Closed and Open Control Loops

A closed control loop exists where a process variable is measured, compared to a setpoint, and action is taken to correct any deviation from setpoint. An open control loop exists where the process variable is not compared, and action is taken not in response to feedback on the condition of the process variable, but is instead taken without regard to process variable conditions. For example, a water valve may be opened to add cooling water to a process to prevent the process fluid from getting too hot, based on a pre-set time interval, regardless of the actual temperature of the process fluid.

Open-loop and closed-loop control

Having defined the term "controlled system" it only remains to give definitions of closed-loop control as contained in standards. First it is useful to fully understand the difference between open-loop control and closed-loop control.

Open-loop control

German standard DIN 19 226 defines open-loop control as a process taking place in a system where by one or more variables in the form of input variables exert influence on other variables in the form of output variables by reason of the laws which characterize the system.
The distinguishing feature of open-loop control is the open nature of its action, that is, the output variable does not have any influence on the input variable.

Example

Volumetric flow is set by adjusting a control valve. At constant applied pressure, the volumetric flow is directly influenced by the position of the control valve. This relationship between control valve setting and volumetric flow can be determined either by means of physical equation or by experiment. This results in the definition of a system consisting of the "valve" with the output variable "volumetric flow" and the input variable "control valve setting" (see Picture 1).

Picture 1: Open-loop control of volumetric flow setting


This system can be controlled by adjusting the control valve. This allows the desired volumetric flow to be set. However, if the applied pressure fluctuates, the volumetric flow will also fluctuate. In this open system, adjustment must be made manually. If this adjustment is to take place automatically, the system must have closed-loop control.

Closed-loop control

DIN 19 226 defines closed-loop control as a process where the controlled variable is continuously monitored and compared with the reference variable. Depending on the result of this comparison, the input variable for the system is influenced to adjust the output variable to the desired value despite any disturbing influences. This feedback results in a closed-loop action. This theoretical definition can be clarified using the example of volumetric flow control.

Deviation

Example: The volumetric flow (the output variable) is to be maintained at the predetermined value of the reference variable. First a measurement is made and this measurement is converted into an electrical signal. This signal is passed to the controller and compared with the desired value. Comparison takes place by subtracting the measured value from the desired value. The result is the deviation.

Manipulating element

In order to automatically control the control valve with the aid of the deviation, an electrical actuating motor or proportional solenoid is required. This allows adjustment of the controlled variable. This part is called the manipulating element (see Picture 2).

Picture 2: Closed-loop control of volumetric flow


The controller now passes a signal to the manipulating element dependent on the deviation. If there is a large negative deviation, that is the measured value of the volumetric flow is greater than the desired value (reference variable) the valve is closed further. If there is a large positive deviation, that is the measured value is smaller than the desired value, the valve is opened further.

Setting of the output variable is normally not ideal:

>> If the intervention is too fast and too great, influence at the input end of the system is too large. This results in great fluctuations at the output;
>> If influence is slow and small, the output variable will only approximate to the desired value.

In addition, different types of systems (control system) require different control strategy. Systems that respond slowly must be adjusted carefully and with forethought. This describes some of the control engineering problems faced by the closed-loop control engineer.

Design of a closed-loop control requires the following steps:

>> Determine manipulated variable (thus defining the controlled system);
>> Determine the behaviour of the controlled system;
>> Determine control strategy for the controlled system (behaviour of the "controller" system);
>> Select suitable measuring and manipulating elements.

Controlled System and Control Algorithm

Control Algorithm

A control algorithm is a mathematical expression of a control function. Using the temperature control loop example, V in the equation below is the fuel valve position, and e is the error. The relationship in a control algorithm can be expressed as:

V = f(±e)

The fuel valve position (V) is a function (f) of the sign (positive or negative) of the error (Picture 1).

Picture 1: Control Algorithm Example


Control algorithms can be used to calculate the requirements of much more complex control loops than the one described here. In more complex control loops, questions such as “How far should the valve be opened or closed in response to a given change in setpoint?” and “How long should the valve be held in the new position after the process variable moves back toward setpoint?” need to be answered.

Manual and Automatic Control

Before process automation, people, rather than machines, performed many of the process control tasks. For example, a human operator might have watched a level gauge and closed a valve when the level reached the setpoint. Control operations that involve human action to make an adjustment are called manual control systems. Conversely, control operations in which no human intervention is required, such as an automatic valve actuator that responds to a level controller, are called automatic control systems.

Controlled System

The controlled system has an input variable and an output variable. Its response is described in terms of dependence of the output variable on the input variable. These responses between one or several variables can normally be described using mathematical equations based on physical laws. Such physical relationships can be determined by experimentation. Controlled systems are shown as a block with the appropriate input and output variables (see Picture 2).

Picture 2: Block diagram of a Controlled System

Example

A water bath is to be maintained at a constant temperature. The water bath is heated by a helical pipe through which steam flows. The flow rate of steam can be set by means of a control valve. Here the control system consists of positioning of the control valve and the temperature of the water bath. This result in a controlled system with the input variable "temperature of water bath" and the output variable "position of control valve" (see Picture 3).

Picture 3: Water bath Controlled System


The following sequences take place within the controlled system:

>> The position of the control valve affects the flow rate of steam through the helical pipe.
>> The steam flow-rate determines the amount of heat passed to the water bath.
>> The temperature of the bath increases if the heat input is greater than the heat loss and drops if the heat input is less than the heat loss.
>> These sequences give the relationship between the input and output variables.


The advantage of creating a system with input and output variables and representing the system as a block is that this representation separates the problem from the specific equipment used and allows a generic view. All sorts of controlled systems demonstrate the same response and can therefore be treated in the same way.

Process Control Terminology

As in any field, process control has its own set of common terms that you should be familiar with and that you will use when talking about control technology.

The Control Loop

Imagine you are sitting in a cabin in front of a small fire on a cold winter evening. You feel uncomfortably cold, so you throw another log on the fire. This is an example of a control loop. In the control loop, a variable (temperature) fell below the setpoint (your comfort level), and you took action to bring the process back into the desired condition by adding fuel to the fire. The control loop will now remain static until the temperature again rises above or falls below your comfort level.

Control loops in the process control industry work in the same way, requiring three tasks to occur:

❑ Measurement
❑ Comparison
❑ Adjustment

Many different instruments and devices may or may not be used in control loops (e.g., transmitters, sensors, controllers, valves, pumps), but the three tasks of measurement, comparison, and adjustment are always present.

Closed-loop Control Technology

 

Variables such as pressure, temperature or flow-rate often have to be set on large machines or systems. This setting should not change when faults occur. Such tasks are undertaken by a closed-loop controller. Control engineering deals with all problems that occur in this connection. The controlled variable is first measured and an electrical signal is created to allow an independent closed-loop controller to control the variable.
The measured value in the controller must then be compared with the desired value or the desired-value curve. The result of this comparison determines any action that needs to be taken. Finally a suitable location must be found in the system where the controlled variable can be influenced (for example the actuator of a heating system). This requires knowledge of how the system behaves.

Closed-loop control technology attempts to be generic – that is, to be applicable to various technologies. Most text books describe this with the aid of higher mathematics. The EMW articles describes the fundamentals of closed-loop control technology with minimum use of mathematics.

Reference Variable

 

In closed-loop control the task is to keep the controlled variable at the desired value or to follow the desired-value curve. This desired value is known as the reference variable.

Process Variable

 

A process variable is a condition of the process fluid (a liquid or gas) that can change the manufacturing process in some way. In the example of you sitting by the fire, the process variable was temperature. In the example of the filling tank, the process variable is level. Common process variables include:

❑ Pressure (Pressure in a pneumatic accumulator; Pressure of a hydraulic press;)
❑ Flow (Flow-rate of coolant in a heat exchanger)
❑ Level
❑ Temperature (Temperature in a galvanizing bath; Temperature of air, water..)
❑ Density
❑ Ph (acidity or alkalinity)
❑ Liquid interface (the relative amounts of different liquids that are combined in a vessel)
❑ Mass
❑ Conductivity
❑ Speed (Feed speed of a machine tool with electrical drive)

The variable that is subject to control is also called the controlled variable. So, the process variable and the controlled variable are two terms refering to one same thing.

Setpoint

 

The setpoint is a value for a process variable that is desired to be maintained. For example, if a process temperature needs to kept within 5 °C of 100 °C, then the setpoint is 100 °C. A temperature sensor can be used to help maintain the temperature at setpoint. The sensor is inserted into the process, and a contoller compares the temperature reading from the sensor to the setpoint. If the temperature reading is 110 °C, then the controller determines that the process is above setpoint and signals the fuel valve of the burner to close slightly until the process cools to 100 °C. Set points can also be maximum or minimum values. For example, level in tank cannot exceed 20 feet.

Manipulated Variable

 

The controlled variable in any system can be influenced. This influence allows the controlled variable to be changed to match the reference variable (desired value). The variable influenced in this way is called the manipulated variable. Examples of manipulated variable are:

❑ Position of the venting control valve of a air reservoir
❑ Position of a pneumatic pressure-control valve
❑ Voltage applied to the electrical heater of a galvanizing bath
❑ Position of the control valve in the coolant feed line
❑ Position of a valve in a chemical feed line
❑ Voltage on the armature of a DC motor


In the temperature control loop example, the measured variable is temperature, which must be held close to the desired value. In this example and in most instances, the measured variable is also the process variable. The measured variable is the condition of the process fluid that must be kept at the designated setpoint.

Sometimes the measured variable is not the same as the process variable. For example, a manufacturer may measure flow into and out of a storage tank to determine tank level. In this scenario, flow is the measured variable, and the process fluid level is the process variable. The factor that is changed to keep the measured variable at setpoint is called the manipulated variable. In the example described, the manipulated variable would also be flow.

Error

 

Error is the difference between the measured variable and the setpoint and can be either positive or negative. In the temperature control loop example, for instance, the error is the difference between the 110 °C measured variable and the 100 °C setpoint—that is, the error is +10 °C.
The objective of any control scheme is to minimize or eliminate error. Therefore, it is imperative that error be well understood. Any error can be seen as having three major components. These three components are shown on the Picture 1.

Picture 1: Components of Error

Magnitude

 

The magnitude of the error is simply the deviation between the values of the setpoint and the process variable. The magnitude of error at any point in time compared to the previous error provides the basis for determining the change in error. The change in error is also an important value.

Duration

 

Duration refers to the length of time that an error condition has existed.

Rate Of Change

 

The rate of change is shown by the slope of the error plot (Picture 1).

Offset

 

Offset is a sustained deviation of the process variable from the setpoint. In the temperature control loop example, if the control system held the process fluid at 100.5 °C consistently, even though the setpoint is 100 °C, then an offset of 0.5 °C exists.

Load Disturbance

 

A load disturbance is an undesired change in one of the factors that can affect the process variable. In the temperature control loop example, adding cold process fluid to the vessel would be a load disturbance because it would lower the temperature of the process fluid.

Controlled System

 

There are complex relationships between the manipulated variable and the controlled variable. These relationships result from the physical interdependence of the two variables. The part of the control that describes the physical processes is called the controlled system.

The Importance of Process Control

PROCESS

Process as used in the terms process control and process industry, refers to the methods of changing or refining raw materials to create end products. The raw materials, which either pass through or remain in a liquid, gaseous, or slurry (a mix of solids and liquids) state during the process, are transferred, measured, mixed, heated or cooled, filtered, stored, or handled in some other way to produce the end product. Process industries include the chemical industry, the oil and gas industry, the food and beverage industry, the pharmaceutical industry, the water treatment industry, and the power industry.

PROCESS CONTROL

Process control refers to the methods that are used to control process variables when manufacturing a product. For example, factors such as the proportion of one ingredient to another, the temperature of the materials, how well the ingredients are mixed, and the pressure under which the materials are held can significantly impact the quality of an end product. Manufacturers control the production process for three reasons:

❑ Reduce variability
❑ Increase efficiency
❑ Ensure safety

Reduce Variability

Process control can reduce variability in the end product, which ensures a consistently high-quality product. Manufacturers can also save money by reducing variability. For example, in a gasoline blending process, as many as 12 or more different components may be blended to make a specific grade of gasoline. If the refinery does not have precise control over the flow of the separate components, the gasoline may get too much of the high-octane components. As a result, customers would receive a higher grade and more expensive gasoline than they paid for, and the refinery would lose money. The opposite situation would be customers receiving a lower grade at a higher price.

Reducing variability can also save money by reducing the need for product padding to meet required product specifications. Padding refers to the process of making a product of higher-quality than it needs to be to meet specifications. When there is variability in the end product (i.e., when process control is poor), manufacturers are forced to pad the product to ensure that specifications are met, which adds to the cost. With accurate, dependable process control, the setpoint (desired or optimal point) can be moved closer to the actual product specification and thus save the manufacturer money.

Increase Efficiency

Some processes need to be maintained at a specific point to maximize efficiency. For example, a control point might be the temperature at which a chemical reaction takes place. Accurate control of temperature ensures process efficiency. Manufacturers save money by minimizing the resources required to produce the end product.

Ensure Safety

A run-away process, such as an out-of-control nuclear or chemical reaction, may result if manufacturers do not maintain precise control of all of the process variables. The consequences of a run-away process can be catastrophic. Precise process control may also be required to ensure safety. For example, maintaining proper boiler pressure by controlling the inflow of air used in combustion and the outflow of exhaust gases is crucial in preventing boiler implosions that can clearly threaten the safety of workers.

Starting of Induction Motor and Speed Control

Some induction motors can draw over 1,000% of full-load current during starting (though a few hundred percent is more common). Small motors of a few kilowatts or smaller can be started by direct connection to the power line. Starting larger motors can cause line voltage sag, affecting other loads. Motor-start rated circuit breakers (analogous to "slow blow" fuses) should replace standard circuit breakers for starting motors of a few kilowatts. This breaker accepts high over-current for the duration of starting.

Picture 1: Autotransformer induction motor starter


Motors over 50 kW use motor starters to reduce line current from several hundred to a few hundred percent of full-load current. An intermittent duty autotransformer may reduce the stator voltage for a fraction of a minute during the start interval, followed by application of full line voltage as shown on Picture 1 above. Closure of the S contacts applies reduced voltage during the start interval. The S contacts open and the R contacts close after starting. This reduces starting current to, say, 200% of full-load current. Since the autotransformer is only used for the short start interval, it may be sized considerably smaller than a continuous duty unit.

Running 3-phase motors on 1-phase

3-phase motors will run on single phase as readily as single-phase motors. The only problem for either motor is starting. Sometimes 3-phase motors are purchased for use on single-phase if 3-phase power is anticipated. The power rating needs to be 50% larger than for a comparable single phase motor to make up for one unused winding. Single phase is applied to a pair of windings simultaneous with a start capacitor in series with the third winding. The start switch is opened upon motor start, as shown on Picture 2 below. Sometimes a smaller capacitor than the start capacitor is retained while running.

Picture 2: Starting a 3-phase motor on single phase


The circuit for running a 3-phase motor on single phase is known as “add a phase” or various other brand names. “Add a phase” supplies a phase approximately midway 90 degrees between the 180 degrees single-phase power source terminals.

Speed control with multiple fields

Induction motors may contain multiple field windings. For example, a 4-pole and an 8-pole winding corresponding to 1,800 and 900 rpm synchronous speeds. Energizing one field or the other is less complex than rewiring the stator coils, as shown on Picture 3 below.

Picture 3: Multiple fields allow speed change


If the field is segmented with leads brought out, it may be rewired (or switched) from 4-pole to 2- pole as shown above for a 2-phase motor. The 22.5o segments are switchable to 45o segments. Only the wiring for one phase is shown above for clarity. Thus, our induction motor may run at multiple speeds. When switching the above 60 Hz motor from 4 poles to 2 poles, the synchronous speed increases from 1,800 rpm to 3,600 rpm. If the motor is driven by 50 Hz, the corresponding 4-pole and 2-pole synchronous speeds would be:

Ns = 120 f/P = 120 x 50/4 = 1,500 rpm (4-pole)

Ns = 3,000 rpm (2-pole)

Speed control with variable voltage

The speed of small squirrel cage induction motors for applications such as driving fans may be changed by reducing the line voltage. This reduces the torque available to the load which reduces the speed. The torque of motor expressed in % is shown on Picture 4.

Picture 4: Variable voltage controls induction motor speed

Electronic speed control

Modern solid-state electronics increase the options for speed control. By changing the 50 or 60 Hz line frequency to higher or lower values, the synchronous speed of the motor may be changed. However, decreasing the frequency of the current fed to the motor also decreases reactance XL which increases the stator current. This may cause the stator magnetic circuit to saturate with disastrous results. In practice, the voltage to the motor needs to be decreased when frequency is decreased.

Picture 5: Electronic variable speed drive

Conversely, the drive frequency may be increased to increase the synchronous speed of the motor. However, the voltage needs to be increased to overcome increasing reactance to keep current up to a normal value and maintain torque. The inverter (Picture 5 above) approximates sine-waves to the motor with pulse width modulation outputs. This is a chopped waveform which is either 'on' or 'off', 'high' or 'low' with the percentage of 'on' time corresponds to the instantaneous sine wave voltage. Once electronics is applied to induction motor control, many control methods are available varying from the simple to complex:

Summary for Speed control:

● Scalar Control: Low-cost method described above to control only voltage and frequency without feedback.
● Vector Control (also known as vector phase control): The flux and torque producing components of stator current are measured or estimated on a real-time basis to enhance the motor torque-speed curve. This is computation-intensive.
● Direct Torque Control: An elaborate adaptive motor model allows more direct control of flux and torque without feedback. This method quickly responds to load changes.

Short Summary for Tesla Poly-Phase Induction Motors:

● A poly-phase induction motor consists of a polyphase winding embedded in a laminated stator and a conductive squirrel cage embedded in a laminated rotor.
● 3-phase currents flowing within the stator create a rotating magnetic field which induces a current, and consequent magnetic field in the rotor. Rotor torque is developed as the rotor slips a little behind the rotating stator field.
● Unlike single-phase motors, poly-phase induction motors are self-starting.
● Motor starters minimize loading of the power line while providing a larger starting torque than required during running. Starters are only required for large motors.
● Multiple field windings can be rewired for multiple discrete motor speeds by changing the number of poles.

Induction Motor Alternator

An induction motor may function as an alternator if it is driven by a torque at greater than 100% of the synchronous speed, as shown on Picture 1 below. This corresponds to a few % of “negative” slip (say, -1% slip). This means that as we are rotating the motor faster than the synchronous speed, the rotor is advancing 1% faster than the stator rotating magnetic field. It normally lags by 1% in a motor. Since the rotor is cutting the stator magnetic field in the opposite direction (leading), the rotor induces a voltage into the stator feeding electrical energy back into the power line.

Picture 1: Negative torque makes induction motor into generator


Such an induction generator must be excited by a “live” source of 50 or 60 Hz power. No power can be generated in the event of a power company power failure. This type of alternator appears to be unsuited as a standby power source. As an auxiliary power wind turbine generator, it has the advantage of not requiring an automatic power failure disconnect switch to protect repair crews. It is fail-safe. Small remote (from the power grid) installations may be make self-exciting by placing capacitors in parallel with the stator phases. If the load is removed residual magnetism may generate a small amount of current flow. This current is allowed to flow by the capacitors without dissipating power. As the generator is brought up to full speed, the current flow increases to supply a magnetizing current to the stator. The load may be applied at this point. Voltage regulation is poor. An induction motor may be converted to a self-excited generator by the addition of capacitors.

Start-up procedure is to bring the wind turbine up to speed in motor mode by application of normal power line voltage to the stator. Any wind-induced turbine speed in excess of synchronous speed will develop negative torque, feeding power back into the power line and reversing the normal direction of the electric kilowatt-hour meter.

Whereas an induction motor presents a lagging power factor to the power line, an induction alternator presents a leading power factor. Induction generators are not widely used in conventional power plants. The speed of the steam turbine drive is steady and controllable as required by synchronous alternators. Synchronous alternators are also more efficient. The speed of a wind turbine is difficult to control and subject to wind speed variation by gusts. An induction alternator is better able to cope with these variations due to the inherent slip. This stresses the gear train and mechanical components less than a synchronous generator. However, this allowable speed variation only amounts to about 1%. Thus, a direct line connected induction generator is considered to be fixed-speed in a wind turbine. See Doubly-fed induction generator for a true variable speed alternator. Multiple generators or multiple windings on a common shaft may be switched to provide a high and low speed to accommodate variable wind conditions.

Power Factor and Efficiency of Induction Motors

Power factor

Induction motors present a lagging (inductive) power factor to the power line. The power factor in large fully loaded high speed motors can be as favorable as 90% for large high speed motors. At ¾ fullload, the largest high speed motor power factor can be 92%. The power factor for small low-speed motors can be as low as 50%. At starting, the power factor can be in the range of 10% to 25%, rising as the rotor achieves speed.

Power factor (PF) varies considerably with the motor mechanical load (Picture 1). An unloaded motor is analogous to a transformer with no resistive load on the secondary. Little resistance is reflected from the secondary (rotor) to the primary (stator). Thus the power line sees a reactive load as low as 10% PF. As the rotor is loaded an increasing resistive component is reflected from rotor to stator, increasing the power factor.

Picture 1: Induction motor power factor and efficiency

Efficiency

Large 3-phase motors are more efficient than smaller 3-phase motors and most all single phase motors. Large induction motor efficiency can be as high as 95% at full load, though 90% is more common. Efficiency for a lightly load or no-loaded induction motor is poor because most of the current is involved with maintaining magnetizing flux. As the torque load is increased, more current is consumed in generating torque while current associated with magnetizing remains fixed. Efficiency at 75% FLT can be slightly higher than that at 100% FLT. Efficiency is decreased a few percent at 50% FLT and decreased a few more percent at 25% FLT. Efficiency only becomes poor below 25% FLT. The variation of efficiency with loading is shown on Picture 1 above.

Induction motors are typically oversized to guarantee that their mechanical load can be started and driven under all operating conditions. If a poly-phase motor is loaded at less than 75% of rated torque where efficiency peaks, efficiency suffers only slightly down to 25% FLT.

Nola power factor corrector

Frank Nola of NASA proposed a power factor corrector (PFC) as an energy saving device for single phase induction motors in the late 1970's. It is based on the premise that a less than fully-loaded induction motor is less efficient and has a lower power factor than a fully loaded motor. Thus, there is energy to be saved in partially loaded motors (1-φ motors in particular).

The energy consumed in maintaining the stator magnetic field is relatively fixed with respect to load changes. While there is nothing to be saved in a full- loaded motor, the voltage to a partially-loaded motor may be reduced to decrease the energy required to maintain the magnetic field. This will increase power factor and efficiency. This was a good concept for the notoriously inefficient single-phase motors for which it was intended.

This concept is not very applicable to large 3-phase motors. Because of their high efficiency (90+%), there is not much energy to be saved. Moreover, a 95% efficient motor is still 94% efficient at 50% full-load torque (FLT) and 90% efficient at 25% FLT. The potential energy savings in going from 100% FLT to 25% FLT is the difference in efficiency 95% - 90% = 5%. This is not 5% of the full load wattage but 5% of the wattage at the reduced load. The Nola power factor corrector might be applicable to a 3-phase motor which idles most of the time (below 25% FLT) - like a punch press. The payback period for the expensive electronic controller has been estimated to be unattractive for most applications, although it might be economical as part of an electronic motor starter or speed control.