Файл: Industrial Control (Students guide, 1999, v1.1 ).pdf

Experiment #4: Continuous Process Control

Questions and

Challenge

1.Give two examples of continuous process control other than those given in the text.

2.How is the drive level in open-loop control determined?

3.What is the primary advantage of open-loop control?

4.What is the primary disadvantage of open-loop control?

5.The ADC0831 will convert a range of analog input to one of 256 possible binary values. The number 256 identifies the _________ of the converter.

6.The purpose of the chip select and clock lines to the ADC0831 are to ____________ the conversion process.

7.If the LM34 were placed in a 98.6-degree environment, the expected output would be _________volts.

8.In pulse-width modulation, the amount of drive action is based on the __________ of time ON over the total time.

9.If a 40-watt heater were pulse-width modulated at a 75% duty cycle, the average power consumed would be __________ watts.

10.When disturbances change in an open-loop process, so does the ____________.

Challenge: Open-loop Control of the Fan

Recall in Experiment #2, the circuit in Figure 4.13 was introduced to control the speed of the brushless motor fan.

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Experiment #4: Continuous Process Control

Figure 4.13: Sample and Hold PWM Drive

Because of the fan’s quick response to voltage fluctuations, the Sample and Hold circuit is necessary to effectively control speed using the PWM instruction. Construct this circuit to be able to vary fan speed. Directly connect the resistor across the +Vin supply. With the fan directly pointed toward the canister, experiment with different PWM drive levels to the fan. What drive level is necessary to cool the canister to 101oF?

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Experiment #5: Closed-Loop Control

An open-loop control system can deliver a desired output if the process is well understood and all conditions affecting the process are constant. However, Experiment #4 showed us that an open-loop control system couldn’t guarantee the desired output from a process that was subject to even mild

disturbances. There is no mechanism in an open-loop system to react when disturbances affect the output. Although you were able to find a drive setting that would yield the desired temperature in Experiment #4, when the fan was moved closer or further from the heater, the fixed setting was no longer valid. Closed-loop control provides automatic adjustment of a process by collecting and evaluating data and responding to it accordingly. A typical block diagram of an automatic control system is depicted in Figure 5.1.

Figure 5.1: Closed-Loop Control

In this diagram, an appropriate sensor is measuring the Actual Output. The signal-conditioning block takes the raw output of the sensor and converts it into data for the Controller block. The Setpoint is an input to the Controller block that represents the desired output of the process. The controller evaluates the two pieces of data. Based on this evaluation, the controller initiates action on the Power Interface. This block provides the signal conditioning at the controller’s output. Experiment #3 discussed several methods of driving power interface circuits. The Power Interface has the ability to control the Actuator. This may be a relay, a solenoid valve, a motor drive, etc. The action taken by the Actuator is sufficient to drive the Actual Output toward the desired value.

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Experiment #5: Closed-Loop Control

As you can see, this control scenario forms a loop, a closed-loop. Furthermore, since it is the process’s output that is being measured, and its value determines actuator settings, it is a feedback closed-loop system. The

The type of reaction that takes place upon evaluation of the input defines the process-control mode. There are five common control modes. They are on-off, on-off with differential gap, proportional, integral, and derivative. The fundamental characteristic that distinguishes each control mode is listed below in Table 5.1.

Table 5.1: Five Common Control Modes

Continue taking more forceful action for the duration the error exists.

This exercise will focus on converting the open-loop temperature control system of Experiment #4 into an on-off closed-loop system. Our system will show advantages and disadvantages to this method of control. The characteristics of the system being controlled determines how suitable a particular control mode will be. Experiment #6 will use the same circuitry to overview and apply proportional, integral, and derivative control modes. Leave the circuit constructed after completing this step.

Figure 5.2 is a schematic of the circuitry necessary for the next two exercises. As you see, this is identical to Experiment #4. The 35-mm film canister provides the environment we wish to control. The heater drive provides full power for developing heat in the resistor. The LED is also driven by Pin 8. Remember that the LED is driven by the +5-Vdd supply, and the heater is driven by the +9-volt unregulated line supply. The LM34 sensor will provide temperature data. In closed-loop control, we will monitor the temperature and use it to determine control levels. The fan’s air currents will act as a disturbance to the process.

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Experiment #5: Closed-Loop Control

Figure 5.2: Closed-Loop Control Circuitry

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Experiment #5: Closed-Loop Control

If the circuit isn’t already on your board, carefully construct it. Use space on the small Board of Education efficiently to allow for the circuitry. Take your time, plan your layout, and be careful not to inadvertently short any wires. Refer back to Experiment #4 for details on the film canister construction, the operation of the LM34, and the use of the ADC0831 analog-to-digital converter.

Double-check the Zero and Span voltages of the ADC0831. Use your voltmeter to set the Zero voltage (Vin(-))

to .7 and the Span (V(ref)) to .5 volts. This will establish a full-scale temperature measurement range from 70 to 120 degrees F.

Exercises

Exercise #1: Establishing Closed-Loop Control

Let’s assume it is our objective to maintain temperature within the canister at 101.50 oF + 1 degree. This would be representative of the requirements of an incubator used for hatching eggs. Maintaining the eggs at the setpoint temperature of 101.5 oF is perfect, but the temperature could go up to 102.50 or down to 100.50 without damage to the embryos. Although it may be hard to imagine an incubator when you look at your film canister, the BASIC Stamp would be well suited as the controller in a large commercial hatchery incubator.

To maintain temperature at the desired value seems like a pretty “common sense” task. That is, simply measure temperature; if it is above the setpoint, turn the heater OFF; and, if it is below, turn the heater ON. The simplest kind of control mode is on-off control. There are drawbacks to this control mode, however. During the following exercise, you will establish on-off control of your model incubator. Pay close attention to the characteristics exhibited by your model. These characteristics would also apply to real control applications.

Procedure

Programming for this application requires data acquisition, evaluation, and control action. Our display routine will also include storing and displaying the minimum and maximum overshoot in the process.

The structure and much of the content of Program 4.1 may be used to acquire and calculate our measurement. Instead of turning the heater on continually, a new subroutine will be added to evaluate and control it. Evaluation will be based on a setpoint variable. Refer to Program 5.1 following.

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Experiment #5: Closed-Loop Control

'Program 5.1: Simple ON/OFF Control with the StampPlot Interface

'This program establishes simple ON/OFF control of the model incubator. 'Program I/O is based on the circuitry of Figure 5.2.

'Zero and Span voltages: Digital 0 = Vin(-) =.70V and Span = Vref = .50V.

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Experiment #5: Closed-Loop Control

DEBUG "!USRS Overshoot/Undershoot Test Ready",CR

MMFlag = 1

RETURN

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Experiment #5: Closed-Loop Control

Run the program and observe the behavior of the system. StampPlot Lite will graphically plot the temperature response of the system and the on-off status of Output 8. Follow the StampPlot Lite procedures of running the program, closing the debug window, opening StampPlot Lite, and pressing the “reset” button.

When you start your system, the heater will be on as indicated by the LED. The heater/resistor becomes quite hot when full power is applied. This heat transfers through the environment and warms the temperature sensor. When the sensor has heated to 101.5, the BASIC Stamp will turn off the heater. For a period after the heater is turned off, the temperature continues to rise. This is called overshoot. At this point, it is important to understand the dynamics of your system. The heat held within the mass of the resistor will continue to dissipate into the air, the air becomes warmer, and the LM34 reports that overshoot has occurred. Similar to the mechanical inertia of a moving object, this phenomenon is called thermal inertia. Overshoot becomes large when the heat energy contained in the mass of the resistor is large, relative to the heat already in the canister. The 35-mm canister is small, but the mass of the half-watt resistor also is small. As a result, the overshoot of your system will probably be less than one degree.

When the temperature does turn around and begin to fall, as it passes the setpoint, the heater is once again turned on. Undershoot will occur for similar reasons as did the overshoot. During the time the heater is coming up in temperature, the ambient temperature has continued downward. Continuous cycling above and below the desired setpoint is typical of on-off control. The rate of this cycling and the degree of the overshoot depend on the characteristics of the system. On-off control is suitable for processes that have large capacity, can tolerate sluggish response, and sustain a relatively constant level of disturbance. If our incubator were large, well insulated, and kept in a constant room environment, on-off control would be acceptable. After the process has had a chance to cycle a few times, record the minimum and maximum overshoot values.

Maximum overshoot _______________ Minimum Overshoot __________________

Using your cursor, investigate time between cycles. Record these times below.

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