ELEC6240 Digital Control System Design : Trends In Advanced Intelligen
2. State the minimum number of printed-circuit cards required to construct a basic controller, naming each of the cards.
3. Produce a sketch to show the construction of the data words on the data highway.
4. Explain the use of the different MODES of control available in a controller of a Distributed Control System.
5. The Instrument Engineer, designing a Distributed Control System, has available to him Basic Controllers and Advanced Controllers. List all the reasons why an Advanced Controller might be used in preference to a Basic Controller, making reference to the full processing capabilities of the advanced controller.
6. Describe the difference between notified status and change detect points and list the types of cards to which this applies.
7. Describe the main purpose of sequence programmes and state what would require to be done in the writing of a programme which was too long to fit in one sequence slot, making reference to the problem which this could cause.
Answer:
1: Components of Direct Digital Control System
Points
The location of information storage within a direct digital control system is called word points. Data can be gotten from software calculations or sensors and logic. information can be transmitted to software calculations, managed devices and logic. Every data storage location has a specific way of its identification.
(DDC) Direct Digital Controls information is grouped in three different ways as follows; flow, type and source of data.
Data Type
Data type is categorized as analog, accumulating or digital (binary or discrete). The value of data is either 0 or 1 and normally describes the state of contact sets. Analog data are ma
thematical, numbers with decimals and normally have changing inputs of functions of electrical pressure, temperature, relative humidity or other known sensed variable of HVAC. Accumulating data also known as pulse input are mathematical, number with decimals, where the output sum is kept.
Data Flow
Information flow in or out of the directed digital control components is determined by data flow element. Information that is input is defined by input points whereas data that is output is define d by output points.
Data Source
External or hardware points are defined so if the information is gotten or shipped to an external gadget. External points are input, output, analog, accumulating or digital. Information created by control software component is represented by Internal (data, virtual, software or numeric) points. These points as well can be analog, digital, or accumulating.
Data sent on the network for utilization by other controllers is described using in-direct or Global points. Analog input points mean hardware point and act as a varying value with time. HVAC application analog inputs include; relative humidity, airflow measurements, and carbon dioxide. Analog outputs consist of control signals for harmonizing positions of the valve, positions of damper and drive speed of variable frequency.
HVAC application digital inputs represent the status of fans, contactors, pumps, lighting, motors, etc. Even though the high limit of a temperature is controlling an analog value it is considered as digital input because the data sent to the controller is in digital condition. Digital outputs are devices or motors turned or switched on or off. Outputs that are digital consist of fans, lighting contactors pumps, two-position valves, among others.
True output of analog is a changing Direct Current voltage or signal of milliamp that is helps drive a controlled device. PWM which is another type of analog output is achieved by controlling a time given of contacts sets. Time taken for contacts closure is proportional to the performance level for the transducer device.
2: Construction of Basic Controller
The minimum number of printed circuit cards required to construct a basic controller is five. Namely; co-processor modules, a power supply, peripheral device, central processing unit (processor or CPU or), and input and output modules (I/O). These components coordinate to bring data into the controller from the field, analyze that data, and transmit data back out to several field. The controller will fail to operate properly if these main components are missing.
3: Construction of the Data Words on the Data Highway
Figure 2: Construction of the data words on the data highway
4: Modes of Control
The following are the different modes of control in the controller of distributed control system; manual mode (MAN), auto mode (AUT), cascade mode (CAS) and primary direct mode (PRD)
MAN mode manipulated variable (MV) can be altered by decrementing or incrementing corresponding keys.
AUT mode manipulating variable value can't be altered. Process value (PV) is displayed on the faceplate of the proportional–integral–derivative (PID) controller. The operator enters the set value (SV) manually. The PID controller on normal activities is in the AUT mode.
Manipulated value is calculated by the system in the Auto mode automatically. This is the result moving to the Final Control component.
In CAS mode the result of primary controller like MV1 proceeds as set point of the secondary controller say SV2. The result of secondary controller proceeds to final control component. In addition, for a cascade mode, the primary controller can be in MAN or AUT mode, but the secondary controller is in CAS loop.
Primary Direct Mode (PRD); this mode is considered to be special. The result of primary controller say MV1 in this mode proceeds directly to primary control component. The secondary controller is omitted and can't be used. Adjusting window of the secondary controller sets the PRD mode.
The following figure shows the general idea of control modes.
Figure 3: General Idea of Control Modes
5: Benefits of Advanced Controls
Advanced controls can be implemented in any process as compared to the basic controls. Thus the instrument engineers would prefer a control mechanism that doesn’t limit him in implementing the control procedures. Research affirms that advanced controls reduces operating costs by up to 6% and generates 1% extra revenue.
In automated batch processes advanced controls facilitates enhancements of control regulations better than basic controls. It will facilitate production of a commodity repeatedly leading to consistency of products produced thus, allowing manufacturers to carry out test under different conditions. This fact, therefore, increases production capacity, enables flexible conditions of operations and reduces batch time. This advantages are unnoticeable in basic controls
Advance controls enhances coordination of events that occur frequently in one loop control system in continuous processes better than basic controls. It also decreases variance in processes and enables industries to operate nearly within their operating constraints.
6: Difference Between Notified Status and Change Detect Points
In the case of notified status several telemetry units are distributed across the entire system to notify operators if anything goes wrong. An example of a telemetry is NetGuardian. These units can notify on communication network, controls and other outside variables like humidity, temperature, pressure among others that may affect the system thus holding operations in the loop. On the other hand, Change-detect points is a tool used to identify if a change has occurred.it provides confident intervals and levels by reporting on changes detected in the system. Change detect points are flexible, controls rate of errors, simple to use and robust to outliers. They both apply in double-sided and multi-layered cards.
7: Purpose of Sequence Programmes
The main objective of sequence programs is to allow continuous, constant and sequential execution of instructions in the order in which they are written. If the written programs were too long to fit into a sequence slot, it should be organized to determine similar sections that may have same functionality so that they can be aligned in consecutive columns. This particular case brings about the problem of inefficiency because similar recursive problems are solved every time.
Reference:
Smith R, Electricity for Refrigeration, Heating and Air Conditioning (Cengage Learning 2015)
Pyrho?nen J, Hrabovcova? V and Semken R, Electrical Machine Drives Control (Wiley 2016)
Considine D and Considine G, Standard Handbook Of Industrial Automation (Springer US 2012)
Guo L and Wang H, Stochastic Distribution Control System Design (Springer 2010)
Mitkowski W and others, Trends In Advanced Intelligent Control, Optimization And Automation (Springer 2017)
Bernard P, Foundations Of Itil (Van Haren Publ 2012)
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