Tag Archives: advanced

Programmable, 2 wire, relay controller

Although the previous posts described the two wire, 4-20 mA transmitters’ internal workings, sometimes a simpler action is adequate.

The first and pretty obvious example is the temperature switch (which is actually a bimetal). It doesn’t need any power supply, it makes (or breaks) a contact if the temperature reaches a certain threshold.

Another example is the float switch used in water tanks. it also makes (or breaks) a contact if the water level reaches the given level.

If the power they can deliver is insufficent, then these “sensing devices” usually operate a relay, and the relay’s contact switches on the higher power mechanism.

For example a reed switch based float switch (used in a water tank) can only switch a maximum of 0.5 A, but the water pump (operated by this float switch) needs multiple amps for its operation. In this case the float switch operates a relay, and the pump’s multiple amps flow through the relay’s contacts.

On the other hand there are more complex issues to be solved, as simple as can be.

Let’s see the following situation: There is a garden with an automatic sprinkler system. This system has a rainwater storage tank, a water pump, sprinklers and a soil humidity sensor. If the soil moisture falls below a certain level then the sprinklers begin to operate, and if the moisture rises to an other certain level the water pump stops.

It is a simple hystheresis function with adjustable low (turn on) and high (turn off) threshold.

Continue reading Programmable, 2 wire, relay controller

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Universal, 4-20 mA, two wire industrial transmitter

This post unifies together the following posts:

Industrial, 4-20 mA current loop, measuring basics I.

Industrial, 4-20 mA current loop, measuring basics II.

Industrial, 4-20 mA current loop, measuring basics III.

Industrial, 4-20 mA current loop, measuring basics IV.

Internal workings of process controllers I

. Internal workings of process controllers II.

There are a lot of types of transmitters nowadays. Usually a separate one (a specific type) is needed for thermocouple measurement, an other one (an other type) is needed for level measurement…

The basic concept of this post is to present a way for a universally usable transmitter, which can accept almost any type of input (Pt100, pH probe, rotation, level, light…) and produce a configurable, standard 4-20 mA output.

Continue reading Universal, 4-20 mA, two wire industrial transmitter

Internal workings of process controllers II.

Continued from the previous post

In practice, all these process controllers incorporate a microcontroller.

But microcontrollers are programmed in assembly or in C language, so to create a function block programmable process controller from a C programmable microcontroller, an intermediate firmware must be written.

Continue reading Internal workings of process controllers II.

Internal workings of process controllers I.

This time the internal workings of a process controller will be shown.

I’ll show it through a Siemens SIPART DR24 process controller.

Its user manual can be found on the Siemens’ website:

Siemens SIPART DR24 manual

Although it is almost obsolete, the operational principles can easily be demonstrated with it.

Continue reading Internal workings of process controllers I.

Predictive digital filtering for first order systems

Today measurements must be fast, cheap and accurate at the same time.
These requirements are not easy to achieve.

This post describes a simple but very efficient software method to make a slow measurement faster.

Let’s look at a simple temperature measurement example.

The temperature sensor sits in a protective metal cover.

This sensor is at room tempeature, and we would like to measure the temperature of boiling water.

The example is very simple but it will clearly show the operation of the computing method.

When we put the sensor into the boiling water, we know that the measured value jumps from 20 °C to 100 °C.

but the sensor’s output rises slowly since the protective cover needs time to heat up.

This heat up phenomena acts as a first order filter function.

The first order filter function is described as:

f
where T(t) is the measured temperature at a given t time, T(final) is the final value (100 °C in this example), T(0) is the starting temperature (20 °C in this example) and Tau is the time constant of the filter.

It may seems a little complicated, but plotting a graph gives a very clear demonstration of how it works.

comp1

The problem here is that we know that the measured temperature momentarily rises as the sensor goes into the boiling water, but the measurement shows a slow rising to the final value.

Continue reading Predictive digital filtering for first order systems

Industrial, 4-20 mA current loop, measuring basics IV.

Continued from the previous post

Although the title says “basics”, here are some advanced circuits about the 4-20 mA current loop transmitters.

In the first example a USB connection is shown. Nowadays almost every transmitter incorporates a microcontroller. The transmitters can be programmed through their display, or through HART protocol, or via DIP switches.

Transmitters with HART communication are expensive, using displays also increases the cost, and using DIP switches provides a not so flexible way of configuration.

Some manufacturers use USB as a programming / configuration interface, but most of them are not loop powered, so they need a separate power supply, which increases the cabling costs.

The following solution shows a way of how to include the USB communication in the transmitter design, while the transmitter is powered from the loop.

Continue reading Industrial, 4-20 mA current loop, measuring basics IV.