Electro Mechanical Control Systems - Coursework Example

TOP-DOWN DESIGN IN ELECTRO MECHANICAL CONTROL SYSTEMS Introduction The top-down engineering approachentails the designing process that starts by specifying or defining the global system state and then assuming that every component is having a global knowledge of the system; the approach is a centralized one. The solution is decentralized via the replacement of the global knowledge using communication (Planchard and Planchard, 2011). From the top-down design methodology, the design begins from the top assuming that the resources are globally accessible by every subcomponent of the defined system, just like in the centralized case. Specifications are then defined using the global systems states and implies that every individual component is able to estimate or retrieve, accurately and usually within reasonable time delay. Under the stated conditions, the properties or characteristics of a classical centralized solution to the defined global specification are anticipated to hold, usually up to some tolerable or allowable performance degradation; this also true in the decentralized environment. Figure 1. Top-down design cycle (Chakrabarti, 2012) Advantages of Top-down design The system realizes focused or full utilization of functionality emanating from the individual subsystems. The first system implementation is the showcase for the identity engineering design solution. When phases are completed for the control system, there is a complex and more mature implementation of functional control system solution. The operation of the system is not initially impacted severely as compared to the bottom-up approach. The top-down design guarantees the systems’ analyst in ascertaining the overall functional objectives of the control system, and they how they are met in the overall system. This ensures that there is no chaos that appears when the system is designed “all at once”. The capability of employing separate systems analysis or design teams performing their duties in parallel on different related subsystems ensures fast system implementation; and also ensures that there is no loss of sight in the system goals due to lengthy details (Hurst, 2012). Therefore top-down design is suitable when: the project is constrained by the tight budget, since top-down approach assists in maximizing savings via thorough planning of the budgets when beginning the product concept design cycle. building large and complex systems. Complex machines and systems benefit from the top-down approach since it allows the breaking down of the project’s goals units or smaller problems for easy solution. Ball Screw Linear Actuator Figure 2. Ball Screw Linear Actuator (Hurst, 2012) Figure 3. How ball movements A linear actuator is an actuator that gives a straight-line motion. The ball screw linear actuator consists of the threaded shaft known as the drive screw or leadscrew that is rotated using an electric motor (Hurst, 2012). A ball screw actuator is utilized in translating the rotational motion into the linear one, and vice versa. The ball screws are utilized in guiding, locating, supporting, accuracy moving components in the control applications. They are useful since they can withstand or apply high thrust loads with high efficiency; usually over 90%. Proper designing and specifications for the ball screw ensure repeatability, and accuracy; and reduction of the production cost. The ball screw drive has the ball nut and ball screw with the recirculating ball bearing. The bearings make an interface between the nut and the screw, the interface rolls in the matching form in ball nut and ball screw. Therefore, the load exerted on the ball bearing is relatively low. Since the system has rolling elements, there is low coefficiency of friction in ball screw drive; hence an increased mechanical efficiency. The “nut” located on the shaft moves in the linear motion when the shaft rotates. The rotary torque emanating from the motor is the transferred via the gear train to the given drive screw. Consequently, the output motion emanates as the nut retreats or advances along the threads; this depends on direction that the shaft is rotating. The nut is constrained from the rotation. In order to reduce the friction and other losses, and the increase of efficiency, there is usage of ball bearings in transferring the force emanating the drive screw to the drive nut; this is utilised in ball bearing screw. In the ball bearing screw, when the system’s shaft rotates, then the balls roll through the grooves. In case the balls move to one of the nut’s end, they are usually recirculate to the other end via small pipes. The ball bearing screws are utilised in eliminating the friction (Otero, 2012). Position Control The ball screw components are selected via the repeatability and accuracy emanating from the assembly, mounting configuration, dimensional constraints, availability if power, and the environmental conditions; as shown in figure 3 (Pahl, Wallace and Blessing, 2009). The positional repeatability and accuracy are determined for the application requirement. Ball screws that allow greater cumulated variations over useful or important length of screw are utilized in applications that normally use the linear feedback only or in the coarse movement for the positional location. Ball screws with the accumulation of the load error in order to avail precise position through the entire useful length of the screw are utilized in the critical repeatable and accurate positioning. Figure 4. Selection Criteria (Pahl, Wallace and Blessing, 2009) Speed The critical speed characterizes the rotary speed which the assembly or system sets-up the harmonic vibrations. This critical speed is proportional to the root diameter of the screw, the end support and the unsupported length configuration. The larger diameter screws are characterized with increase in the load capacity and increased speed rating. The small loads are characterized with decreased linear speed, when there is constant motor speed; increased motor speed when there is constant linear speed; and decreased input torque requirement. The higher leads are characterized with increased linear speed when there is a constant input motor speed, and decreased input motor speeds in case of constant linear speeds. Figure 5. Torque versus the Thrust at the Ball Screw (MT Precision, EDrive Products, 2012) Using the chart from EDrive Products, figure 4, the approximate motor torque needed for the production of a given force is determined via the adjacent chart: matching the peak force on the X-Axis and draw a perpendicular line the value via the data line. At the intersection point, a horizontal line is drawn to the Y-Axis legend on the left; therefore this is the torque input required for the production of the peak axial force. Figure 6. Position and Speed control in Ball Screw Linear Actuators Figure 7. Discrete Position and Speed control in Ball Screw Linear Actuators Figure 8. Continuous Position and Speed control in Ball Screw Linear Actuators Merits of Ball Screw Linear Actuators The ball screw linear actuators are the ideal candidates for positioning of loads that are loads that are supported, pivoted or externally guided. They are useful in environment that is full of airborne contaminants concentrations, since the rodless actuators are less well protected. The ball screw linear actuators are replacements for the pneumatic and hydraulic cylinders because: The ball screw linear actuators are precise, hence can be utilized for handling and manufacturing applications. They have broad range of options with the accessories; hence there is easy finding of the actuators for the applications. The ball screw actuators are the direct descendants of the pneumatic and hydraulic cylinders; they have the same unique characteristics designs of the pneumatic and hydraulic cylinders i.e. simplicity, clean and energy efficient power transmissions. The ball screw linear actuators are easy to integrate on the PLCs, less noisy and have greater accuracy when compared to pneumatic and hydraulic cylinders. The ball screw linear actuators are made of the protections that have robust designs, hence suitably utilized in the harsh environment. Figure 9. Tool Actuator Features (MT Precision, EDrive Products, 2012) PLC’s Programming Figure 10. The internal block Diagram of a register (the DAC8760) The first step for the PLC programmer in the development of the PLC control program is the proper description or definition of the Registry control task. The registry control task is important since it specifies what to be done and the parameter involved in the specified control operation. With the knowledge of the registry, a control strategy is determined as the second stage of the control programme development; this ensures that the sequences within the programme translate the inputs to a desired output control. When the input and outputs are well known, there exists proper programme guidelines followed in the programme implementation, hence ensure the development of an organized system. Figure 11. Modification of the register values Through the knowledge of the register input and outputs, the designer can come-up with new applications or upgrading the existing equipment based on the programme internal components. In addition, the designer is able to arrange the PLC system information through the internal address assignment table, I/O assignment table, and register address assignment table. A clear knowledge of input device programming allows the program translations, for example, of the normally closed input devices, or the bidirectional power flow, and complicated logic rungs. The PLC input and output modules are categorized according to the output and input signals related to specified applications. Register type is one of the modules. The PLC-Subgrouping design avails the guidelines in the proper designing of the Programmable Logic Controllers for proper functionality and safety i.e. demand mode and de-energized in tripping. Some of the input and output knowledge for the register design selection entails: System or task requirements. The application requirements. The input or output capacity is requirements Types of inputs or outputs required The size of memory is required. The speed of CPU required. The electrical requirements. Operator interface. The speed of operation. Software. Communication requirements. The register output and input modules transfers 16 or 8 bits information words to and from a PLC. The words are usually numbers; binary or BCD that are generated from the thumb wheel switches or the encoders systems for the input or the data for the output display of the PLC. Through the knowledge of the design of the register, proper monitoring of the operations is evident. In addition, the internal registers’ data can be altered for the finer operations. The internal relays are usually simulated via bit locations in the registers. Registers are typically assigned in general data storage and in case the power is removed, it ensures same content when power returns, and in addition, they are utilized as temporary storage for the data or math manipulations. The designer can utilize registers in minimizing the number of isolations by configuring them with two DAC in the daisy chain mode. In addition, the designer can use the registers as the I/O expander. Designers can utilize registers as in the controlling of the opto-switches, which are utilized in switching between voltages into current input modes. The registers when designed when can be loaded with appropriate bit for turning off or on the burden resistance. Registers are useful in the zero error correction and in the gain correction. When the designers know the register inputs, internal programme, and outputs, there are three principal types of data that are generated: Viable testing criteria. Testing will be carried in the form of a specific result or test data sets, in qualitative criteria, and in the quantitative performance constraints on the system’s behaviour. These avails the basis for the inductive arguments used in meeting system requirements. Design justifications. When the designer is knowledgeable of the inputs, internal programme, and outputs, they are capable of justifying arguments from the outcomes if the processes match the specifications given in the previous stages. Usually these justifications are conducted in a deductive manner, that is, using input and output knowledge. The process conformance. From the knowledge of the inputs, internal programme, and outputs by the designer leads to the proper selection of the personnel and training that generate the final product of the required or desired quality. The designer’s knowledge argues if the PLC register design process is conforming the required process or the correct procedure is being followed. Examples of Ladder Figure 12. Programming a timer sequence From figure 10, timers starts running when the IPO is closed. In a span of 5 seconds, the timer’s contacts are closed hence switching on the output OPO. In case IP1 is closed, the timer resets. Hardware Ladder Diagram (Software) Figure 13. An electric oven temperature control via16-bit PLC for temperatures of approximately 100°C. Hardware Sequence file Figure 14. Sequencer program for the Washing Machine References Buede, D. (2013). The engineering design of systems. Hoboken, N.J.: Wiley. Bystrom, M. and Eisenstein, B. (2005). Practical engineering design. Boca Raton: Taylor & Francis. Chakrabarti, A. (2012). Engineering design synthesis. London: Springer. Hurst, K. (2012). Engineering design principles. London: Arnold. Madsen, D. and Madsen, D. (2012). Engineering drawing & design. Clifton Park, NY: Delmar, Cengage Learning. Misra, K. (2009). Handbook of performability engineering. London: Springer. Otero, C. (2012). Software engineering design. Boca Raton, Fla.: CRC Press. Pahl, G., Wallace, K. and Blessing, L. (2009). Engineering design. London: Springer. Planchard, D. and Planchard, M. (2011). Engineering design. [Mission, Kan.?]: SDC Publications. Read More