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Introduction to Hydraulic Logic Systems in a Controls Course

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  1996 ASEE Annual Conference Proceedings Session 3266Introduction to Hydraulic and Logic Systems in a Controls CourseJoey K. Parker, Dale SchinstockDepartment of Mechanical EngineeringThe University of Alabama, Tuscaloosa, ALAbstract A four week long module on industrial hydraulics and logic control systems is described. This module istaught in an otherwise conventional senior level controls course that emphasizes feedback control systems.Students are introduced to hydraulic system components and circuit design considerations. Logic controlsystems, including programmable logic controllers (PLC’s), are introduced next. Pneumatic systems areintroduced as a special form of hydraulics. Some tutorial introduction to the material is given along with severalspecific design guidelines for the students. A representative student design project is also described. Introduction Many mechanical and electrical engineering programs include a required or elective course in controlsystems. The topical outlines for these courses typically follow the approaches used in the numerous textbooksavailable. Closed loop, feedback control system analysis is greatly emphasized, both from the transfer functionand state space points of view. Little or no mention is made of another broad class of topics, logic controlsystems, which are very commonly found in industrial applications.Logic control systems are used with sensors and actuators that operate in a simple “on/off“ mode (forexample a light switch). A great deal of simple automation can be accomplished with inexpensive pneumaticcylinders and a few solenoid valves. Unfortunately, many students are never exposed to even the most basicconcepts of this powerful and widely used control system technique. Additionally, many students are simplyunaware of the wide variety of actuators and sensors that are available for use in control systems. One reasonfor this is the faculty's lack of familiarity with these subjects. Another is the perception that there is not enough“room” in the course for adding this material. A third reason is that texts and other supporting materials are notas readily available for logic control as for feedback control systems.The purpose of this paper is to attempt to address some of these concerns by providing a description of the relative simple, but very practical, material that we use. We have successfully incorporated a four week long hydraulics and logic control systems component into our required senior level control systems course(outline given in Table 1). Our focus is on hydraulic and logic control system design, not the selection of specific components, i.e. we work at a “schematic” level. We introduce hydraulic system components first forthree reasons.1. Students learn the available components and can quickly create a schematic of systems to be controlled.2. Most mechanical engineering students have a better “feel” and appreciation for hydraulic actuators thanelectric motors.3. The incompressible nature of hydraulic fluid system makes them somewhat easier to understand thancompressible fluid systems (pneumatics), although the much lower cost of pneumatics makes them veryattractive in many applications. P  a g e1 .2  8  8 .1   1996 ASEE Annual Conference Proceedings The topics in this paper are presented in essentially the same order as in the course. Some portions arepresented in an almost tutorial fashion. Other portions give some specific details of examples that we use in ourcourse.Table 1 - ME 475 Control Systems Course OutlineWeek MaterialWeekMaterial1Intro, Industrial Hydraulic Components7Block Diagrams2Hydraulic Circuit Design, Logic ControlSystems8Stability3Logic Control Systems, IndustrialPneumatics9Steady State Errors4Industrial Hydraulic System DesignConsiderations10-11Root Locus5System Modeling, System Representation,12-13Design via Root Locus6Time Response14-16Frequency Response MethodsNote that our mechanical engineering students are required to take a dynamic systems modeling course prior tothe controls course, which means that the topical material for week 5 is a review. The final 11 weeks of thecourse closely follow the textbook by Nise 1 . Industrial Hydraulic Components Drive elements (actuators) for industrial automation fall into one of three categories: electrical,hydraulic, or pneumatic. As a very general rule, electrical drive systems are used in high precision, relativelylow load applications where control flexibility is paramount. Hydraulic drive systems tend to be used wherelarge loads must be manipulated. Pneumatic drives are generally limited to simple two position operationswhere low cost and simple programming are necessary.Hydraulic Drive SystemsA large percentage of heavy-duty industrial applications require the use of hydraulic actuators. Bothlinear and rotary actuators have large force- or torque-to-weight ratios, which is the primary benefit in manyapplications. Many of the problems with hydraulic systems are due to the complex system of componentsrequired. The minimum set of equipment required to drive even a single axis includes: hydraulic pump and oilsupply, electric motor to drive pump, water cooling system for pump, pressure relief valves, safety shut-off valves, filters, directional control valves, hydraulic hoses, and at least one hydraulic actuator. Additionalcomponents such as accumulators, manifolds, oil-cooling heat exchangers, or additional reservoirs may berequired in some applications. Although the correct selection of many of these components (hydraulic fluidtype, hoses and tubing, filters, tanks) is vital to the system operation, they are not directly considered in thiscourse.Hydraulic PumpsHydraulic pumps are required to generate the high fluid pressures of 500 to 3000 psi (3500 kPa to 21000kPa) used in industrial applications. Three basic types of hydraulic pumps (and motors) are available; pistonpumps, vane pumps, and gear pumps. Pumps use an external source of energy (typically an electric motor) topressurize the hydraulic fluid.Hydraulic ActuatorsThree types of actuators are common in hydraulic applications: hydraulic motors, linear cylinders, androtary actuators. Hydraulic motors are similar in design and construction to pumps, except that the high-pressure fluid is used as the energy source to drive an external load. In fact, most pumps will act as motors if  P  a g e1 .2  8  8 .2   1996 ASEE Annual Conference Proceedings the flow direction is reversed. Linear cylinders are probably the most common type of hydraulic actuator.Rotary actuators are essentially a hybrid between hydraulic motors (with continuous rotary motion) and linearcylinders (with finite linear motion), since they provide a limited rotary motion.ValvesSeveral different types of valves are used in hydraulic systems. Valves can be categorized 2  as either1. pressure control (pressure relief, sequence, unloading, counterbalance, pressure reducing),2. flow control (fixed restriction, variable (needle), compensated, flow divider),3. directional control (check, shuttle, two/three/four way, manual (shut-off), electrohydraulic servovalve)Pressure relief valves are used to limit the system pressure to a preset limit, and serve a vital safety interest.Pressure control valves that pass flow to other portions of a hydraulic circuit are called either sequence,unloading, or counterbalance valves depending upon the application. A pressure reducing valve is used toprovide a downstream source of fluid at a reduced pressure, much like a voltage regulator in an electrical circuit.Flow control valves are used to limit the amount of hydraulic fluid flow. Directional control valves are used tocontrol the path that the hydraulic fluid flow uses as it flows from the pump back to the reservoir. Two-, three-,and four-way directional control valves can be actuated by a variety of different means including manualoperation (levers, pedal, or palm buttons), electrical solenoid, cam operation, spring returns, and pilot operation.Check valves are flow control devices that allow fluid flow in one direction, but completely prevent flow in thereverse direction (which is very similar to the action of a diode in an electrical circuit). Hydraulic Circuit Design Many useful hydraulic circuits can be constructed by assembling basic building blocks, once theunderlying principles are understood. The only example shown here is the manual position control of a double-acting hydraulic cylinder (Figure 1). Four-way, threeposition valves are commonly used in these applicationssince the center position allows the cylinder to be stoppedat any intermediate position. While the valve is in thecenter position, the fixed displacement, single directionpump is unloaded. All flow from the pump is immediatelyreturned to the tank reservoir under a relatively lowpressure. If the valve is shifted to either of the two endpositions, the flow from the pump is routed to the selectedend of the cylinder. The pressure will then start to buildrapidly until it reaches a pressure that is sufficient to movethe load against gravity and friction. The purpose of thepressure relief valve is to set the maximum operatingpressure in the system. When the force created by thepressure in the pilot inlet line (shown dashed in the figure)balances the spring force, the relief valve “cracks” and flowis diverted from the cylinder to the tank. This operation issomewhat similar to that of a voltage regulator in an electrical circuit. Different types of pressure relief controlvalves are available to provide various pressure-flow characteristics. Additional information on hydrauliccomponents and systems can be found in a series of handbooks by Hedges 3 . Logic Control Systems To begin the discussion of industrial logic control systems, consider the simple hydraulic system shownin Figure 2. Pressurized hydraulic fluid is available to the simple two position, four-way solenoid valve, as Pumprelief valvePressureThree-position, tandem center,directional control valveReservoir (tank)Double acting cylinder Figure 1. Simple Hydraulic Circuit P  a g e1 .2  8  8 . 3   1996 ASEE Annual Conference Proceedings indicated by the dark arrow symbol. In the configuration shown the hydraulic cylinder will retract fully. Thesolenoid valve shown is activated by an electrical current passing through the solenoid coil. This type of simpleON/OFF programming has traditionally been done by relay control systems, like that shown on the right. Thisschematic diagram represents a type of “programming” frequently referred to as ladder logic. The two partsof a relay (coils and contacts) are both shown in this diagram. Electrical current passing through the coil of therelay (denoted by the circle element CR-1) closes one of these sets of contacts (CR-1B) which allows current toflow through the solenoid, SOL-A. Another set of contacts, CR-1A, is used to hold the contacts closed oncethey have been energized. A momentary contact push-button PB-1 (normally open or N.O.) is provided forinitiating motion. When PB-1 is pressed, current flows through the actuating circuit of relay CR-1, which closesthe output contacts (CR-1A and CR-1B). Relay CR-1 remains energized until the limit switch, LS-1, isactivated by the cylinder. When this limit switch is activated, the current flow through the control relay CR-1 isinterrupted, and the contacts CR-1A and CR-1B both open. The solenoid SOL-A is de-energized, therefore thespring shifts the solenoid back to the right position, which causes the cylinder to retract. The circuit is inactiveuntil a subsequent pressing of the push-button PB-1. PB-1CR-1CR-1ACR-1BLS-1SOL-ASol. A120 VAC Limit switch, LS-1Double acting cylinderTwo position, solenoid operated,spring return directional controlvalveHydraulic supplyReservoir (tank)Normally Open contactsSolenoid coil Control relayNormally ClosedNormally Open pushbutton contactslimit switchcontacts Figure 2. Simple hydraulic and logic control systemProgrammable ControllersOne of the disadvantages of the relay logic systems of the previous section is the difficult nature of the programming. The program logic is hard-wired by the interconnection of the relays, limit switches, andpushbutton inputs. Changing the task performed by the simple system of Figure 3 requires that the circuit bephysically rewired. For circuits with only three or four components this is not difficult. However, systemscontaining ten to several hundred individual components are not uncommon in industrial automation systems.The programmable logic controller (PLC) was developed in the early 60's to overcome the deficiencies of relaylogic systems. Programmable logic is implemented using a microcomputer instead of the hard-wired logic of theconventional hard-wired relay system. The major advantage of PLC's (frequently referred to as justprogrammable controllers or PC's) is that the programming can be done in ladder logic, just like relay logicsystems.The major criteria for specifying PLC's are the number of input contacts that can be read and the numberof output switches that can be controlled. Small PLC's might have 8 to 12 inputs and outputs, while largermodels can use 100 or more I/O (input/ output) points. Inputs are typically 0-120 volts AC or 0-24 volts DC.Output options frequently include relay contacts, triac (120 VAC) or 24 volt (open collector) outputs. Some of the newer PLC models have advanced features such as analog inputs (0 - 10 Volts), PID (proportional-integral-derivative) control loops, and serial (RS-232) communications capabilities. P  a g e1 .2  8  8 .4 
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