Showing posts with label Process-Control. Show all posts
Showing posts with label Process-Control. Show all posts

Sunday, February 05, 2017

Proportional Analog Dial Temperature Controller

This is a Low cost controller, Analog Dial Temperature Controller. It is also called Blind Controller. This essentially means Open Loop, just control the fuel or energy input to the system to regulate heat. This is not a Blind Controller that way, it only cannot display the temperature value, that could be another reason it is called blind.

Blind Temperature Controller

Dial cyclic timers were used to control heat, these were purely mechanical clockwork devices. They could regulate well, when the material flow (liquid) is constant and mains power is regulated. But when the job to be heated, varies in quantity, control temperature is close to ambient or when a precise control is required; closed loop controllers are used. Even a thermostat is like closed loop, as the bimetallic sensor is temperature dependent. But not good enough.

Blind Dial Proportional Temperature Controller

This controller is closed loop, precision controller, only the digital display of temperature is absent. Fine one deg variations may not be easy in this.

Thursday, May 12, 2016

Analog PID control using OpAmps

The Measured Value and The Setpoint are two inputs to a Control System. The Measured Value is the Amplified input of a Transducer or Sensor for some Parameter that needs to be controlled. It could be Pressure or Temperature...etc.

The Setpoint is the User Defined Input using a Potentiometer, Thumbwheel, EPROM or Flash Value. This is the value at which the process has to be maintained for that parameter.

Analog PID control using OpAmps

Industrial Process Control Circuits

The difference of these two is the Error, this is the input for this PID Analog Computation Stage. The three Opamps are configured as Proportional, Integrator and Differentiator Amps.  The Addition or Summation of these Values is the PID Control Output.(These days it is Math in the Firmware on a MCU, DSP or Software Application in SCADA)

This Analog PID Control Output can now be translated to a 4-20 mA Control Signal, that means 0-100% of power to the Actuator, which could be a Heater, Pump, Fan, Motor using AC/DC Drives. It could be a Steam Valve, Pneumatic or Hydraulic Motorized/Solenoids. The Actuator Size/Array must be right for the Process, a tiny fan cannot cool a Large Furnace, a small solenoid valve cannot fill a Big Tank. An effective Proportional or PID  control depends on choosing or designing the Sensor, Actuator and System Environment prudently.  

The Auto Reset is needed to ensure the Integrator does not dampen the Process so much that it fails to even raise to the Process value fast enough (Diffrentiator). So in the Proportional Band the Integrator is Active.

If the Setpoint is 1000 deg C, the proportional band is 10%. The Raise of temperature till 950 deg is Undampended. After that Integrator is called in by the Window Comparator made of two opamps, the integrator prevents OverShoot, Undershoot, Ringing and Oscillations.

The PID control output can also be a Time Proportional Output like PWM. With a large cycle time of 20 or More seconds. Like 2 Seconds on and 18 Seconds off for 10% Control.Fast Cycle times may be needed for small systems with less inertia.

Sunday, April 10, 2016

Voltage to Current Convertor using LM723

his Circuit converts a voltage control output from a Process Controller to be converted into a Current Control if the AC-Drive or Valve needs a Current Control Signal.

Significance of Current Loop 4 to 20 mA Standard

Voltage to Current Convertor using LM723

This is a three wire voltage to current loop converter. The 1-5 V DC is attenuated and fed to pin 5 LM723 opamp section which tries to maintain the same voltage at pin 10 across the 10 E, thereby producing a open collector constant current sink proportional to the 1-5V input. By trimming the attenuator you can scale-calibrate 1-5V input to 4-20mA output for looping many instruments in series, like a controller, recorder or PLC. With a supply voltage upto 24V, three instruments can be looped. The connection to pin 6 is required to convert 0-1 input to 4-20mA.

All the transmitter circuits can be seen here. Industrial Process Control Circuits

This circuit was designed by me in the eighties, the 555 was for negative supply, The whole thing went into the anodized cast aluminuim head of a sensor.

How 4-20mA Works

Sunday, January 17, 2016

Voltage to Current Source 4-20 mA

Here is a circuit that is a voltage to current converter but with a current source. That means it supplies a current from a positive terminal and can drive a remote shunt in a Measuring Instrument. The other end of the shunt returns to gnd or low.

More than one instrument can be connected in series if the the sum of the voltage drops in the instrument shunts are much less than the voltage 12V of the current source.

The 0-1V to 4-20 mA Converter published earlier is a current sink,

Voltage to Current Source 4-20 mA

You can use a LM358 or LM324. The first opamp is a Voltage to Current with a sink output. That current creates a varying voltage w.r.t the 12V DC supply, this varying voltage is mirrored by the second opamp across the source output resistor. This way a constant current is obtained with a sourcing output. The control elements are small signal high gain transistors. Any suitable equivalent can be used. Even the opamp can be chosen by the precision and application you want.

Industrial Process Control Circuits

In this form of feedback. way to understand .... "Op-Amp drives the output to maintain both inputs at the same level" and also the "Output takes the polarity of the dominant input" and lastly "dominant means, more positive". +5 is more Dominant than +3 or 0 or -2. Then -3 is more dominant than -12. See which is more positive.

Long distance of current loop may need higher voltage and lower source resistor value. Then the output transistor needs to change, if you use 24V DC then that voltage should not reach opamp. Design needs to foresee all possibilities of I/O troubles, as these are wired by a customer, mistakes happen. Hence, Industrial Designs have to be rugged.

Wednesday, October 14, 2015

Linearizing Circuit for Thermocouples

This circuit changes the gain of opamp U1B in four steps or segments. It can be used to get a linear output from most transducers to 1% levels.U1A is a amplifying buffer use it to boost the signal to the required level.
Linearizing Circuit for Thermocouples

The resistor values i have put are for an imaginary transducer, you have to design them. The buffered input signal is compared to reference switching points by LM339.

Temperature Measurement and Control

LM339 changes the gain resistors of U1B thru the mux switch 4066. JP1 to JP4 can select either amplification or attenuation of signal. The resistor switched by 4066 can be across R1 or R2 based on JP1 to JP4.

You may have to input transducer values into a spreadsheet and draw a graph. Then divide the graph into 5 segments and deduce the switch points and gain.

Wednesday, October 05, 2005

0-1V to 4-20 mA Converter

Ensure +5/-5 dual supply for chip TL062 IC3. Gnd is common ps ground, let grounds radiate from ground plane in one side of PCB. R3-R8 is an attenuator that may need to be designed or modified.
0-1V to 4-20 mA Converter

In output R23 is for protection from shorting of +5V supply, R23 can also go to an unregulated or external. supply upto 24V DC which is referenced to this circuits gnd. More voltage more distance.

Q2 is the current control device, and R22 50E is the shunt for taking a sample of current. 4-20mA in the output (provided suitable load is connected) means 200mV- 1000mV across 50E shunt. This is fed to close loop control system of IC3a inverting pin.

An opamp on this type of feedback tries to drive the output in such a way, so as to maintain both the inputs at same level.

If there is 1V at pin 3 and no current is flowing pin 2 is at 0V so output goes positive and drives Q2. this results in a flow of current till a 1V builds across shunt, if it exceeds then output of opamp falls This reduces drive to transistor and hence current reduces. That is the part of V to I conversion with open collector output.

Now we need 200mV to 1000mV to get 4-20mA 4mA is good for 0 as low level measurements are more noise prone. that is the reason 4mA and not 0mA.

Now we need to convert 0-2 V to 0.2 - 1.0 V using IC3B. R14 is a representation of that 200mV offset set by R16 pot. the opamp IC3B adds both the input and this offset to get 200mV to 1000mV. for that the opamp IC3B is an analog computer, summer, subtracter. Try to now calculate the values for that.

Circuits by Application

Analog Circuits

  1. Battery Level Indicator
  2. Simple Sample and Hold
  3. Sample and Hold Standby
  4. Voltmeter Attenuator
  5. Precision Current Source
  6. Opamp Supply Virtual Ground

SCR and Triac

  1. Solid State Relay
  2. Normally Closed AC SSR
  3. AC-AC-SSR
  4. DC-DC SSR
  5. 2N2646 based Pulser
  6. Drive SCR thyristor

Mains Power

  1. Flashing Neon Lamp
  2. Dimmer power control
  3. Edison Bulb Life Extend
  4. Mains Current LED
  5. Mains Voltage LED

Digital Circuits

  1. Simple Digital Counter
  2. Running Lights
  3. Frequency Divider
  4. Crystal Oscillator
  5. Simple High speed switch
  6. Differential TTL converter

Measureall DMM

  1. Ohmmeter Measure Resistance
  2. Precision Digital Attenuator
  3. Precision Amplifier

Mixed Circuits

  1. Monostable Multivibrator
  2. Digital to Analog
  3. LM311 Oscillator
  4. PLL using 4046
  5. VCO with LM331
  6. BCD Thumbwheel to Analog
  7. V to F Converter ICL8038
555 Circuits
  1. OR gate with two 555
  2. fixed frequency duty cycle
  3. Pulse width modulation
  4. Astable Multivibrator
  5. uC Reset Generator
  6. LM555 Voltage Doubler
  7. 555 Power Oscillator
Discrete Circuits
  1. Isolated dual supply
  2. Sound to light converter
  3. Water operated relay
  4. Telephone Indicator
  5. Passive volume control
  6. RS232 Opto-Isolation
  7. Voltage Level Indicator
  8. Relay Driver
  9. Constant Current LED
  10. Voltage Doubler
  11. FET Current Source
Opamp Circuits
  1. Three Opamp Differential
  2. Two Opamp Differential
  3. Buffer Opamps
  4. Differential Op-Amp
  5. Inverting Opamp
  6. Non Inverting Opamp
  7. Digital gain control
  8. Square Triangle Oscillator
  9. Dual Polarity Output Amps
  10. Ammeter Precision Rectifier
  11. Voltage / Current 4-20 mA
  12. Current Source for RTD
Power Electronics
  1. Dual Power Supply
  2. Single Power Supply
  3. Battery Backup Supply
  4. 5V 1A Supply LM2575
  5. 5V Power Supply L296
  6. Dual Power Supply
  7. Tubelight Electronic Choke
  8. Voltage Doublers Multipliers
  9. White LED Lamp on Ni-Cd

uC and uP

  1. PC RS232 with MAX232A
  2. Battery Backup SRAM
  3. watchdog uC uP systems
Instrumentation Circuits
  1. Mains monitor LM3914
  2. Simple Mains monitor
  3. single digit voltmeter
  4. High Resistance Meter
  5. Diode Thermometer
  6. Function Generator
  7. Diode Leakage Tester
  8. Analog LED Ohm Meter
  9. Millivolt Source Current Loop
Process Control
  1. AD590 - temperature
  2. Thermocouple Amplifier
  3. Linearizing Thermocouple
  4. Thermocouple Amplifier
  5. 0-1V to 4-20 mA
  6. 1-5V to 4-20 mA
  7. InfraRed - Optical Switch
  8. InfraRed Detector