Process Control Laboratory Report

The construction and operation of industrial plant requires an understanding that the processes involved in the plants should be properly controlled to come out with the desired production output. More importantly, the processes must be carried out under specified operating conditions so that the result will be as expected. Invariably, this calls for the introduction of control devices to curtail the plant operations within the required conditions. The devices prompt for the necessary adjustments to production variables (especially the input variables) in cases of deviations from processing conditions.

In effect, the production variables are kept on target (Montgomery et al. 1994). The importance of control equipments to processes has therefore necessitated a good understanding of the principles behind their design and operation. The choice of device should be such that control signals and responses are generated as quick as possible to forestall production losses (Leveson et al. 1994) More importantly, operating personnel needs to have an in-depth understanding of the functionality of these equipments most of which come as electronic devices (Parkinson and Smith 2002).

This is the focus of this work. The laboratory work intends to i. conduct a laboratory investigation into the operations of a typical process control unit using a process simulator ii. investigate the manner by which control devices respond to variations in process conditions and prompts for adjustments iii. determine systems behaviour under continuous and on-off control operations with the aid of the laboratory simulation iv. examine systems response under stepwise variations in the pre-determined control conditions v.

investigate systems response to proportional derivative and integral actions The work involves setting up a self-controlled air heating system with the use of a Process Trainer (PT 326) combined with a Process Control Simulator (PCS 327) in other to simulate typical industrial control operations. The heating process is akin to that of a typical combustion or other similar plants. Here the input is the electrical energy converted to heat energy like that supplied form combustion fuel. The output on the other hand is the temperature of the heated air.

This is similar to the temperature rise identified with the combustion plant end products. Literature Review Process control is a field that has grown over the years (Schuppen 2000, Parkinson and Smith 2002). Schuppen (2000) further described process control as involving the use of electronic or other similar forms of “sensors”, to generate “feedbacks” on systems performance (see figure LR1). On the long run the feedbacks initiates the necessary modifications to tailor the system back to the predetermined specifications.

The ultimate aim of process control is to optimise production inputs to obtain the desired result (or output). Figure LR1: showing a typical feedback loop for a controller. Source: Cooper (2007) System control has been generally useful in the engineering and allied fields. Schuppen (2000) pointed out that early application of system control covers the fabrication of radar detection devices, radio amplifiers, aircrafts and so on. Presently, process control devices are used in various manufacturing plants and even in controlling robots mobility (Parkinson and Smith, 2002).

Furthermore, Schuppen (2000) gave an outline of the processes involved in the design of control device. It starts with model formulation. These may be numerical models, physical (or analogue) models, or a combination of these. Thereafter, boundary conditions are specified to serve as a guide. Leveson et al. (1994) explained that these conditions are not just specified; there are rules guiding the specifications as well. Controls are of two different types – on-off or control and proportional control (Parkinson and Smith, 2002; Kuipers 2004).

The introduction of the overlap at a constant throttle control (constant air supply) caused some changes in the output. Figure 9 of the oscilloscope’s channel A (and the other drawings in the appendix) shows that it is quite possible to obtain a net value for both positive and negative deviations respectively. As obtained from the results, figure D1 below shows that the amplitude of the measurements from channel A increased from 2 to 3, then to 4 as the overlap changed from 0 to 2, and to 4 respectively (table 1). This is as expected.

The overlap indicates the amount of deviation from the set value that can still be accommodated or tolerated. With this, it means that if the deviation that is still within the range of tolerance the system is still assumed to be at the set point. However, in actual sense, this is not the case. What the records from the oscilloscope channel A displays is simply the deviations from the set value. The more the overlap, the more the system is allowed to deviate from the set point before being controlled and consequently, the more the amplitude as recorded by oscilloscope channel A (table 1).

Invariably, the two-step controller is more effective at curtailing deviations from the set point if there are no overlaps. Moreover, in case there are overlaps, the more the overlaps the more ineffective the controller becomes. Besides this, if one looks at the amplitude for channel B in table 1 and figure 10 (for instance), it is discovered that it is when the signal amplitude gets to 12 volts that it sends the message to control the deviation at channel A back to the set point.

Even as the value of the overlap increases causing the deviation to increase, the signal amplitude will still get to 12 volts before it sends a message to curtail the deviation. This further supports the fact that the control becomes less responsive to deviations as the overlap increases. Figure D1: the results varying the overlap for the first experiment (two-step control) as displayed in a graph (this excludes the amplitude for the signal). Note that the rate of heat supply is constant as the throttle setting is varied. In a similarly way, the period increases as the overlap increases (figure D1).

The period is a equal to twice the amount of time it takes to increase from the set point to the point of maximum deviation (which is the amplitude as recorded by channel A) and back to the set point (as a result of the control measure). Therefore, if the maximum deviation (amplitude) is more before it is controlled, it takes more time for the system to return back to the set (or pre-determined) point. The reverse happens for a small amount of amplitude. Since the period is a reverse of the frequency, as the period increase, the frequency decreases.

The time taken for the control to be put off also, as expected follow the same trend. High amplitude is indicative of the fact the temperature has risen relatively high above the set point and it will take more time to switch off the heat supplier for the system to cool down back to the set point. With low amplitude, it takes less time. However, in case of a negative deviation (going below the set point), because the heat is provided at the same rate, it will take the same rate for heat to be supplied to bring up the temperature to the set point no matter the amount of the amplitude.

Therefore, the time taken to put on the controller should remains at almost the same value as depicted in the result (figure D1). Of a major importance is the issue of varying the amount of load (air) supplied to the heating device at a constant rate of heat supply. The rate at which deviations from the set temperature occurs reduces as the air load (throttle valve setting) increases (see table 1 and figure D2). This is because more air load implies that the amount of substance to which heat will be supplied has increased but the heat rate remains the same.

Therefore, the temperature increase or deviation from the set value is less rapid compared to the case when a lesser amount of air is being heated by the same amount of heat. With the same amount of heat transferred to a lesser amount of air, the temperature rises rapidly above the set point in no time before the controller could bring it back to the set temperature. This is the reason for high amplitude measured from channel A in the case of a lower throttle control. Invariably, a lower maximum deviation is observed for a high throttle control.

This indicates that the more the air load the more efficient is the controller at curtailing deviations. Other observations follow the same trend of variation with the amplitude as the in first case when the overlap was the varying factor. The period and the time taken to put off the heater reduce as the amplitude reduces. However, the frequency follows a reverse trend as usual (figure D2). The time to put on the heater remains relatively the same for the same reason given earlier.

Figure D2: the results varying the throttle control setting for the first experiment (two-step control) as displayed in a graph (this excludes the amplitude for the signal). Note that the rate of heat supply is constant as the throttle setting is varied. In varying the heat power supply from 100% to 50% at the constant throttle setting of 9 in each case, it was discovered that the amplitude remains the same (at 1 volt). It shows that change in the rate of heat supplied does not have any effect on the ability of the controller.

The ability remains the same in this case as indicated by the amount of maximum deviation that occurred in each of the two cases (figure D3 and table 1). It can be concluded that if the throttle settings remains unchanged, the amplitude remains unchanged as well even if the rate of heat supply changes. This could be a as result of the fact that it is the same amount of air that is heated and, whatever the rate of heat may be, the same amount of maximum deviation is realised. However, it is faster reaching the maximum deviation for a high rate of heat supply than for a low rate because the rate of heat transfer is high.

This is the reason for a short period recorded for a high rate of heat supply (figure D3). The period was increased to 1 seconds when the heat rate was reduced. A similar reasoning can be drawn for the case of time when the supply is on. At a high rate, the maximum deviation is reached faster and the period of putting the supply on before shutting it off is short compared to when it gets to the maximum deviation at a reduced pace. The slower pace means longer time to put on the heater. The time when the supply is put off is almost the same in each case because the rate of cooling is expected to be the same.

Since we are dealing with the same amount of air and amplitude, air masses in each case will cool down at the same rate from the same amount of temperature. So, this means they will get back to the pre-set temperature within the same time frame. In summary, it can be concluded that varying the rate of heat supply will not have any effect on the efficiency of the control device; it makes no difference. Figure D3: the results varying the heat rate percentages for the first experiment (two-step control) as displayed in a graph (this excludes the amplitude for the signal).

Note that the throttle setting is kept constant at the value of 9. Experiment 2: Continuous (proportional) control The graph of offset against band width shows a linear direct relationship (figure 11). Thus, the higher the bandwidth, the more the offset. However, the higher the offset the more the likelihood the one that will have the system in a more steady state (figure 12). Moreover, a plot of offset versus gain (or sensitivity) brings about an inverse relationship (figure D4 and table D1 below).

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