Wednesday, February 11, 2009
This is a Proportional Controller where the setpoint is derived from a Thumbwheel switch.
The conversion of Thumb-wheel Digital Data to Analog mV is similar to R-2R Weighted Resistor Network. In this case it is a 1-2-4-8 Binary Weighted Resistor Network. It has no Digital Components.
You can see an example circuit below for digit weights you just use like 10K-100K-1M etc. There is a problem of procuring 8M Resistors, so use series parallel combinations, avoid open presets. Trimpots can be used but then it raises the BOM cost.
1-2-4-8 Kilo Ohms may load opamp for high output levels. 1-2-4-8 Mega Ohms may be ok in the lowest digit. Greater than 10M designs are possible only in lab, not in commercial or industrial domains.
Make such R networks, solder array on thumbwheel, in some thumbwheels remove diodes or other connections. Club all of them, thumbwheels. One opamp will do. Use -2.5 V for positive mV output. The resistors should be close to 0.3% at least.
This Binary Resistor Network can also work with Digital CMOS Chips like CD4029. Use these chips on a separate supply, which is just a LM336 - 5V device. A digital thumb-wheel also can be used.
In this controller you can see a sensor open indication. When the sensor breaks, the temperature controller may continue to turn on the actuators or heaters, It may even Oscillate. So when a high impedance is detected in the sensor input terminals, the output relay is shut off and a LED is turned on to simplify operator's diagnosis.
Mount the controller a distance away from heaters, ac-drives and vibrating parts. Avoid direct sunlight on controller, fix controller in a sealed control panel. Earth the point where the thermocouple senses heat. Some heaters leak. The machine has to be earthed.
Proportional Process Controller Specifications
Front Panel Controls
Display - Process Value PV
This is a 3 1/2 digit display which shows the process value from 000 to 1999 as chosen by user it could be deg C, Kg/cm2 or Lts/hr.
Push Switch - Displays Setpoint
This is a push switch which when pressed, makes Display read SetPoint as required by user to control his process.
LED Indicates Relay ON or OFF in Relay and Pulse Output Units. Units with 0-10V or 4-20mA, Output LED is always OFF.
Vary SetPoint SP
This is an endless 10 Turn Potentiometer this is used to adjust SetPoint (SP) in tandem with Push Switch. Turning Clockwise it Increments SP till max. value, then turn anticlockwise to bring SP to Zero. Use a Plastic Screwdriver or Trimmer.(Tester).
Press Push Switch to make Display read SetPoint (SP), this is the user defined point at which process is controlled. Now holding Push Switch pressed, adjust SP Pot to change Set Point to required value, use a small screwdriver.
Back Panel Terminations
INPUT : 4 to 20 mA, Fused, Polarity is Important. From Sensor, DC Current.
OUTPUT : 4 to 20 mA, Polarity is Important. To Output Device or Actuator, which by using DC Current, drives 0% 4mA, 50% 12mA and 100% 20mA in Linear fashion.
SUPPLY : 230V 50Hz - L, N, E. Connect Proper Earth.
BAND : This is a Single Turn Potentiometer (300 deg) it sets the
Proportional Band of Control. Wide and Narrow are the ends of pot.
RESET : This is a Ten Turn Potentiometer (endless) it sets the
Integral or Reset or Error Cal. Inc. PV and Dec. PV are the two ends of pot.
Electrical Precautions -
- Wrong Connections are the cause of most equipment failure.
- Use Servo stabilizer for extra protection.
- Use a Very good earth connection.
- Use Fuses where appropriate.
- Crimp and Solder all Lugs.
- 4 to 20mA, 4mA 0% & 20mA 100%. < 4mA OFF, > 20mA ON.
- 4-20mA Maximum Voltage @12V DC ie Shunt 10 Ohms to 500 Ohms.
- Protection - use external fuse of 0.1A to safeguard from high energy circuits.
4 to 20mA, 4mA Displays 0% of Process Value '000' & 20mA Displays 100% of Process Value. This is to be specified by user as ... an example 750 Lts/hr.
Input @ 20 Ohms Shunt in STC1000PFC. Protection - Internal fuse of 2A and Semiconductor Protection Circuits to
safeguard from high energy circuits.
Environmental Limitations :
- Delicate Precision Instrument
- Maximum 50 Deg C, 70% RH Non-Condensing
- Acid-Alkali-Salt Free area.
- Equipment may fail if environment is Hot / Corrosive / Vibrating.
Cleaning and Maintenance : No Maintenance required, Clean front panel with moist sponge & Mild Detergent.
Power Supply : 230V AC 50Hz +/- 10%, Sine wave UPS & DG Sets.
Proportional Temperature Controller
A Temperature controller made with above process controller has the following changes in specs.
LED Indicates SSR ON or OFF. Pulse Output goes High Max 15V or Low 0V. High Pulse turns on SSR. and this is Indicated by LED turning ON.
Display - Process Value PV
This is a 3 1/2 digit display which shows the Temperature value in Deg. Centigrade
INPUT : Thermocouple, Polarity is Important.
OUTPUT : DC Pulse Polarity is Important. To SSR. Pulse Maximum Voltage 12V DC at a Load of 250 Ohms.
Read More here - Analog Process Control Circuits