PROCESS CONTROL
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range.
For example, heating up the temperature in a room is a process that has the specific, desired outcome to reach and maintain a defined temperature (e.g. 20°C), kept constant over time. Here, the temperature is the controlled variable. At the same time, it is the input variable since it is measured by a thermometer and used to decide whether to heat or not to heat. The desired temperature (20°C) is thesetpoint. The state of the heater (e.g. the setting of the valve allowing hot water to flow through it) is called the manipulated variable since it is subject to control actions.
A commonly used control device called a programmable logic controller, or a PLC, is used to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of analog and digital outputs. Using the example in the previous paragraph, the room temperature would be an input to the PLC. The logical statements would compare the setpoint to the input temperature and determine whether more or less heating was necessary to keep the temperature constant. A PLC output would then either open or close the hot water valve, an incremental amount, depending on whether more or less hot water was needed. Larger more complex systems can be controlled by a Distributed Control System (DCS) or SCADA system.
In practice, process control systems can be characterized as one or more of the following forms:
· Discrete – Found in many manufacturing, motion and packaging applications. Robotic assembly, such as that found in automotive production, can be characterized as discrete process control. Most discrete manufacturing involves the production of discrete pieces of product, such as metal stamping.
· Batch – Some applications require that specific quantities of raw materials be combined in specific ways for particular durations to produce an intermediate or end result. One example is the production of adhesives and glues, which normally require the mixing of raw materials in a heated vessel for a period of time to form a quantity of end product. Other important examples are the production of food, beverages and medicine. Batch processes are generally used to produce a relatively low to intermediate quantity of product per year (a few pounds to millions of pounds).
· Continuous – Often, a physical system is represented through variables that are smooth and uninterrupted in time. The control of the water temperature in a heating jacket, for example, is an example of continuous process control. Some important continuous processes are the production of fuels, chemicals and plastics. Continuous processes in manufacturing are used to produce very large quantities of product per year (millions to billions of pounds).
Applications having elements of discrete, batch and continuous process control are often called hybrid applications.
JOB PRODUCTION
Job production or jobbing is characterized by the manufacture of one or few numbers of a single product designed and manufactured strictly to customer’s specifications, within the given period and within the price fixed prior to tile contract. Some typical examples of industries engaged in jobbing production are: general repair shops; special purpose machine tool manufacturers; workshops to manufacture jigs and fixtures for other units; building contractors; tailoring shops manufacturing made-to-measure suite of clothes; manufacturers of ships, cranes, furnaces, turbo-generators, pressure vessels; and others manufacturing articles made to customers’ orders.
Characteristics of Job production:
· Disproportionate manufacturing cycle time
· Large work-in-progress
· Limited functions of production planning and control
· Process planning activity is almost absent. Drawings and specifications are directly given to the supervisor who is expected to decide work methods, select optimum process, fix up machine tools to be used and estimate time required completing an operation.
· Materials are indented and purchased on receipt of orders unlike in batch or mass production where material requirements are planned well in advance.
Importance of Job production:
· Small production runs
· Discontinuous flow of materials
· General purpose machines and process layout
· Highly skilled labor
· Highly competent knowledgeable supervision
· Simple Mechanism
· Decentralized Process
Advantages of job production:
· The advantage of job production is that each item can be altered for the specific customer and this provides genuine marketing benefits. A business is likely to be able to ‘add value’ to the products and possibly create a unique selling point (USP), both of which should enable it to sell at high prices.
Disadvantages of job production:
· Whether it is based on low or high technology, Job production is an expensive process as it is labor intensive (uses more workers compared to machines). This raises costs to firms as the payment of wages and salaries is more expensive than the costs of running machines.
BATCH PRODUCTION
The primary characteristic of Batch Production manufacturing is that all components are completed at a workstation before they move on to the next one. Batch production is mostly popular in bakeries and in the manufacture of sports shoes, pharmaceutical ingredients, inks, paints and adhesives. In the manufacture of inks and paints, a technique that is called a color-run is used. A color-run is where the production process manufactures the lightest color first, such as light yellow followed by the next increasingly darker color such as orange, then red and so on until reaching black and then starts over again.
This helps to minimize the cleanup and reconfiguring of the machinery between each batch. White (by which is meant opaque paint, not transparent ink) is the only color that cannot be used in a color-run due to the fact that only a small amount of white pigment can adversely affect the medium colors.
It is important to note that there are inefficiencies that are associated with Batch Production Manufacturing. The production equipment must be stopped, re-configured, and its output tested before the next batch can go on to be produced. The time between batches is known as "down time" and can be costly for a manufacturing company.
Batch production can be very useful for a factory that makes seasonal items or products for which it is difficult to forecast demand. Batch production is best used for products that are made, very similar and without too much detail. One of the best examples in Batch Production Manufacturing is cars. This is because all cars would have the same body shape and therefore, all can be made at the same time and with no extra cost.
Batch Production Manufacturing does have many "pros" and "cons" but is effective and used worldwide, mainly by larger businesses on higher profit margins.
One of the major advantages of Batch Production Manufacturing; is that it can reduce initial capital outlay because a single production line can be used to produce several products. Batch Production Manufacturing can be useful for small businesses who cannot afford to run continuous production lines. Also, many companies can use Batch Production Manufacturing as a trial run. Then if a retailer buys a batch of a product that does not sell then the producer can cease production without having to sustain huge losses.
A major disadvantage of Batch Production Manufacturing is the lack of flexibility in production. All production equipment, employees and other resources will be focused on completing the current batch of production. Should an unforeseen complication come up it becomes very difficult for manufacturers to switch everything over quickly. The lack of response time can potentially affect the bottom line and adversely affect overall profitability.
