the human operator, but this is without doubt the most important link between the automated system and the real world. 1.1 Manual Switches The most familiar sensor of all is the manual switch. Most people do not think of an electric lamp switch as a sensor, but the switch is the link between the lamp and the person who desire the lamp to be turned on or off. In the same fashion, an automation system is linked to the operator, who may desire to turn the system on or off or make adjustments to the automated cycle. Virtually all manual switches are electric, and we shall ignore any exceptions to this statement. Fig. 2. The different electric switches. For a switch to be on means that the circuit it controls is closed or "made". When a switch is off, its circuit is open or "broken". Most switches have two stable states: on and off. However, many switches have only a single stable state. Such switches have a spring action that returns them to the normal state whenever they are released from an outside force. That normal state can be in either the open position or the closed position, which leads to the terms normally open (ON) and normally closed (NC) used to describe switches. The ordinary wall switch is an example of a toggle switch, and in its simplest form it merely opens or closes a single circuit. Such switches are designated single-pole, single-throw (SPST) and are diagrammed as in Fig. 2a. The term single-pole, single throw implies that multiple poles and multiple throws are possible, and in fact these types must be examined in order to understand the simpler SPST designation. Another switch is shown in Fig. 2b, it is toggled to the left makes circuit A, toggled to the right makes circuit B, and the center position holds both circuits open. Such a switch is designated single pole, double throw (SPDT). The switch is single pole because of the common lead on one side of the switch, but it is double throw because it can complete either of two circuits on this same common pole. It is possible also to have two leads on both sides of the switch, which enables the switch to make two different circuits that do not have a common pole. In effect, such a switch is making two different circuits with a single mechanical throw. Such a switch is called a double pole, single throw (DPST) and is illustrated in Fig. 2c. In still another configuration, each contact on one side of the switch can be connected to either of two contacts each on the other side of the switch. Such a switch is a double pole, double throw (DPDT) and is displayed in Fig. 2d. It is possible to add as many poles as desired, but to go beyond two throws the use of a toggle switch becomes impractical. For multiple throws, a rotary switch becomes appropriate. Figure 3 shows a rotary switch that could be classifieds a single pole, five throws switch, although it is usually designated simply as six- position rotary switch. Fig. 3. Rotary switch. The safety of robots and other automated systems demands that most operator control switches have only one stable state: off. Thus, a positive operator action must be maintained to keep the switch on. The advantage of single stable state switches is that control can be exercised whenever and wherever it is necessary to change the operational state of the system. In the automation industry, these momentary off switches located at various points about the equipment are called emergency power off or simply EPOs. The most popular physical configuration of a momentary switch is the familiar push button. Push button can be either "making" or "breaking" that is, normally open or normally closed respectively. Figure 4 displays the standard momentary push button diagram in both configurations. Fig. 4 Standard momentary push button switches. 1.2 Limit Switches Like manual switches, limit switches are actuated mechanically, but limit switches are automatic inputs from the manufacturing process, the material, or the automated system itself, without intervention by the operator. There are literally thousands of styles and models of limit switches, even more variables than their cousins, the manual switches. The reason for this is that limit switches must be designed to be exactly correct in size, lever travel, force of actuation and ruggedness for the specific automation application desired out of the multiple automation applications that might be feasible. By comparison, manual switches are designed for human operators who have relatively similar physical characteristics. In addition, many limit switches are used in situations for which they must be "industrially hardened" or built to withstand severe industrial environments to which a human operator might not be exposed. Limit switches are actuated by levers, toggle, push button, plunger, rollers, cat whiskers and just about anything else the inventor can devise to make an automation application feasible. Fig. 5 illustrates a wide variety of limit switches used in automated systems. Fig. 5. Different styles of limit switches. Robot systems employ limit switches both in the construction of the robot itself and in the peripheral equipment. Limit switches can be used to limit the travel of a robot arm on any of its axes of motion. When the limit is reached, a circuit is opened (or closed) that removes power from that axis of motion either directly or via the robot controller. As an example, in the peripheral equipment a limit switch can be used on a gate in the perimeter barrier to act as an interlock (an automatic EPO) whenever someone enters the gate. 1.3 Proximity Switches Some switches do not require physical contact or light radiation to "feel" or sense an object. Such switches are called proximity switches because they can sense the presence of a nearby object without touching it. Figure 6 illustrates two types of proximity sensors, the first of which acts only upon presentation of a ferrous metal object within its sensing range (Fig. 6a). The other type is capable of sensing both ferrous and nonferrous metal objects (Fig. 6b). At first it might seem that the robotics and automation engineer would always prefer the switch that functions with all metals. Fig. 6. Proximity switches. There are physical bases for proximity switches that can respond to any object metal or nonmetal. One type uses an electromagnetic (radio frequency) antenna specially designed and placed to fit the application. The antenna receives a signal transmitted by another strategically placed antenna, but the reception of the signal is disturbed by the intrusion of any object into the field. This disturbance is detected by the antenna that trips a switch when the disturbance reaches a specified level. The sensitivity of the antenna is related to the electrical properties of the material of the object being detected. The size of the object to be detected also plays a role. The system can be turned to be somewhat selective to specific items. Another type of proximity switch that works for nonmetallic objects is the sonar type. Sonar systems transmit and receive reflections of pressure waves to detect object presence. These pressure waves are called sound waves when their frequencies are within the audible range. Most sonar systems use ultrasonic radiation which has frequencies higher than audible waves. 1.4 Photoelectric Sensors In wider use than proximity switches are sensors that are sensitive to light radiation: photoelectric sensors. Two basic approaches for employing photoelectric s are in use. The first approach merely uses a photocell to detect the presence of light radiating naturally from some object in the process. A good example is the use of photocells to turn on lighting systems automatically at dusk and to turn them back off again at dawn. Increasing emphasis upon energy costs has generated additional interest in this kind of automated system. The second approach to photoelectric employs a beam of light emitted by an artificial light source. The principal purpose of this approach is to detect the presence or absence of objects in the path of the beam. The beam emitter can be a separate unit or can be incorporated into the sensor. The combination variety requires some type of natural or artificial reflector to direct the light beam back to the emitter/sensor. Fig. 7. Three types of reflective surfaces. Reflective surfaces for photoelectric systems are of three types: diffuse, specular reflective and retroreflective, as shown in Fig. 7. The diffuse reflective surfaces is the lowest in cost and describes most reflective surfaces. Even an ordinary white object acts as a diffuse reflective surface in that it reflects light but not images. Diffuse reflectors scatter so much light that only a small fraction makes its way back to the photocell sensor. Therefore, the savings in the diffuse surface may be lost in the necessary provision for a more sensitive and perhaps more sophisticated sensor. Photocell systems that use diffuse reflective surfaces are also more susceptible to stray signals. Specular reflective surfaces are most often associated with the word reflective and include mirrors and very shiny surfaces. Specular reflective surfaces obey the physical law that the angle of incidence equals the angle of reflection. It is obvious that the source and sensor must be more closely aimed for specular reflective surfaces than for diffuse surfaces. For systems in which the emitter and sensor are mounted in the same unit, the plane of the specular reflective surface must be perpendicular to the direction of the incident beam or the reflected beam will be lost. Once again, this can be either a disadvantage or an advantage, depending upon the objective of the inventor or automation engineer. Retroreflective surfaces are the most complex and expensive of the three types. Retroreflectors are capable of reflecting back to the source a large percentage of the light beam regardless of the angle of incidence. Basically, the retroreflective surface violates the physical principle that angle of incidence equals angle of reflection, except when the plane of the surface is perpendicular to the incident beam. A common example of a retroreflective surface is the red reflector on the rear of a bicycle. Although the reflector is not illuminated internally, it still glows brilliantly from the retroreflected light of the head lamp of a vehicle behind it. The light is concentrated in a beam back to the source regardless of its direction within certain operational limits. Certain highway signs, highway paint stripes, reflective tape strips, and movie screens are coated with a surface of tiny beads that cause a portion of incident light to be bounced back to the source. At least partially, these surfaces can permit photoelectrics to take advantage of their retroreflective properties. Obviously, alignment is not as important as for specular reflective surfaces. Sensitivity of the photocell can be lower than for diffuse reflcetive surfaces, so retroreflective surfaces somewhat combine the advantages of specular and diffuse reflective surfaces, but at a price-the retroreflector is the most expensive of the three types. As useful as photo cells are, the automation engineer should beware of conditions that can foul, confuse, or even ruin the photoelectric system. Stray ambient light, even from worker-carried flashlights, can be a problem in confusing the system. Stray light from welders arcs can be even more destructive, perhaps permanently damaging the sensor. High temperatures from manufacturing processes may also damage equipment. If the manufacturing environment presents a dirt or vapor problem, dust or condensation may accumulate on lenses or mirrors. If manual cleaning of these surfaces is necessary, some of the benefits of automation may be defeated. Vibrations from adjacent machines must be anticipated and dealt with in automation planning. Vibrations may cause source sensor misalignment problems and may lower lamp reliability, especially for incandescent lamps. There are ways to overcome most problems with photoelectrics, especially of the automation engineer plans for these problems from the beginning. For example, ambient light can be shielded and mirrors can be used to redirect the beam. Dust and condensate problems can be alleviated by automatic air jets. Another trick is to use overdesign when adverse environmental conditions are expected. For example, if the beam system is rated at 8 to 10 feet, use it at distance of 2 to 3 feet. To solve problems of vibration caused misalignment, a retroreflective system may be the answer. Other problems may be alleviated by use of infrared beams or fiber optics, as will be discussed in the sections that follow. 1.5 Infrared Sensors Sometimes it is useful to detect electromagnetic radiation outside the visible range. Infrared sensors respond to radiation in the range of wavelength just beyond the visible spectrum at the red end. Hot objects emit infrared radiation, and thus infrared sensors are useful for location heat sources in a process. Such applications in which "natural" infrared radiation is sensed are useful for monitoring systems to detect malfunctions. Infrared sensors are also very useful when used with artificial beams to detect the presence or absence of objects, even more so than are photocells systems. Since infrared radiation is invisible, there are some advantages of using infrared beams and receivers instead of ordinary photocells. Also, infrared sensors are virtually unaffected by stray ambient light-with obvious advantages. A strategy that is gaining popularity is the use of a modulated infrared beam, in which the source is pulsed to provide much greater intensity and the sensor is modulated to receive at the same frequency. The modulated beam permits a much greater range for otherwise weak beam sources such as LEDs (light-emitting diodes) are solid state and have great advantages over incandescents in terms of power requirements and reliability. 1.6 Fiber Optics A convenient supplement to photoelectric or infrared sensor systems are fiber optic light tubes, which are flexible pipes of glass or plastic that can be used to bend light beams around corners. When bundles of fibers are used together, whole images can be transmitted. However, the typical automation application is to use one fiber to transmit a light beam that is sensed by the system as either present or absent. An advantage of the fiber optics is their surprising efficiency. Fiber optics are so efficient that it becomes worthwhile for the telephone industry to convert communications circuits from electrical signals to modulated light signals for transmission via fiber optics and subsequent reconversion at the receiving end. 1.7 Lasers Before leaving the subject of photoelectric sensors, laser light should be considered. Lasers are concentrated, amplified beams of collimated light. They are capable of delivering over a distance a large amount of energy into a tiny spot and thus have obvious industrial applications. In automated systems, the laser is useful in providing very long, precise light beams. The precision of these beams makes them excellent for detecting tiny objects that are capable of breaking the beam at large and varying distances. The presence or absence of a continuous beam then can be used as a logic input to an automated control system. Such precision also makes the laser a good tool for dimensional measurement. 2 Analyzers Once information is sensed by an automated system, it must be registered and analyzed for content, and then a decision must be made by the system as to what action should be taken. This function can be quite complex, and the system components that perform it are generally too complicated to discuss in detail in this chapter. But some of the components deserve mention here to enable the reader to understand the components of NC machines, robots, programmable controllers, and other manufacturing automation devices discussed in the remaining chapters. 2.1 Computers Digital computers are the primary means of analyzing automation system inputs. Computers are extremely versatile in that the ways they can be programmed to manipulate data are limitless. The continuing miniaturization of computer circuits along with decreasing costs made possible by technological breakthroughs has created a continuing increase in the number of applications of manufacturing automation. No other development has had more impact upon the growth of industrial robots. 2.2 Counters It is frequently useful for an automated system to determine how many of various items are present or path through an automated system. This function can be handled either internally by a computer or programmable controller or externally by a separate device called a counter. The counter can be mechanical, but most automatic systems employ solid-state electronic counters. If the counter is a separate unit, it will usually have a display to report current status of the count in progress. The quantity counted is usually a series of voltage pulses that have been generated by a sensor detecting some physical quantity of importance to be automated system, for example, glass bottles coming down a conveyor line. The curved glass would be somewhat specular, and at a precise position the angle would be exactly right to reflect a pulse of light from a positioned source upon a photoelectric sensor, as shown in Fig. 8. The sensor would convert the light pulse to a voltage pulse, which would be transmitted to the counter.  Fig. 8. Automatic counting system using photoelectric. Note in Fig. 8 that the spacing of the voltage pulses is not uniform. It is also possible for the peaks themselves to be of varying width (time) and amplitude (voltage). This is entirely satisfactory-within limits, of course- because most industrial counters are some tolerance in the voltage required to generate the count. In Fig. 8, the counter shown has two display registers. The top is an electronic display (LED type) and represents the current count in progress. The bottom register is mechanical and represents a present number that acts as a target to trigger an output signal when the current count register reaches the target value. This feature enables the counter to cause something to happen in the manufacturing process. A good example would be the automatic batching of manufactured products into preset lot sizes. Another useful feature of many counters is bidirectionality, which enables them to accept two kinds of inputs: count up and down. This can be useful in automatic industrial quality control applications and in material handling. 2.3 Optical Encoders The capability of rapidly scanning a series of bars makes possible additional automation opportunities when light and dark bars are placed in concentric rings on a disk. Figure 9 shows a portion of such a disk that can be rigidly attached to a shaft and housed in an assembly consisting of optical sensors for each ring. The assembly is called an optical encoder (see Fig. 10) and is useful for automatically detecting shaft rotation. The shaft rotation information can be fed back into a computer or control mechanism for controlling velocity or position of the shaft. Such a device has application for robots and numerical control machines. Fig. 9. Sample portion of optical encoder disk. Fig. 10. Absolute optical encoder. Optical encoders can be either incremental or absolute. The incremental types transmit a series of voltage pulses proportional to the angle of rotation of the shaft. The control computer must know the previous position of the shaft in order to calculate the new position. Absolute encoders transmit a pattern of voltages that describes the position of the shaft at any given time. The innermost ring switches from dark to light every 180 deg., the next ring every 90 deg., the next 45 deg. and so on, dependent upon the number of rings on the disk. The resulting bit pattern output by the encoder reveals the exact angular position of the shaft, as can be seen in the following example. Example 1 Absolute Optical Encoder: An absolute optical encoder disk has eight rings and eight LED sensors, and in turn provides 8-bit outputs. Suppose the output pattern is 10010110. What is the angular position of the shaft? We can state the following equation for the encoder computation as n A   mi Ai (1) i 1 where i = ring number mi = 0 if ring i is white (clear) = 1 if ring i is black (opaque) Ai = angular value for ring i n = total number of rings 12-bit ring optical encoders are available that have resolutions of over 4000 counts per turn. 3 Actuators Once a real-world condition is sensed and analyzed, something may need to be done about it. It is at this point that the automation of many systems ceases because it is believed that a human operator must intervene and apply judgment for taking some kind of physical actions. Such systems may be called "process monitoring" if they merely sense and display or record data or "on-line assist" if they also analyze data and give advice or prompts to the operator suggesting specific actions to be taken. However, more and more automated systems are closing the loop by taking physical actions automatically without operator intervention. Actuation may be a direct physical action upon the process, such as a sweep bar that sweeps items off a conveyor belt at the command of a computer or other analyzer. In other cases, an actuator is simply a physical making of an electrical circuit, which in turn has a direct effect upon the process. An example would be an actuator (relay) that turns on power to a electric furnace heating circuit. 3.1 Cylinders When a linear movement is required in an automation application, a cylinder usually is chosen to accomplish it. The most popular are the pneumatic types because of the convenience of piping compressed air throughout a manufacturing plant. Shop air is generally regulated to the range 80-100 psi, which is adequate for most grippers, movers, positioners and tool-stroking devices. Figure 11 shows the use of pneumatic cylinders in an automatic work station. The control of air cylinders is accomplished by valves that may be driven by electrical impulses or by air logic devices. when the manufacturing process requires forces to be applied automatically in excess of 200 pounds, the more powerful hydraulic cylinder is usually selected over the pneumatic cylinder. Hydraulic pressures in excess of 2000 psi are readily available; compare these pressures with the 80-100 psi commonly used in pneumatic systems. Given the mechanical advantage of a large enough cylinder, pneumatics can deliver as large a force as the hydraulics, but space and convenience tend to favor hydraulics for the large forces. The most powerful industrial robots are driven by hydraulic actuators. Fig. Use of pneumatic cylinders in an automatic workstation. Besides being powerful, hydraulic cylinders have the advantage of being well controlled throughout the stroke. In addition, they are quiet, although the pump and reservoir can be quite noisy. Disadvantages are high initial cost, maintenance and problems from leaking cylinders. A caution to observe in the design of either pneumatic or hydraulic actuators is that both pressure and volume requirements must be met. A system may have sufficient pressure to actuate cylinders or other actuators, but may not be able to maintain that pressure during high-speed operations. This mistake has been observed especially in partially automated factories in which pneumatic systems are used to power mechanized screwdrivers, staplers and handling equipment. System design that anticipates the demands that will be placed upon the pneumatic or hydraulic equipment and actuators during peak periods will avoid this drawback. 3.2 Solenoids When a small, light, quick linear motion is desired in an automated system, an electrical solenoid is a logical selection. In basic physics, we learned that the principle of the solenoid operation is the creation of a magnetic field set up by passing an electrical current through a coil. Thus, the core of the solenoid can be selectively drawn into the coil in response to an electrical current. In the absence of the coil current, the core can be automatically returned by spring action. The stroke motion of a solenoid is not very controlled in comparison, for example, with a hydraulic cylinder-but many automation applications require only a short quick, discrete action, not a smooth, controlled stroke. 3.2 Relays The most popular solenoid of all is one that is used to switch an electrical circuit- that is, the common relay. Switching-type circuit usually operate at lower voltages and especially at lower amperage than power circuits. The output of the switching logic network then can be used to trip one or more relays to close or open a power circuit. Figure 12 shows the use of relays to close electrical circuits under the automatic control of process sensors. Compare the logic of the two circuits shown. In Fig. 12a relays from both sensors must be energized to make the power circuit. In the arrangement in Fig. 12b the action of either relay A or B is sufficient to make the circuit. The myriad ways in which relays can be combined in switching networks from the basis for the classical approach to automating manufacturing systems. A relay can be described as either "latching" or "nonlatching" a comparison of which is shown in Fig. 13. A latching relay needs only an electrical impulse to pull and hold the power circuit closed. Another impulse is needed on a different switching circuit to release the latch. Fig. 12 Using relays to automatically switch power circuits. Fig. 13. Comparison of ordinary versus latching relays. Nonlatching relays hold only while the switching relay is energized and thus require a continuous electrical signal. The discontinuation of that signal permits the relay to release the switch immediately. Thus far, we have described relays that make circuits when energized, but relays can also break circuits when they receive an electrical signal. When the energization of a relay coil makes a circuit, the relay is designated "normal open" (the relays of Fig. 12 are normally open). Conversely the relay that breaks a circuit when energized is designated "normally closed". It follows, then, that the normal state of an electric relay is the deenergized state. The typical relay and solenoids in general operate on low-voltage direct current. But the convenience and availability of 110 volt alternating current (AC) have given rise to the ac relay and ac solenoid. Another advantage to the higher voltage ac solenoids is their relative immunity to induced voltages from power conductors in the manufacturing environment. A sensitive low-voltage solenoid can be easily tripped inadvertently by a stray voltage induced by a powerful current in an electrical cable that passes close to a relay circuit, especially if the conductor paths are adjacent, parallel and long. As the amperage level of the power circuit increases, the nomenclature for relays changes. Small motors, handling devices and automated tool actuators usually are served by power circuits of less than 10 A, and the relays that trip them are indeed called relays. However, as the amperage ranges upward between 10 and 30 A, the nomenclature is power relay. At higher amperage, the relay may be called contactor. Still the basic principle of the simple relay is being employed and automation engineer should not be confused by these terms. A special need for a relay is in the tripping of power circuits for electric motors. The automation engineer will hear reference to "motor starters"; these devices are either contactors or relays that in addition provide overload protection to open circuit if a heavy mechanical load begins to cause the motor to carry too much current. A familiar period of overload is when the motor is first turned on, but a motor starter should permit a tolerable overload during the startup period. For this reason, the overload protectors are usually thermal, allowing them to be somewhat forgiving of a temporary overload. 4 Drives Like actuators, drives take some action upon the process at the command of a computer or other analyzer. For puposes of classification the distinction being made here between actuators and drives is that actuators are used to effect a short, complete, discrete motion- usually linear- and drives execute more continuous movements typified, but not limited to rotation. Actuators may turn drives on and off and drives may provide the energy for the movement of actuator. Some automation devices such as genevas seems to belong both categories. 4.1 Motors The quintessential drive is the motor. The automation engineer must have a broad perspective of the term motor to include not only electric motors but hydraulic and pneumatic motors as well. Internal combustion engines, also called motors are relatively insignificant in automated manufacturing. Hydraulic and pneumatic motors are the converse of their corresponding pumps. When hydraulic or pneumatic pressure is conveniently available throughout a manufacturing system or automated work station it may make more sense to tap this pressure to impart motion to a subsystem by means of a hydraulic or pneumatic motor. Hydraulic motors are capable of delivering a large amount of power in a confined space. Compared to hydraulic motors, pneumatic motors are more noisy and less powerful, but they may be more practical in many automation applications, especially considering the availability of compressed air in an automated manufacturing environment. Both pneumatic and hydraulic motors have some advantages over electric motors in systems in which electric motors may be hazardous either from an electrocution standpoint or from the ignition of flammable vapors or gases. Even when referring to electric motors, automation engineers need to know to which type they are referring. The majority of service motors are ac motors. There are two special types of dc electric motors used for drives for robots and manufacturing automation machines, however, and these deserve special emphasis. 4.2 Stepper Motors For several reasons, the stepper motor is a very useful drive in automation applications. For one thing, it is driven by discrete dc voltage pulses, which are very convenient outputs from digital computers and other automation control systems. The stepper motor also is ideal for executing a precise angular advance as may be require in indexing or other automation applications. stepper motors are ideal for open-loop operations where the control system gives a specific output command and expects the system to react properly without monitoring results in a feedback loop. Some industrial robots use stepper motor drives, and stepper motors are useful in numerically controlled machine tools. most of these applications are open-loop, but it is possible to employ feedback loops to monitor the position of the driven components. An analyzer in the loop compares actual position with desired position, and the difference is considered error. The driver can then issue voltage pulses to the stepper motor until the error is reduced to zero. 4.3 DC Servo Motors DC servo motors are useful in numerically controlled machine tools and industrial robots for the control of motion. By using a feedback loop, the controller can deliver to the motor dc voltage that is proportional to the observed error. When the error is reduced to zero, the voltage goes to zero and the motor stops. More sophisticated automatic servos can supply dc servo-motor voltage proportional to the rate of change of error and/or to the summation of accumulated error over time. One important characteristic of the dc servo motor that is also true of the stepper motor is that both hold their torque when they come to rest under power. Therefore, the power is useful not only for rotating the shaft but also for holding it motionless when no movement is desired. Numerically controlled machine tools and industrial robots are examples of automation equipment that must be able to hold axis positions between commands to change positions. 4.4 GENEVAS A Geneva mechanism used to drive an indexing table intermittently. Figure 14 reveals the drive mechanism (usually on the underside) of an indexing table. Note that there are two wheels: a driver and the driven indexing table. The actual driving force is delivered by a pin on the driver, which slides in a series of slots in the driven indexing table. The driver rotates continuously, imparting motion to the table while passing through angle B of its motion (see Fig. 14). During the remainder of the revolution of the driver, the driven indexing table is at rest. This at-rest (or dwell) period is the during which work is accomplished at each work station of the indexing table. In Fig. 14, note that the path of driver pin is precisely radial (orthogonal to tangential) to the driven wheel both at the times it enters and exits the slot. This is necessary to impart a smooth start and complete stop to the indexing table. In fact, any coasting of the table during the station execution (wheel-a-rest) phase will cause the driver pin to miss its next entry slot. This is a serious problem that will jam the drive for the machine and produce other mishaps that may occur on top of the table. Note also that the index and dwell time are dependent upon the driver speed and are constant from cycle to cycle. Fig. 14. Geneva mechanism. It is possible to have various numbers of stations in a Geneva indexing table setup, although the number cannot be less than three. More than eight stations should be considered rare. The following equations describe the relationships between number of stations, dwell time, index time and driver speed. Note that at four stations angle A = angle B, and driver and driven wheel diameters are the same.  angle A  angle B  180 deg (2) angle B  angle C  360 deg (3) angle B 60 Index time  x sec (4) 360 rpm angle C 60 Dwell time  x sec (5) 360 rpm 1 60 Cycle time  index time  dwell time  min  sec (6) driver rpm rpm Example 2 An indexing table driven by a Geneva mechanism has six stations and a driver speed of 12 rpm. Calculate: index time, dwell time and ideal production rate per hour.