Open Loop and Closed Loop Control System Block Diagram

Block Diagram Illustrating a System:

Control System:

Control systems can be classified into two types depending on whether the output (controlled variable) effects the actuating signal or not. They are:

1. Open loop control system
2. Closed loop control system

(a) The Open Loop Control System:

A block diagram of system is shown an open loop system is shown in Figure. An open loop control system is that in which the control action is independent of the system input. Therefore an open loop system in utilizes a controller or controlling device to control the system process. The controller controls in such a way as to obtain the desired output without considering the actual system input.
Block Diagram of  Open Loop System
The controller uses an external power source to provide proper control action to the controlled system process. The open loop control system is characterized by definite relationship between the desired system output and actual system output without monitoring the actual system.

(b) The Closed Loop Control System:

In a closed loop control system the control action is dependent on the system output. In a closed loop control system the actual system output will be measured. This output is compared with the input. The resulting error signal is used for controlling the system output to obtain the desired value. The sequence of operations in a closed loop system is illustrated in Figure.
Sequence of operations in a Closed Loop System
(c) Features of the Open Loop Control System:

The outstanding features of the open loop system are
1. Moderate accuracy
2. Sensitive to surrounding conditions like vibrations voltage aging etc.
3. Slow response to input command and changes there by making the system slow and sluggish.

(d) Factors for the Accuracy of Open Loop Control System:

1. Accuracy of the designed input output relationship
2. The performance of the calibration over long intervals of time
3. The varying effects of environmental conditions

(e) Advantages of Open Loop System:

The following are the advantages of open loop system

1. Simplicity
2. Low cost.       

(f) Performance and Characteristics of the Closed Loop System:

A block diagram of the closed loop control system is shown in Figure. The operations in the closed loop system are performed by output measuring devices, comparators, amplifiers, controllers and system plant. The measuring devices produce signal proportional to the actual system output.
Block Diagram of a Closed Loop System

This signal is compared with the reference input. An error signal is generated. This error signal is amplified to produce an actuating signal. The actuating system operates the controller applying the control signal to the system plant. This changes the controlled system output to the desired value.
High accuracy fast responses to input signals and relatively independent to operating conditions are the characteristics of the closed loop control system.

(g) Accuracy of Closed Loop System

The accuracy of the closed loop control system is governed by the following factors:

1. Accuracy of the closed loop system.
2. Accuracy of the comparing device.
3. Accuracy of control element and its sensitivity.                                               

Regulated Power Supply with Short Circuit Protection

The simple way of providing over current protection is to use another transistor that controls the series transistor. The circuit is shown in Figure
Regulated Power Supply with Short Circuit Protection
In this circuit a current sensing resistor is connected in series with the series transistor. The voltage drop across the current sensing resistor, R4 will be the base emitter voltage of the transistor T3.
The value of this resistance R4 is chosen to be small. For example we connect a 1 W resistor and if the load current is less than 500 mA; the voltage across the resistor will be less than 0.6 V. Therefore T3 will not function. The circuit works normally. If the load current increase beyond 600 mA the voltage drop across the resistance R4 , will be sufficient to turn on the transistor T3.
The collector current of T3 flows through R3. It decreases the base voltage to the series transistors T2. This in turn decreases the output voltage preventing further increase in load current. If we represent circuited load current to be ISC, the voltage drop across R4 is ISC R4. This is equal to the VBE of the transistor T3. Therefore;

VBE = ISCR4 ----------------- (1)

ISC = VBE/R4 ----------------- (2)

By properly choosing the value of resistance R4, the level of current limiting can be adjusted.
The disadvantage of this circuit is that large power dissipation takes place at the series transistor, when there is a short circuit at the output terminals.

Transistor Shunt Voltage Regulator Working

Transistor Shunt Voltage Regulator
Transistor Shunt Voltage Regulator Working is explained below,

In the transistor shunt voltage regulator shown in Figure, the regulating transistor is in shunt with the output terminals. If the output voltage Vo increases, the base-emitter junction of the transistor T1 becomes more forward biased so that its collector current increases. This collector current is the base current for the transistor T2. An increase of base current of T2 raises its collector current which is an amplified version of the collector current of T1. The collector current of T2 is the base current of the transistor T3. An enhancement of the base current of T3 increases its collector current which is a still further amplified form of its input current. An increase of the voltage V0 thus causes an increase of the collector current of T3, increasing thereby the voltage drop across the resistance R. The circuit is thus self-correcting, the increased drop across R reducing the voltage V0 which is thus stabilized. The transistor T3 has to handle a large collector current. So, it must be a power transistor.

Transistor Series Voltage Regulator Theory

Regulated Power Supply for continuous variation of Output:

The circuit of a continuously variable regulated power supply employing negative feedback is shown in Figure. The unregulated dc supply is fed to the voltage regulator. The circuit maintains constant output voltage inspite of variations in load or input voltage. T1 and T2 are the two transistors connected in the circuit, as direct coupled feed back amplifier. In this circuit the output voltage variations are returned as feed back to oppose the input changes. The base of transistor T2 is connected to the variable tap of the potentiometer R3.

Supposing the output voltage increases by any reason, it causes an increase in the voltage across the P & Q points, which is the part of the potential divider, consisting of R2, R3 and R4. This raises the base emitter voltage of the transistor T2. This increases the collector current of T2 and most of it flows through R1. This makes the voltage at the base of T1 to decrease. This low voltage on the base emitter junction of T1, tends to decrease the output voltage, resorting it to the original value. The reverse will happen if the input voltage decreases. The zener diode is used for providing a reference voltage and the difference is amplified by the transistor T2. The potentiometer R3 serves as the voltage controller. The output can be made to vary from zero to the maximum by varying the potentiometer R3. The Circuit Diagram of Transistor Series Voltage Regulator is shown in the circuit below. Also Working is explained.
Transistor Series Voltage Regulator circuit diagram
The Transistor Series Voltage Regulator Theory is explained as follows.

From the circuit of Figure, we can observe that the feedback voltage VF, is the voltage developed across the part of the potential divider P & Q. If we represent that resistance by Rf, the feed back fraction will be, Rf/(Rf + Ra) where Ra is the part of the potential divider R & P as shown in figure.
The closed loop voltage gain of the transistor T1 is proportional to the reciprocal of the feedback fraction.

Therefore, the closed loop gain A' = (1/Rf)/ (Rf + Ra) = 1 + (Ra/Rf) ----------------- (1)

Considering the closed loop of the Zener diode which is the reference, the base emitter junction and the part of the potential divider, the sum of the voltages must be zero.

Hence -VZ - VBE + Vf= 0 or Vf = VZ + VBE ----------------- (2)

The feedback voltage is given by feedback fraction multiplied by the output voltage. If we represent the feed back fraction by ‘b’, we have

Vf = bVout = VZ + VBE ----------------- (3)

Therefore, Vout = (Vz + VBE)/β

As the closed loop gain is equal to 1/b we can write Vout = A' (Vz + VBE) ----------------- (4)

Where, Vout is the regulated output voltage.
A' is the closed loop voltage gain.
VZ is the zener voltage.

VBE is the base emitter voltage of T2

Power Rating Disadvantage: 

Normally the series regulator uses a power transistor as its series transistor. This transistor must be capable of withstanding the power dissipated during full load current. A proper heat sink is to be provided. The power dissipation is given by the product of the collector current (load current) and collector to emitter voltage.

When heavy load currents are expected, power transistors may be connected in darlington pair to act as the series transistor for the regulator circuit. This enhances the power handling ability of the regulated power supply of this type.

Though the series regulator type regulated power supply works satisfactorily, it has the disadvantage that it has no short circuit protection.

In any regulated power supply or in ordinary power supply the series diode will carry the load current. Accidental short circuiting of the output terminals or over drawing of current from the power supply leads to the excessive dissipation of power in the series transistor or diode, resulting in the destruction of the transistor or and the diode of the power supply circuit.

To prevent the destruction of the series transistor or the rectifying diode when accidental short circuit occurs or excessive current is drawn, we have to incorporate a current limiting device. When such a device is incorporated we call the circuit to be consisting of short circuit protection.

Load Regulation Formula


Load effect or load regulation indicates the extent to which the load current changes effect the output voltage. It is expressed as a ratio of the difference between no load voltage to full load voltage and no load voltage. It can be expressed as a fraction or as percentage. 

Load Regulation Formula

% load regulation = (No load voltage — Full load voltage) / No load voltage

The smaller the percentage the better is the performance of the power supply.

(a) Line Regulation or Source Effect:

It is the extent to which the output voltage of a regulated power supply will be held constant under the influence of the mains voltage variations. Typical value of line regulation is 0.01%.

The point to be noted here is that there must be a reasonable limit to the ability of the regulator to maintain output constant for line voltage variations. Therefore a range will be specified for the input voltage.

Example: 230 V, ± 10%.

That is the output of the regulated power supply will be constant even if the supply voltage has its changes between 207 V and 253 V.

(b) Periodic and Random Deviation:

The periodic and random deviation and ripple factor express the quality of the DC output from a regulated power supply. Periodic and random deviation (PARD) includes all periodic and random variations in the output voltage of the power supply. It is expressed as millivolts RMS or millivolts pp. Typical value is 1 mV RMS.

(c) Overload Protection:

It describes the ability of fixed and adjustable current limiters employed in the regulator circuit, from protecting the supply, or the circuit under test from excessive current. An overload indicator is usually provided that indicates the overload condition.

(d) Warm up Time and Drift:

Warm up time is the time the supply takes to stabilize its circuits to present a constant output. It is expressed in minutes.

Drift is the deviation from the rated output voltage with respect to time. It is expressed as a minimum percentage of change in the output voltage from the specified voltage over a time period.

Example: less than 3% in 12 hours.

Regulated Power Supply Specifications

Regulated Power Supply: 

The output voltage of a power supply stage changes due to the line supply voltage variation as well as the load current variation. When the line voltage decreases the output decreases and when it increases it increases. The output voltage of the power supply gets reduced with increasing load currents. The reason for the decrease in output voltage in a power supply stage with increase in load current is the drop across the rectifying diode, the drop across the transformer and the drop across the filter choke or the resistor.

From the characteristics desired out of a power supply stage we know that the output must be maintained constant irrespective of the line voltage change or load current change. In order to keep the output voltage constant for changing line voltage or changing load current we include a regulator circuit in a power supply stage.

A regulated power supply is one which generates constant supply voltage irrespective of the load variations or input variations.

Voltage regulation and current regulation also is carried on in the regulator circuit. From simple zener voltage regulator to feedback controlled voltage regulator there can be many types of voltage regulator circuits. Switched mode regulator circuits are also used.

Voltage regulation is to maintain the magnitude of the output voltage constant. Current regulation is to maintain a specific amount of load current and to limit the maximum magnitude of the load current. The regulator circuits will also be provided with short circuit protection.


The following are the important specifications of a regulated power supply. The specifications differ from manufacturer to manufacturer. However, majority of them are common.

1. Input voltage: 230 V 50 Hz mains voltage
2. Output voltage: As per requirement of the circuit
3. Load regulation: 0.01% or even better
4. Ripple: Less than 1 mV RMS or even better
5. Protection: Short circuit protection for indefinite period set by current limit control.

Output Voltage:

The output voltage may be a fixed value. This is the condition for power supplies used with equipment where the output voltage desired may be for example 48 V, 24, V 12 V, etc. Further there can be more than one independent output voltages derived from the same circuit.
Output voltage in case of laboratory type power supplies has a facility for coarse and fine variation of voltage and in such cases the maximum voltage will be specified. Ex. 0-30 V, This voltage will be maintained constant, by virtue of the regulator circuit, irrespective of variation in line voltage or load current variation.

Power Supply Characteristics

Any power supply ideally must have the following characteristics

1. The magnitude of the output must be presented accurately and precisely.
2. The ripple content of the output must be zero.
3. The output voltage must have high stability irrespective of load current and line voltage changes.
4. The source impedance must be zero.
5. It should be portable.

Sources of DC Voltage: 

The following is the list of DC voltage sources and shows the application of the type of source:

Primary cells (Dry cell)
For use in low power applications like radios, Calculators, Multimeters, Tape recorders, etc.
Secondary cells (Wet Batteries)
For use as backup batteries for PA amplifiers, Inverters, UPS etc
AIkaline batteries
Low power sources mostly with portable instruments.
Nickel Cadmium cells
For large load currents.
Lithium Sulphur dioxide
For large load current requirement
Lithium iodine cells
For large load current requirement
DC Voltage standard
For calibration of instruments
The Laboratory Power supply
To conduct laboratory experiments
Line operated Power supplies
For low medium and High power applications
Uninterrupted Power supplies
Computer centers, telephone exchanges, transmitters, Microwave stations, repeaters etc

Power supply:

The commonly used power supply, with electronic circuits is the line operated power supply. In this the mains voltage is utilized in a transformer or directly, to work a rectifier. The rectified output, will be filtered using a filter circuit, that eliminates the ripple content of the rectified output. The output of the power supply will be pulsating unidirectional current. The block diagram of the power supply is shown in Figure.
Block Diagram of a line operated power supply

A generalized block diagram of the power supply stage is shown in Figure. The transformer isolates the load from the mains power supply ground. It provides either a step up voltage or step down voltage in its secondary. Taps or center tap or both can be provided at the secondary of the transformer.

A rectifier converts the alternating current to pulsating unidirectional current. Mostly Full wave rectifier, or bridge rectifier circuits are used as rectifier circuits.

The alternating component of the rectified voltage is called the ripple voltage. The ripple should not be present and it should be minimized. A filter circuit is used to filter off the ripple contained in the output of the rectifier.

The bleeder resistor is connected across the output terminals of the rectifier, to present a light load on the power supply. It helps in maintaining constant DC voltage in the output of the rectifier. Further it can be of a potential divider type to tap the different voltages required as per the circuit conditions.

The magnitude of the output voltage and the amount of current that can be drawn from the power supply depend on the design of the power supply and the demand of the circuit.

Power Supplies Introduction

Power Supplies Introduction: 

For any electronic equipment to function a power supply stage is necessary. The power supply stage provides the different voltages required by the various devices incorporated in the equipment. The electronic equipment requires D.C. power supplies, with varied specifications. The specifications depend on the type of equipment or the instrument. Currents ranging from few microamperes to several hundreds of amperes and voltage from as low as 1.5 V to several kilo volts are required.

In addition steady voltage (DC voltage) is also required to set the level in digital circuits to establish the high level (1 state) differentiating it from the low state (0 state). This voltage level will be dependent on the digital system requirement.

Further control voltages for functioning of several systems in automatic control systems are also required.

Initially use of primary cells (dry cells) and secondary cells (wet batteries / lead acid cells) were used to supply the DC requirement in electronic circuits. The primary cells are of the disposable type; where as the secondary cells are rechargeable. Disposable alkaline cells were used in portable instruments. Nickel Cadmium cells are used when the current requirement is large. They are rechargeable Lithium based primary cells will find their use in miniature equipment. Lithium iodine and lithium sulphur dioxide cells are of the rechargeable type, are used where the current requirements are large.

It is to be noted that the cost of the batteries plays an important role in their use. Mostly disposable types are not economical. Rechargeable types can be used for low power equipment. Solar cells are used in satellites to energize the electronic circuitry. They are also used in miniature equipment like calculators, digital clocks etc, that take minute current for their operation.

The most common method of supplying power to electronic equipment is to use line operated power supply. There are power supplies for low, medium and high power equipment, use rectifier circuits that take the alternating voltage, either from the single phase, three phase or poly phase transformer and work a rectifier circuit designed as per the requirement.

Improvement in the performance of a power supply can be achieved using regulated power supply circuits that work on feed back principle. The present trend is to use a switched mode power supply that has many advantages over the conventional type.

Uninterrupted power supplies are now in use for continuous power supply to electronic equipment.

Computer Supervisory Control System

Implementation of the Process-Control Loop using a Computer in a Supervisory Capacity.
The expression supervisory digital control is used to describe a situation in which digital techniques are employed to supervise analog control loops. Generally, in these cases, a computer is employed to adjust the setpoint of an analog controller. This approach is employed to take advantage of the well-known ad trusted performance of analog control, while at the same time using the various advantages of computer supervision. In particular, the computer can examine many variables and solve complicated control equations to determine and then set optimum set points of several analog loops. This is particularly important when interaction exists between variables, such as changing the temperature set point and causing the pressure to vary. The block diagram of a supervisory system is shown in figure. In general, the logic or computer supervisor will input the measured quantity and programming and output a calculated set point. In the analog loop described, a hybrid conversion could be developed by using ADCs and DACs to provide temperature input and reference voltage output to analog loop.

Direct Digital Control Block Diagram

The method of process control described by the term direct digital control (DDC) applies to those cases in which digital logic circuits or a computer are an integral part of the loop. In figure 1, we see a block diagram of this approach to processing. Essentially, the evaluation and controller function is taken over by digital logic circuits or programming of a computer. We distinguish these two approaches to DDC by calling logic circuit methods hardware program controlling and computer methods software program controlling. Figure 2 shows how the temperature-control problem defined earlier is implemented by DDC. The control function; set point, and deviation about all nominal are all defined by the program.

Direct digital control has the capacity to control multivariable processes with interaction between elements. This approach is gaining in acceptance as the reliability of computers improves and backup methods are being developed to avoid process shutdown because of a computer failure. The development of the so-called computer on a chip in the form of a single integrated circuit (IC) microprocessor has given considerable impetus to the use of DDC.
Figure 1. Block Diagram of a Process-Control Loop which is Under Computer Based, Direct Digital Control (DDC)
Figure 2. Implementation of the Problem using a computer.

Digital Processing

In digital processing, all data passed in the process-control loop is encoded into a signal. Binary refers to a numbering system with a base of 2, that is, 0 and 1 are the only possible counting states.

The electrical signals on a wire are represented either by a binary "0", denoted also by low (L) and corresponding in transistor-transistor logic (TTL) to about 0 volts, or a binary "1", denoted by high (H) and corresponding in TTL to about +5 volts. Thus, in TTL, electrical voltages are only 0 to 5 volts and hence cannot be an analog representation of the dynamic variable. The importance of the dynamic variable signifies as various encoding of the binary levels. The encoding itself is a correspondence between a set of binary numbers and the analog signal to be encoded. The set of binary numbers is commonly referred to as a word, which may contain many binary counts called bits.

Binary/decimal encoding : 

Let us consider the encoding between a 4-bit binary word and a decimal voltage signal. Such a coding can be simply the representation of numbers between the two systems, as shown in table. Thus, if we wished to encode a 5-volt signal, where each bit corresponds to i volt, we would represent this in a 4-bit binary words as "0101".

Signal transmission:

The encoding of a signal into a binary word implies that a sequence of 0 and 1 levels corresponds to the value of the signal. There are two methods by which a digitally encoded signal can be transmitted through the process-control loop. One method referred to as the parallel transmission mode, provides a separate wire for each binary number in the word. Thus, a 4-bit word requires 4 wires, an 8-bit wires, and so on. The alternate method is serial transmission mode of binary numbers over a single wire, where the binary levels are provided in a time sequence over the wire. These two methods are illustrated in figure for the digital encoding of the 5-volt level. In the parallel mode, the levels can remain set for any time on the lines, but in the serial mode, the levels appear only as a pulse train and must be interpreted in turn as they appear, introducing timing requirements.

Table, Decimal-Binary Encoding

Binary word

Analog and digital converters:

The sensor is most often a device that converts a variable to an electrical or pneumatic analog. Furthermore, the final control element is typically a device that converts a controller analog signal to some effect on the controlled variable in the process. If digital processing is to be employed in the process-control loop, must have a means of converting between the analog and digital representations of the variable and controller outputs. The analog-to-digital converter (ADC) performs the function of conversion of an analog input to a digitally encoded signal. These devices are designed to output a digital word coded to a specified range of signal inputs. Thus, an ADC might accept a 0-10 volt input encoded into a 4-bit word. A 4-bit can represent 15 spans; hence, each bit represents

10 volts/15 = 0.666 volts/state or volts/bit.
In figure, Digital information may be transmitted on Parallel Lines, One for each bit, or erially, where the information appears on a single line in a time sequence.

(a) Parallel Transmission Mode
(b) Serial Transmission Mode

If more voltage resolution is necessary, we may use a word with more bits. A 6-bit word would have 63 span and 10 volts/15 = 0.159 volts/bit.

The digital-to-analog converter (DAC) provides the reverse action of converting the digitally encoded word into an appropriate analog output; that is, it decodes. Here again, each bit, by design, will correspond to a certain level of output, and thus the resolution, or the smallest increment, is the level of one state change. The example of ADC from voltage is only one example of the type of analog signal that can be used. Other converters may use analog signals of frequency variation, current, or even resistance as the primary analog signal.

There are two approaches to digital processing in industrial-control situations. One involves using digital logic circuits or computers to supervise analog process-control loops and the other digital control of a dynamic variable.

Digital Process Controller


The development of process control has seen the blend of electronics technology into almost every aspect because of low cost, consistency, miniaturization and case of interface. It is natural that the further development of digital electronics and associated computer technology has brought about the rapid introduction of digital techniques in the field of process control. Some aspects of process control, such as the initial transduction of a controlled Variable into electrical information, will probably always be of analog nature. It is inevitable, however, with the continued development of computers, miniaturized digital electronics and associated technology that the evaluation and controller phase of process control may be digitally performed.


Consider the process-control loop shown in figure, where a process temperature is to be controlled. A thermistor, whose resistance R is proportional to and an analog of temperature, is used to measure the temperature. The resistance change is converted to a voltage V via some unspecified signal conditioning. This voltage is compared to some reference voltage (set point) by a differential amplifier (evaluation), the output of which activates either a heater or cooler. The range of allowed temperature is determined by the differential amplifier swing necessary to trip either relay. In this loop the temperature is represented by a proportional electrical signal. We then say that the electrical signal is an analog of the temperature. In the case of the thermistor, the resistance is an analog of the temperature.
In a similar fashion, in a pneumatic system, we may have a fluid pressure that is an analog of a variable. The analog relationship between processing signals and dynamic variables need not be linear and in many cases is not. The significant factor here is that the processing signal is a smooth and unique representation of the dynamic variable. In figure we see two examples of analog proportionalities between a variable c and an analog signal b. In case 1, we have a linear relationship and in case 2, a nonlinear relationship, but both are still analog representations of c.

The relationship between a Variable c and its measured equivalent b.

Digital Data Recording System

Magnetic tapes are used as the storage media in computers. There are two types:

The incremental method of recording
The synchronous method of recording

In the incremental digital data recording system, the arrangement is such that for each digital character, to be recorded, the tape steps ahead. Therefore storing of data in this method is very slow. The data may also be discontinuously recorded. The characters of the data are equally and precisely spread along the length of the tape.

In synchronous method of recording a start stop technique is used to run the tape. The tape moves no doubt at a constant speed of around 75 cm/s but it will be brought up to the speed and will then be stopped. During the run data inputs at precise rates, to the tune of tens of thousands of characters per second are recorded. A block of characters are recorded at one time.

The different characters of data in a block are equally spaced over the length of the tape. The blocks of data are separated by erased portion of the tape termed as record gap. The coded combinations of one bit represent the characters of the data.  They occupy appropriate tracks over the width of the tape.

The IBM accepted format of Non Return Zero technique is universally accepted for instrumentation recorders that use the magnetic tape as the media. In this method of recording data the tape will always be saturated either in positive or negative direction. The change in flux direction over the tape will indicate the '1' bit. No change in flux direction indicates a '0’ bit.

Advantages of Digital Data Recording System:

1. The accuracy will be high.
2. The equipment required is simple.
3. Digital recorders are not sensitive to tape speed.
4. The output can be interfaced to a computer for further processing or control.

Disadvantages of Digital Data Recording System:

1. Rich quality of tape is required for recording.
2. The analog information is to be converted in to the binary format.
3. Analog to digital and digital to analog converters are required.
4. Longer lengths of tape are required for storage of information.
5. Only sequential recording and reproduction is possible.

A recorder is an electromechanical instrument that produces a permanent record of one or more parameters as a function of another parameter.

XY Recorder Block Diagram & Working

An XY recorder plots the instantaneous relation between two variables. The writing pen will be deflected in both X direction and Y direction on a stationary chart paper. Depending on the desired application one or more write pens are used.

XY recorders are also employed using proper transducers for recording of physical quantities as function of other physical quantities. The motion of the pen in X and Y directions is obtained by servomotors. A sliding pen and moving arm arrangement is used with X-Y recorders.

(a) Description of the Block Diagram of an X-Y Recorder: 

The block diagram is shown in Figure. From the block diagram we find that the X input and Y input are supplied to the error detector in series with the standard reference voltage offered by the internal reference source.
The output of the error detector is given to a chopper. The servo amplifier is driven by the chopper. The amplifiers output drives the pen. The Y amplifier's output drives the arm. Square shaped graph paper will be used. It is fixed over a pad by electrostatic attraction or by vacuum.

(b) Working: 

The input signals are attenuated to around 0.5 mV which is withir the dynamic range of the recorder. Both X and Y signals are compared with the internal reference source. This is done in the balancing or Error detector block. The X and Y channel error output will be the DC error, which is the difference between the input signals and the reference voltage.
The DC error signal of both channels is used in the choppers to convert it in to an AC signal. The magnitude of the AC output of the choppers is insufficient to drive the motors of the pen and the arm. Therefore the output of the two choppers will be amplified in the servo amplifiers. The servomotors drive the pen and the arm. The pen and the arm execute motion in proper direction to reduce the error. The movement of the pen and arm is to bring the system to balance. The variation of X and Y signals, move the pen and the arm in the appropriate directions to keep the system in balance. This movement produces a record of the signal components on the paper. It is to be remembered that the both X and Y channels and the total system works simultaneously.

(c) Range:

 Input range variable from 0.25 V/cm to 10 V/cm
Accuracy ± 0.3 % to ± 0.1 % at full scale.
Slew rate and acceleration are important. Slew rate is expressed as displacement in second. Accelerating is expressed in cm/s. Slew rate refers to the movement of the pen in Y direction. Acceleration refers to the movement along X direction. Typical values of slew rate and acceleration are 97 cm/s and 7620 cm/s respectively with respect to high speed recorders.
Sensitivity obtainable is around 10 micro volt/mm.
Frequency response is around 6 Hz in both directions.

(d) Variations in Design: 

The X-Y recorder described above is of analog type. Digital X-Y plotters are available. These digital X-Y recorders employ stepper motors of the open loop type, instead of the servomotors used in analog recorders. Digital X-Y plotters have many advantages.

(e) Advantages of Digital Recorders: 

The following are the advantages of digital recorders
1. Number of input channels can be provided for sampling and storage simultaneously.
2. Number of desired trigger modes can be incorporated.
3. They have the provision to display pre-trigger data.
4. Multi-pen Multi Ink plotting is possible.
5. Analysis of records with respect to data, time and setup condition is possible.
6. They will be able to draw grids and axis.
7. Communication interface facility with other digital equipment is possible.
8. Desired specifications and functions are obtainable by the use of specially programmed software packages.

(f) Applications: 

X-Y recorders are used in recording:
1. The speed torque characteristics of motors.
2. Regulation characteristics of power supply
3. Characteristics of electronic devices like transistors and diodes etc.
4. Hysteresis curves, stress-strain characteristics.
5. Electrical characteristics of material. Ex: Resistance versus temperature.

Circular Chart Recorder with Diagram

It is clear from the name itself, that this recorder uses a circular medium for recording. A flat circular chart will be used for recording data in this recorder. The assembly of circular chart recorder is shown in Figure.
circular chart recorder diagram
The following are the component parts of this recorder:

1. Measuring element
2. Pen Lift
3. Pen Mounting
4. Base End
5. Chart Hub
6. Case
7. Chart Drive
8. Chart Plate
9. Door
10. Recording Pen 'V' Type
11. Time Indicator
12. Pen Arm
13. Operating mechanism

Circular Chart Recorder Working Principle:

All the above components are mounted on a single panel. The chart will be mounted on a flat supporting plate. Curling will be prevented by the use of spring clips. Measuring element may be a helical pressure tube or the like. Levers 'a','b' and 'c' convey motion from the measuring element to the recorder. To obtain best recording light uniform pressure and smooth chart surface are desirable. A timing device drives the chart at a uniform rate. To get the best results the pen is to be accurately fixed and locked. The block diagram of circular chart recorder is shown in figure.

Strip Chart Recorder with Block Diagram

The block diagram of a strip chart recorder is shown in Figure. The data will be recorded on a roll of chart paper. The paper continuously moves at a constant speed.

block diagram of strip chart recorder

The basic components of a strip chart recorder are:

1. Stylus [pen]: to mark on the paper
2. The stylus driving system
3. Chart paper drive system
4. Chart paper speed selector.

Generally a pointer will be attached to the stylus. This permits measurement of instantaneous value of the quantity under measurement directly on a calibrated scale.

A servo feedback system will be used to see that the displacement of the pen over the paper tracks the input voltage in the desired frequency range.

Commonly potentiometer system will be used to measure the position of the stylus. The uniform movement of the chart paper will be controlled by a stepper motor. The following data recording techniques are used:

1. Pen and Ink stylus
2. Impact printing
3. Thermal writing
4.  Electric writing
5. Optical writing

Potentiometric Recorder

The disadvantage of the galvanometer recorder can be eliminated by using an amplifier between the test terminals and the moving coil meter. However this reduces the accuracy of the record. In a potentiometric recorder the accuracy is improved by a process of comparison of input signal with a reference voltage. The reference voltage will be supplied from an internal source of the recorder itself. When the input signal is given it is applied in series with the reference signal such that the difference of the input signal and the reference signal will produce an error. If the input is lower than the reference voltage the error can be taken as negative. If the input is more than the reference voltage the error can be taken as positive. Only in one condition when the input equals the reference voltage the error will be zero.

The error signal will be given to the amplifier. The amplifier output will control a motor. The speed and direction of rotation of the motor depend on the output of the amplifier which is the error signal. Self balance is obtained by sliding the slider of the potentiometer to get a null output. Reversible motors are used in the D.C. system. In A.C. system a two phase motor is used.

(a) Constructional Details of Self Balancing Recorder (Block Diagram): 

Potentiometric recorder block diagram
The self balancing type recorder is also called the potentiometric recorder. T he constructional details are shown in figure. From the block diagram shown, it can be seen that the input signal is applied to the amplifier in series with the part of the potentiometer. The potentiometer is supplied with a reference voltage derived from an internal power supply. The output of the internal power supply is made stable.

The field coil of the motor is energized by the output of the amplifier. The output of the amplifier is the error signal. The variable arm/slider/wiper of the potentiometer is connected to the armature of the motor. The wiper carries a pen. The paper feed mechanism moves the paper with a constant speed.

(b) Potentiometric Recorder Working: 

It can be seen from the block diagram that the input signal and the part of the voltage across the potentiometer are in series. The difference between these voltages is the error signal. This error signal is available at the input terminals of the amplifier. The field coil of the motor is connected to the output of the amplifier. The construction of the motor is such that, it turns in a direction that rotates the wiper of the potentiometer to reduce the error The balance is obtained by the current through the armature of the motor flowing in one direction or the other depending on whether the input is higher or lower than the reference voltage. When the error reduces to zero the motor slows down and stops. At the instant of zero error the motor stops giving null balance.

As- the wiper of the potentiometer is driven by the motor's armature and as a pen is arranged over the wiper, the pen executes motion in the direction of movement of the wiper. As the armature moves in either direction depending on the error the pen moving in synchronized direction records the waveform. The paper feed motor will be synchronized with power line frequency. Capillary action of a recorder is defined as the process of establishing flow of ink from the reservoir through the tubing and into the hallow of the pen.

(c) Range: 

The bandwidth is around 0.8 Hz
The sensitivity is 4 mV/mm with an error of less than ± 0.25%

(d) Applications:

Mostly used to record process temperature
They are also used in the control of process temperature.

(e) Variations in Design:

Instruments that record variations of one variable under measurement are termed Single Point Recorders. There are multipoint recorders that record the waveform of many inputs. The maximum number of channels can be 24 and can extend up to 36. Colored recording in six colors is possible. The frequency ranges from 0 Hz to 5 kHz in improved models.