delabs Circuits

Showing posts with label process-control. Show all posts
Showing posts with label process-control. Show all posts

Tuesday, September 09, 2014

Fluid or Water Level Indicator with Reed Relays

This is a way by which you can monitor the level of fluids like water in a tank. Based on data from the sensor you can control things like solenoids or motors as required or even turn on a buzzer, hooter or annunciator in a control panel.

Fluid or Water Level Indicator with Reed Relays

Look at the picture on the right, the float can be a Styrofoam type with a couple of ceramic magnets stuck inside. The float must move freely up and down a long plastic pipe of around 10 mm dia. If the fluid is not water but a solvent then design a non-soluble float or coat the float you made with something like an epoxy resin.
Then you need to assemble a small circuit within the long plastic pipe. Many reed relays with a chain of resistors in series has to be inserted in the pipe. The resistor and the glass reed relays should be in different levels of the pipe where you need an alarm or indication. Seal the pipe hermetically at both ends with epoxy resin after taking out a shielded cable of just 2 wires + 1 ground shield.
When the fluid level changes, the float moves up or down with it, when the float moves over a section of pipe where the reed relay is mounted, the magnets on the float make the reed operate and the resistance of the sensor changes.

The resistance is measured and the alarm can be operated as you like. The advantage of this arrangement is the electric circuit does not come in contact with the fluid, no sparking risk.

Reed Relay. It was invented by Dr. W. B. Ellwood at Bell Labs in 1936. Contacts are in a sealed glass tube filled with inert gas like in a bulb, so they do not corrode. It is faster than regular relays and as no spring is used it has a longer mechanicalreed relay life. The two contacts are ferromagnetic blades plated with rhodium. When you take a magnet close by, the contacts touch each other. You can also put the reed in a coil, then when you energize the coil the reed operates. Used in telecommunication.OKI is one manufacturer of this type of component.

Design and Caution.

When you drive inductive loads you have to use RC snubbers, freewheeling diodes, varistors or zeners. when you drive lamps the cold current is high so use thermistors. The Reed relay is best used in telecom and instrumentation and avoided in power electronics. It can handle high RF frequency as the path of current is straight and footprint small.

Here is a Circuit to Build and Learn this Concepts -

Fluid or Water Level with Reed Relays

"When you use the DMM or digital multimeter, try not to use it for high voltages and currents, use Solderman Talks 1702external shunts and attenuators instead. If you try to measure 230V AC in the Ohms mode or in the Current mode with the probe in the current socket then you will see fumes in your DMM and a hole in your pocket !!" 

- Solderman Talks 1702

Monday, December 26, 2011

Control system design Notes

Proportional Controllers not only save energy, they can give a controlled finish or treatment to a job. An on-off controller or thermostat limit protection has to be used in series, this is to to stop a Runaway Process.

Control system design Notes

There was a real life instance when a Temperature cycling oven maker (environment chamber), faced large liabilities as an 8086 based profile controller and chart recorder just locked up and all the electronic equipment were roasted. Here even a bimetallic thermostat would have saved the day. Nowadays we have a watchdog timer to prevent such uP mishaps, but still use an analog controller in series as an over temperature trip.

Other Reading

The basics of control system design: Part 1 - Moving beyond PID

"This series of six articles will explore the use of six variations of P, I, and D gains: Proportional Control, Proportional-Integral Control, Psueodo-derivative feedback with feed-forwared (PDFF), PID control, PID+ control, and Proportional Derivative Control."

Wednesday, February 18, 2009

Electronics for Measurement Systems

Feedback and Temperature Control

System Model, Types of Feedback Control, On-Off Control, Proportional+Derivative Control, Proportional+Integral+Derivative Control, Proportional+Integral Control, Third-Order Systems..
 Capacitive Pressure Sensors

"Capacitive pressure sensors use a thin diaphragm, usually metal or metal-coated quartz, as one plate of a capacitor. The diaphragm is exposed to the process pressure on one side and to a reference pressure on the other. "

Monday, January 21, 2008

Embedded Process Control

Microcontrollers are versatile for process control instrumentation, The Instruments need to be small and power efficient, they have to be immune to electrically noisy environments.

When such a instrument is mounted closed to big machines, it has to be resistant to vibration failure due to harmonics. When it is used in ships or chemical industries, they need to be corrosion resistant. This makes Industrial Instruments a challenge to design, as the reliability is next only to Medical and Military instrumentation.

In applications like data loggers, scanners and PID Controllers Microcontrollers are inevitable. The analog and digital counterparts that were built years back were difficult to maintain and very big in size. With a uC a 48*48 1/16 - DIN PID Box is possible, as shown here Newport Autotune PID.

Modular Automation like PLC uses many uCs. There are Multiplexers, Analog and digital modules controlled by uC. Modern Computer based SCADA have wireless interfaces and transmitters using uC based intelligent circuits.

delabs Technologies 06:48 20-Apr-07


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