Multi Zone Process Monitor

Here is an ancestor of the product in the earlier post. It is a Process Scanner and Indicator. There was no control, but there were individual alarms for each channel.

Multi Zone Process Monitor

This is a original design of mine, obsolete now due to size and technology. The inspiration of the product concept was from a Omega Catalog Item. Even today i use the Omega catalog for inspiration, but i do not design complete products, only circuit sections.

The Omega catalog was introduced to me by the MD of a firm i worked in as a R&D Engineer in my young engineer like days. He used to call me and show a catalog product and ask me if i could build something like that. I would then design it working both at home and office, sometimes even in the night, it was just the enthusiasm or creating something new.

I even built the first Pneumatic Tyre Inflater with the help of the CNC director for that firm, this product won a national award in the nineties. I left the firm on a trivial issue which i never disclosed. It was Exhibition time, many new products were not complete, they asked me to make Mockup Dummy Instruments for demonstration only, overnight. I ran away.

Read more about Control Instruments at my pages here

Flow Measurement and Control

There are three Controls to be Adjusted to make a Proportional Flow Controller Perform Properly. This method has to be practiced and experience gained from it can be used to get very good and stable Control of the Flow or Velocity of a Fluid.

1. Set Point. (SP)

This is the Flow Rate at which you require the Fluid to be controlled at. Adjust the rate at which the fluid flow is expected to be controlled .

2. Process Value. (PV)

This is the Actual Flow Rate of the fluid in the flow sensor or its path. It is very important that the Flow Sensor is placed at a position in the fluid circuit in such a way to avoid cushions which may lead to oscillations around the Setpoint.

3. Proportional Band or Dead band. (PB)

Dead band or H % or Hysterisis are terms used in on / off Controllers in proportional controller we use the term proportional band.

4-20mA Control Signal

The Flow rate zone in which the Output is 4.1mA to 19.9mA which in turn Drives the AC-Drive >> Motor from 0% to 100% is the proportional Band. It is Given in % e.g. 50% PB of 200 lph SP is 100 lph. Band 150 lph – 250 lph

eg : The Motor is at 100% Drive ‘on’ till 150 lph and ‘off’ above 250 lph. ` Between 150 to 250 is the PB. A little above 150 the Motor reduces power gradually till at 250 it turns off. When SP=PV the output is 12mA (ideal) here the motor runs at 50% of the total power.

Selection of Motor / Drive and circuit Capacity:

The Motor / Drive Combination must at 12mA Control signal give a flow Rate at which the system is used for most of the time this gives good stability. The max flow rate setting required by system must be achievable at 80% of Motor Time Axis Power this is to make allowance for load and line regulations. The Flow Circuit should have normal resistance to flow to reduce Oscillations.

STC1000PFC – P = Proportional Control, F = Flow, C = Current Output.

Zoom Image

Flow Measurement and Control

Tuning or Adjusting a Proportional Flow Controller

Step # 1

Ensure Flow Sensor Output 4-20mA is properly connected to the Flow Controller polarity reversal will show reducing reading in the Display as Flow rises. The Motor / Drive used and Power selected must be able to bring the Flow more than the maximum required Flow rate directly without control (open loop).So when in doubt connect motor/drive and run at max power and observe maximum flow rate e. g. if max. flow obtained with Motor at 100% Power (direct) is
300 lph the STC1000PFC can control flow rates upto 260 lph.

Step # 2

Keep PB in middle position and power on system e.g. set Flow rate to 200 lph. Now Observe maximum overshoot. and adjust proportional band as in table. (PB Control is a 300 lph Single Turn Control with Ends.) SP 200 lph, PV (Process Value).

PV overshootProportional Band
10 % 220 lph or moreNear Maximum fully clockwise till end.
5 % 210 lph to 220lphMiddle of the PB Control or towards max.
2% 204 lph to 210 lphLittle above present setting.
Less than 200 lph Droop e. g. 190PB is Critically set Do not Change.

After each change turn on system again to see response till 2 % or less variation or overshoot or oscillations are obtained.Flow Control Curve

Thumb Rule ! –

  • Increment PB to Decrease Overshoot.
  • Increment PB to Decrease Oscillations.
  • Stop adjustment when PV droops around SP with no oscillations.
  • Adjust EC to match SP = PV after PV is stable at a point less than SP.

If proportional band setting is maximum, fully clockwise turn till end (single turn pot) the motor will slowly ramp up to full flow speed and ramp down slowly to reduce flow.

If proportional band is at the minimum, the motor will go full speed till it comes close to the set flow rate and turn off abruptly almost like an on / off controller the motor may pulsate on & off near setpoint.

Step # 3

There is an additional control called Reset or Error Cal EC ( Integral) which is factory set for SP=PV 50% Power Output i.e. Output Control = 12mA. In certain cases after stable reading is obtained after adjusting or tuning PB the Flow may stabilize say at 195 lph for a set point of 200 lph the process is stable but a ten lphrees process error is present. this can be Corrected with EC or RESET POT at the rear panel (this is a endless 10 Turn Pot).

Adjust Error Cal provided in the back panel to increase Flow rate to 200 lph from 190 lph. when this is done give some time for system to respond after every 1/2 a turn 180 lph of the RESET (EC) control pot .

The RESET control is a Ten turn potentiometer like the SP potentiometer after 10 turns the direction of turning must change. Clockwise Increases Flow rate Anticlockwise decreases Flow rate. (at min. PB setting EC pot sets the On / Off or Operating Point).

Analog Mux for Data Acquisition Systems

Here is a 4-20 mA In/Out Analog Mux with Cascade option. This is a simple circuit i designed to make a Automation System within a budget.

Analog Mux using 4051

This takes 4-20mA from many Transmitters and gives out just one 4-20 mA output. The Mux is done with a digital byte or word. This is a slow scanner as process is slow, that way many analog inputs can be multiplexed and sent into one analog input of a D/A. In near real time systems a faster mux could be used or mux totally avoided. This was made in some numbers, so the pcb is better than others.

4-20mA Multiplexer Circuit – pdf

RTD 3-W Mains Power 4-20 mA Transmitter

This is the Photo of a RTD 3-W Mains Powered Temperature 4-20mA Transmitter. The Circuits and PCB are here

RTD PT100 Transmitter and Multiplexer.

From Soldermans Basic Electronics

Now with new Technologies like Zigbee and Modbus, We can classify Transmitters as shown below. The Measured Parameter Temperature, Flow or Events Has to reach an Intelligent Data Storage and Analysis System. It may just be an Human Operator who jots the data on a Notepad and Turns a Few Dials based on his Experience or an embedded controller. It could even be a Computer Network or a Web Application used by many, like Monitoring the Weather Attributes.

  • Analog Transmitters – Like 4-20mA Loop.
  • Electrically Isolated Analog Transmitters.
  • Transmitters that need to be Intrinsically Safe.
  • Digital Transmitters and Optical Interface.
  • Wireless Transmitters and TCP-IP.

The job of the transmitter is to take the weak analog measured parameter information from sensor, be close to it, amplify, clean, linearize the signal if required and send strong – error free data over a long distance to an operator or system.

Pressure Measurement and Control

Pressure is the next most important parameter after Temperature. In Hydraulics and Pneumatics, the measurement, indication, control and logging-charting of pressure is indispensable.

Popular Pressure Sensors are based on either Piezoresistive or Strain-Gauge Technologies. Strain-Gauge types can be Metal Foil or Semiconductor Film.

The Analog Strip Chart Recorder is used in Simple Systems where a Record or Report has to be generated. Data Loggers and PC based Data Acquisition Systems expand this capability by making the Data Electronically Accessible for Charting-Analysis and Reporting using database applications.

The Liquid-Fluid Level in containers of uniform cross-section can be computed by measuring the pressure of the fluid head. In cases where levels of semi-volatile fluids are measured, sensors with Micro-watts of power usage or data transmission is required to protect from sparking related accidents. Even Zener Barriers can help reduce risks due to wrong connections or cross connection due to cable damages.

In the field, due to various constrains many safety practices can be overlooked or circumvented. The Sensors and System design must be over-engineered in some cases to safeguard man-machine assets.

From Elex Quna – Electrical Circuits FAQ

Related Reading

RTD PT100 Transmitter and Multiplexer

This is a RTD Pt-100 Transmitter, It can Convert Thermocouple mV or RTD mV to 4-20 mA Current Transmitter. The part numbers on Circuit and silk screen may not match. But the PCB may be usable. The output is a current source and not a current sink. It goes to a 4-20mA Mux with a common ground. The Circuit RTD-Pt-100-Transmitter Circuit.

This low-cost transmitter was made to send RTD information in a 4-20mA over long distance. The 4-20mA was the input to a Analog-Mux which interfaced to a GE-Fanuc PLC system and a 486 Computer. The Analog Multiplexer Built around CD4051 was was controlled by PLC to get data from numerous channels. This reduced the cost of the System. This is ok when Real-Time Acquisition is not required. The Analog 4-20 to 4-20 Mux-Buffer circuit can be seen at Industrial Process Control Circuits.

This was done as customer budget could not afford dedicated analog input modules for every channel. There were more than 50 Inputs. The thermal inertia of system did not need a real time or fast correction.

RTD Transmitter 4-20 mA

The PNGs of PCB is 600dpi 16 color. It is to be scaled appropriately.

Related Resources –

Mini RTD Pt100 Three Wire Transmitter

These are the circuits and boards of a Mini Temperature Transmitter for a Platinum hundred ohms temperature Sensor.
RTD-3W-Transmitter Circuit. See my Transmitter Application Note for this. 3 Wire Transmitter RTD

The customer wanted a two wire system, this needs low power opamps which were ten times the price. If this transmitter works at 3mA it could have been two wire. Those parts were in short supply and the budget of customer was low. So i made it with regular opamps but three wire. The customer was satisfied as he got a cost effective solution. But now the situation of markets and products are different.

If you need to transmit the temperature from a high voltage area or toxic environment. You will need to hermetically seal the transmitters, convert the Voltage to frequency, frequency to IR LED flashes or RF. This can be then remotely monitored. Then the transmitter has to be battery-solar operated, low power too. ICL7135 is a simple solution. It has a serial output that can be used to Drive IR Leds. Remotely sense these flashes in a Micro-controller and you have a reading. You may be able to use optic-fibers too. Where volatile liquids are present so that the risks of sparks can be eliminated.

The PNGs of PCB is 600dpi 16 color. It is to be scaled appropriately.3 Wire Transmitter RTD

This is the same Mini RTD Pt-100 Transmitter but in its case. Encapsulated in epoxy, hermetically sealed against harsh industrial environment. This will work well even near fumes of Ammonia with no corrosion. But not near vapors which can be ignited by sparking as terminals are still open. The side view shows zero and full scale ten turn bourns trimpots heads, for calibration. After cal it can be sealed with RTV compound.