After publishing the article "Basic principles of Design", we've received a few reviews with the request to consider this issue more detail. I'll try to do that in a series of articles, beginning with this publication.
Using of Flowcharts
By Vladimir Koren
Flowchart is a graphical display of the sequence of actions or algorithm of a device. It can be built using the Functional Flow Block Diagram (FFBD) , Fig.1 and 2, or Time Diagram.
Graphical representation of working sequence is very helpful, and even more, required in all stages of design. Using this kind of diagrams will help you to avoid a huge quantity of mistakes during your work. Well-designed flowchart can help you build not just a good circuit. It can also help you determine how realize the operating sequence, using the discrete components or programmable controller. And thus, decrease the system cost and increase its reliability and efficiency.
As an example, let’s build a flowchart for hypothetical "Streaming Water Heater" using a time diagram.
Main Power Supply is 3-phase, 400V but a device can be used with other power supply standards.
The heater that used in the device, is an inductive-type heater of 10 kW rated power.
The heating process must be temperature-controlled.
The heating element has a built-in temperature switch, which protects it against overheating.
The heater must be protected against “dry functioning”, i.e. cannot be switched on without the liquid flow.
Three-phase Process Pump of 1.5 kW rated power is used.
Moreover, it needs to provide a mode when works just a pump, without heater - a pumping process.
So, we have all necessary data for beginning a work. Let’s draw two axes, "x" and "y". Axis "x" is a time line and "y" will be used for arrangement of device elements. An internal area of a diagram, we will use for description of the operation sequence. First of all, we place all outward components at the “y” axis, vertically, beginning from the bottom point. After that we put all other elements used for realization of the algorithm, Fig.3.
OK, we have described the device operating sequence, let’s examine how it will work.
Device OFF – Power Switch is switched off, all components of our device do not work.
Waiting Mode – Power Switch switched on. Mode Selector is in Neutral Position . Temperature controller begins to work but the device is waiting for operator decision: which of the modes is currently needed to use, Pumping or Heating.
Pumping mode - Mode Selector is in Pumping Position . Process Pump and Flow Switch are activated.
Waiting Mode – Mode Selector is in Neutral Position . Process Pump is stopped, Flow Switch deactivated.
Heating mode – Mode Selector is in Heating Position . Both Pump and Heater are running. However, to prevent the functioning of the heater without water, it should be switched on, for example, 10 seconds after activation of the flow switch. In this case, it is better of all to set the flow switch at the output heater. The final temperature of heated water we can control using a digital thermostat in which two control values are set. First of them is a final temperature set point, which used to stop the heating process (TSP). The second value is a differential (THY) between a TSP and required minimum of the water temperature (TRM). We will use this value to determine when the heating process must be initiated again (HON point). For example, we need to support the outlet water temperature in a range of 70 to 90ºC. So, the temperature set point (TSP) should be set on 90ºC, and differential (THY) on 20 degrees Celsius.
HON = TSP – THY, or 90 - 20 = 70ºC
After the temperature of water reaches 90ºC, heating mode turns off. However, when the temperature will be lowered to 70ºC, heating mode will turn on again. Thereby, we provide a necessary temperature mode. This is one heating cycle, which is repeated as required, during the heating mode.
Heating mode OFF – Mode Selector is in Neutral Position  (Waiting Mode). The heater switches off immediately but pump - just in a few seconds. This delay is needed in order to remove the afterheat of the heater element.
Device OFF – Power Switch is switched off.
Now we need to determine all possible malfunctions and decide how to protect the system. Let’s to invent, what bad may occur during device's work.
There are two reasons when our Stream Heater must be stopped immediately. Otherwise either pump or heating element can say "good bye" exhaling a light bluish smoke.
The pump motor current overload.
The drop or complete termination of the water flow rate.
Another group of the malfunctions does not require the full stop of the device. This is the heater's faultiness.
If we will stop whole device immediately, then the heating elements can be irretrievably lost. It can be caused by heat, which is accumulated in the heater's body and coils during the work. Therefore, we need to eliminate this residual heat. We can do this by delaying the pump switching off. The water stream will cool the heater off.
Now we have a full understanding on how the device should work, and we are ready to start selection of electrical components. At this stage, we have to decide how we can realize the scheme: using discrete components or it makes sense to use a programmable controller (PLC).
At first glance it might seem that the use of PLC controller is expensive. However, the analysis of prices shows that cost of three time-relays is comparable with the cost of one controller. In addition, because a large part of the device's algorithm will be implemented through the controller's program, we can reduce an assembling time and thereby reduce a cost of works. According to cost and quantity of required inputs/outputs, we can choose the controller XD-10, produced by Crouzet company. The controller's program diagram, based on the above mentioned flowchart (Fig.3), is shown at Fig.4 (incidentally, this is another kind of flow chart). This program secures practically all functions needed for operation of our device, including an indication of the working and the emergency states on the controller screen.
Programming of the industrial controllers is a separate, rather complex, but very interesting theme. Therefore, with more details, this issue will be reviewed in our following publications.
So, we have significantly progressed in our project:
We have fully familiarized with the given task;
We have comprehended (primarily for ourselves), how the device should work;
We have defined which components can be used to realize this algorithm, and outlined ways to solve the problem.
All this became possible owing to using the flowchart.
Thus, we can make a conclusion:
Using the flowcharts is a very important stage in designing.
The author is indebted to Dr. Boris Menin and Michael Glazov, for their interest and partial support of this work.
*Our next publication will be dedicated to drafting of elementary (schematic) diagrams.