Probably useful to those who study automation or who have a similar task.
Formulation of the problem:
It is necessary to develop a control system based on OWEN PLC160 equipment with verification of elements of technological equipment. Recycle the control circuitry into program control sequences in Codesys v2.3. Make screen forms with remote control.
The technological scheme of the ice accumulator is presented on the graphic sheet (functional diagram).
In the “dry” well there are pumps (4) and (5). To prevent freezing of water in idle pumps in the winter season, heating of the well is provided (maintaining a positive temperature). When constructing an ice accumulator, many issues regarding the construction site, materials, equipment used, must be addressed taking into account local conditions.
The simplicity of the design and the operating principle of the installation allows you to vary widely the materials used, while fulfilling some of the mandatory conditions without which the installation will not work effectively:
- the ice accumulator must be located near the dairy department of the farm;
- Depth is chosen so as to exclude the passage of groundwater under the ice massif. If groundwater comes close to the surface of the site, you can build a non-buried ice accumulator with concrete walls;- a structure for freezing and storing ice should have an overlap, reliable hydro and thermal insulation;
- the bottom of the storage should have a slight slope for the flow of water into the collection ("wet") well;
- in the upper part of the structure, it is necessary to provide a compartment for installing fans and hatches, providing entry and exit of cold air supplied by fans. In the summer, hatches close;
- concrete beams are laid at the bottom of the ice storage that support the ice mass during melting.
The principle of operation of the ice accumulator is as follows. In the mode of ice freezing in winter, the pump (5) delivers water from the “wet” well tank to the pipeline located in the upper part of the ice accumulator. Water is sprayed by nozzles (6). In the cooling mode of milk in the warm season, the pump (4) delivers water from the “wet” well tank to the flow cooler, the heated water from the cooler is piped to the far end of the ice accumulator, and, flowing down the bottom, it is cooled. Thus, the melting of ice from below is ensured. From the under-ice space, cold water is collected through pipes (8) into a container (13). As the flow of ice in the lower part, the entire array is supported by concrete supports laid on the bottom. When using capacitive coolers, the pump (14) must be included in the pipe cut so that the capacitive cooler is in the suction line. Valve (11) is used to drain water into the sewer when washing the ice accumulator.To ensure reliable operation of natural cold installations in winter, the exclusion of freezing of pipelines is of importance. Pipes in the freezing zone must be isolated or freed from water when the water supply pump is turned off. All lines are mounted with a slope for water flow. Also, to ensure the release of the pipeline from water, it is proposed to include an air-hydraulic valve in the line.
The main factor determining the battery capacity of the ice storage is its capacity. The amount of ice packaged (Ml) is determined by the amount of milk produced in summer (Mm).
Mm = (Wr * N * n * K1) / 365,
Where Wr is the average annual milk yield per cow, Wr = 4800 kg / year;
N - livestock served by the milk processing line, N = 400 goals.
Development of process control algorithms for an automated system for the accumulation and storage of ice use. The process of ice accumulation, from the point of view of automation, is the most complex. In this case, a layered method of freezing ice is recommended, which allows to obtain high-quality ice without water lenses and voids.The moment of ice formation can be determined both taking into account the dependence of the time of freezing the water layer on the temperature of the outside air, and using an ice formation sensor. According to the data, at outdoor temperatures above -5 неС it is not advisable to freeze ice due to the low intensity of the process, and, most importantly, due to the low quality of the obtained ice. The freezing process can be intensified and the freezing time of the water layer can be more than halved by supplying water with a temperature of 2-3 ° C to freezing and creating strong flows of cold air above the surface of the poured water using fans.
Below is a diagram of the control algorithm for the process of layer-by-layer freezing of ice, taking into account the dependence of the time of freezing the water layer on the temperature of the outside air.
Description of the algorithm:
1. If the thickness of the layer of accumulated ice is Hl ˃ 3m, then the operation of the algorithm ends; otherwise, go to step 2.
2. If the outdoor temperature tнв ˂ -5 ̊С, then the operation of the algorithm is suspended, otherwise, go to step 3.
3. Turn on the water supply to the battery. Estimated water supply time Tpr
Tpr = (L * b * h) / Q,
Where L is the length of the capacity of the ice accumulator, m;
b is the width of the tank, m;
h is the thickness of the water layer, m;
Q - feed hearth, m3 / s.
4. If the water supply time is Tp ˂ Tpr, then go to step 4 of the algorithm; otherwise, go to step 5.
5. Turn off the water supply to the battery.
6. Turn on the fans.
7. If the outdoor temperature is tнн ˃ -8 ̊С, then go to step 8 of the algorithm, otherwise go to step 9.
8. Set the freezing time of the water layer Tz = 2.9 hours.
9. If the outside temperature is tnv ≥ -11 ̊С, then go to step 10 of the algorithm, otherwise go to step 11.
10. Set the freezing time of the water layer Tz = 2.1 hours.
11. If the outdoor temperature tnv ˃ -14 ̊С, then go to step 12 of the algorithm, otherwise go to step 13.
12. Set the freezing time of the water layer Tz = 1.4 hours.
13. If the outside temperature is tнв ˃ -17 ̊С, then go to step 14 of the algorithm, otherwise go to step 15.
14. Set the freezing time of the water layer Tz = 1.1 hours.
15. If the outdoor temperature tnv ˃ -20 ̊С, then go to paragraph 16 of the algorithm, otherwise go to paragraph 17.
16. Set the freezing time of the water layer Tz = 0.9 hours.
17. Set the freezing time of the water layer Tz = 0.8 hours.
18. If the water freezing time Tzv ˂ Tz, then go to step 18 of the algorithm, otherwise go to step 19.
19. Turn off the fans. Go to step 1 of the algorithm.
The process control system consists of several low-voltage complete devices (NKU), providing process control in manual and automatic modes, and the processes in the ice accumulator are controlled by NKU-3 and NKU-4.
NKU-4, which controls the processes of maintaining the water level in the “wet” well and heating the “dry” well (ensuring a positive temperature in the well), it is advisable to place it in an ice accumulator. In this case, the cable lines from the cabinet to the level sensor, temperature sensor and electric heater will be of minimum length.
The electrical circuit diagram of NKU-4 is shown in the drawing.
The control cabinet is connected to AC 220V via a packet switch SA1 and a circuit breaker QF1, which protects the line from short circuits. The lamp HL1 signals the supply of voltage to the circuit.
The well heating mode selection (manual or automatic) is carried out by the SA2 package switch. In automatic mode, the temperature controller PA1 receives power. The temperature sensor RK is installed in the “dry” well of the ice accumulator. If the air temperature in the well is lower than the setpoint of the regulator A1, the contacts of the regulator A1.1 include a magnetic starter KM1, which with its contacts KM1.1 turns on the electric heater ЕК1. The voltage supply to the EC1 is signaled by the lamp HL2. When the air temperature in the well of the regulator A1 setting is reached, the heating is turned off.
The choice of the mode of water supply to the “wet” well (manual or automatic) is carried out by the SA3 packet switch. In the automatic control mode, the inclusion of the electromagnetic valve is carried out by the relay contacts KV1. The KV1 relay is connected in series with the electrodes of the contact water level sensor located in the "wet" well.
The primary water level transducers are isolated electrodes installed at a certain level. Relay KV1 is powered by a DC voltage, obtained using diodes VD1 ... VD4, and for safety reasons, voltage to the electrodes is supplied from a step-down transformer TV1.
KV1.1 turns on valve YA1, the water supply to the well is cut off. Relay KV1 loses power if the water level in the well is lower than the lower level electrodes B2.2 (L). In this case, the contacts KV1.1 are closed, valve YA1 is turned off, water is supplied to the well.
The voltage supply to YA1 is indicated by the lamp HL3.
In the cold season, it is possible to freeze the level sensor and violate, as a result, its performance. Therefore, heating of the level sensor is provided, with the help of an electric heater ЕК2, located in the central electrode of the sensor.
Switching on the heating of the sensor and step-by-step regulation of heating is carried out using the SA4 package switch. The HL4 lamp signals the heating of the level sensor. To prevent condensation on the inner walls of the control cabinet, an internal heating of the cabinet is provided, which is switched on by the SA5 switch. The degree of heating is adjustable. You can turn on the heating element EK3 or series-connected EK4 and EK5. The HL5 lamp signals the inclusion of cabinet heating.
NKU-3 manages the processes of layer-by-layer freezing of ice in an ice accumulator, supply of a coolant to a cooler from under-ice space and controls the operation of fans.
The automation system for layer-by-layer freezing of ice is simplified if the moment of ice formation is determined using a special sensor. The moment of ice formation can be determined by the change, during the phase transition of water into ice, of mechanical, electrical, optical or other environmental parameters.The problem of creating an ice formation sensor is rather complicated due to the fact that the controlled layer moves during the freezing process. Therefore, the sensor must either move simultaneously with the growth of the ice mass, or be distributed, multipoint.
A promising direction is the development of a sensor based on the difference in electrical conductivity of ice and water. The resistance of ice is greater than the resistance of water by 500 or more times. This means that it is possible to create a sensor on this principle. The sensor consists of an electrical part located in the ice massif and an electronic part used to analyze the resistance of the medium and provide a relay control signal to the control system of the water supply pump for freezing at the time of ice formation. This sensor has several disadvantages. False alarms of the sensor occur due to freezing of metal electrodes when the bulk of the water has not yet frozen. Therefore, it is necessary to refine the sensor, aimed at ensuring
automatic setting change as ice builds up.
The graphic sheet shows the electrical circuit diagram of NKU-3, developed taking into account the dependence of the time of freezing the water layer on the temperature of the outside air.
The control cabinet is connected to the three-phase AC 380 / 220V circuit through a circuit breaker QF1, which protects the line from short circuits. The voltage is signaled by the lamp HL1. Through the circuit breaker QF2, power is supplied to the NKU-4. The HL2 lamp signals the power supply to NKU-4, NKU-3 controls the electric motors: M, M2 and m3 - fans 1, 2 and 3; M4 - freezing pump; M5 - cooling pump. The control is carried out using magnetic starters KM1 ... KM5. All electric motors are protected from overloads by thermal relays KK1 ... KK5. The selection of the freezing mode (manual or automatic) is carried out by the SA1 batch switch. In the "manual" mode, the on and off control of the electric motors of the fans M1, M2, M3 is controlled by the SB2 "start" and SB1 "stop" buttons.
The required number of working fans is set by switches SA2, SA3, SA4.
The electric motor M4 of the freezing pump is controlled by the SB4 “start” and SB3 “stop” buttons. The signaling of voltage supply to the electric motors of the fans and the freezing pump is carried out by HL3 ... HL6.
The chiller pump is switched on for milk cooling using the SB7 (SB8) “start” buttons and the buttons are turned off
SB5 (SB6) “stop”. HL7 lamp signals the voltage supply to the electric motor of the cooling pump.
3. Created an automatic process control scheme
The rationale for choosing a controller for an automatic ice-making system.
The task of automatic control of the freezing system is such factors as maintaining the required temperature in the required areas, ensuring the correct process sequence, displaying process parameters for the installation operator. The control controller must have the necessary speed, the required set of ports, the possibility of further integration into the SCADA system. In addition, the cost of the controller should be below average for this type of device. Currently, the market is widely represented by controllers of various manufacturers. In addition to Western models, there are also many developments of Russian production, which have recently significantly supplanted foreign analogues, primarily due to significantly lower cost, all other things being equal. Therefore, the choice of expensive models may not be economically feasible if the required functionality is present in simpler models in the line.
For comparison, the average performance controller for systems of small and medium degree of integration S7-1200 manufactured by SIEMENS has almost the same set of functions as the domestic controller manufactured by OWEN - PLK160, the cost of which is in ruble terms a little more than half the cost of S7-1200. This model includes a sufficient number of input / output ports, analog inputs for connecting various sensors, standard communication interfaces - RS485 and Ethernet TCP / IP, and most importantly - a visualization system built into the software shell that allows you to organize the operator’s workplace without purchasing expensive SCADA - systems.
The main technical characteristics of the PLC-160 are shown in the tables below.
Main characteristics of the PLC-160
Communication and programming interfaces
The controller’s executable program is functionally decomposed into two modules, each of which performs a certain part of the algorithm and is written in its own language. Such a partition is justified by the fact that it is advisable to use special tools for various types of tasks. So, for processing input states and performing logical operations, it is more suitable (and more visual) - the language of functional blocks CFC, which contains a large set of ready-made blocks, each of which performs a specific function. It is enough for the programmer to choose the necessary set of blocks for the implementation of a particular program, set the specific properties of these objects, and compile the source code. With certain skills, this approach significantly speeds up the process of creating programs, since it makes it possible to use ready-made developments.
The first part of the software is made in a high-level language - ST (structured text), which is not as clear as CFC, but, in contrast, is more flexible, it allows you to create compact but complex conditions. In this program, the ST code implements the function of reading values from a temperature sensor connected to the PLC analog input and, based on them, selects the setting for the operation time of the freezing timer. This provides weather-dependent control. The program code for ST is shown in the figure below, part of the CFC module is shown in the following figures.
The time calculated to start the timer has a format in minutes, but for a visual (accelerated) demonstration of the work, a software correction has been introduced, in which one minute is equal to one second of real time.
ST code module
The second part of the algorithm is performed in the language of functional blocks - CFC. Due to the significant amount of code, a complete listing of the module program is possible only with a slight reduction in scale, therefore, the figures below illustrate the main branches of the algorithm.
Full listing of the CFC module at a scale of 30%
The physical discrete inputs of the controller (input ports) are connected via software configuration to logical assemblies on the AND and OR elements, which perform the function of switching (selecting) control signals from the buttons on the panel in manual mode, or from control signals of the program during the operation of the algorithm. Let us explain the operation of the selection module using the example of turning on the fan of the freezing zone, the remaining loads are controlled in the same way.
If the Automatic \ Manual switch is set to - Automatic, then the log. 1 from it goes to one of the inputs of the elements And - 0 and 4. Element 4 has an inversion compared to 0, since it has an inverse input. The second input of these elements receives a signal from manual and automatic control. Thus, the state of the inputs (1 and 1) in which the control can be transmitted through the element will be alternately, either through zero or through the fourth element, depending on the position of the type of work selection switch. Next, the enable signal falls on element 1 - OR, which sums the signals from the sources and passes them directly to the load control. After starting the start in automatic mode (if it is not blocked by the conditions of the algorithm), the start sequence is activated, which is implemented on timers with a turn-on delay - TON. These modules (TON_1 ... TON_4) delay the establishment of the output signal relative to the input for a given time. In this program, the modules are used to prevent the simultaneous inclusion of powerful consumers - induction motors, which have a significant inrush current.
The freezing cycle itself is implemented on another type of timers - TP_1 (timer for an arbitrary time interval), which takes the calculated value of the freezing time from the program on ST and, when the automatic mode is turned on, gives an output pulse equal to the specified interval. On the trailing edge of the pulse, all consumers are switched off - the ice-making cycle is over.
In addition to the marked elements, the program uses other blocks that perform auxiliary functions. For example, pulse generators BLINK_1 and BLINK_2 are needed to “blink” visualization elements, and the incremental counter CTU_1 provides animation of the movement of air flows in the freezing zone. The X coordinate of the position of the flow direction arrows is attached to the current state of the counter, which causes them to move along the longitudinal axis.
A chain of sequential launch modules.
Description of the operation of the concept.
Since the control logic is implemented in the controller program, the electrical circuit of the freezing system is extremely simple, without a large number of installation elements - delay relays, measuring transducers, and pulse generators. Actually, the entire circuit is a set of contactors for switching powerful loads, a controller and a terminal block for connecting external connections.
Below is a schematic electrical diagram of the installation
Contactor control signals are removed directly from the controller outputs, without the use of intermediate relays. However, it should be noted that when using starters with windings consuming more than 20 W, the use of intermediate relays becomes appropriate to protect the built-in relay of the controller from overloads and extend their life.
The controller receives control discrete signals from remote buttons on the wall of the control box (or from the operator panel on the PC), as well as from an analog temperature sensor and, in accordance with program logic, gives control actions to the outputs, controlling the operation of various actuators.
Let us consider the operation of the circuit using the example of a channel for switching on a fan for blowing a dry well, other loads are switched on in a similar way. When the manual mode is on and the signal from the blower switch (or from a button on the PC panel) is received, the controller gives a signal to turn on, an internal relay is triggered, which connects the starter coil to one of its phases with its contacts. The starter operates and supplies power to the email. blower fan motor. In series with each starter, thermal relays are turned on, which are designed to turn off the electric. engines under continuous loads close to critical. The characteristic of the thermal relay must be matched to the specific consumed rated power of the engine at idle. The dry well is heated by three groups of heating elements included in the “star” scheme, which ensures uniform distribution of loads across the supply phases. The controller outputs, through which the starters are switched on, are duplicated by external, mechanically connected Start and Stop switches located on the front door of the control cabinet, which makes it possible to manually quickly manage loads (for example, if the controller breaks down and is sent for repair). When installing the control cabinet, one should take into account possible adverse conditions for equipment placement at the facility (low temperature, high humidity) and use structural elements with a degree of protection not less than IP 60.
An approximate view of the control cabinet is shown in Fig. 5 To protect against low temperatures, it should be possible to heat the internal space with low-power heating elements or incandescent lamps of low power.
Variant of waterproof design of control cabinet
The workflow is displayed in the Codesys 2 integrated system (it is necessary to connect the controller to the PC via one of the communication interfaces). The operator’s workstation is a mimic diagram with the functional elements of the freezing system, controls and controls placed on it.
Process Control Window
After loading the workspace, you must press the F5 key to start the program. By default, the manual control mode is enabled, which makes it possible to check the functioning of various actuators before starting the automatic mode. On the right side of the mimic diagram, a sectional view of the installation is conventionally shown — dry and wet wells, an ice accumulator chamber, circulation and spray pumps, fans and communications.
On the left side are the controls for the installation in manual mode. The panel also has a master controller that simulates the sensor of the external temperature of a dry well, which can be used to set the required temperature to check the operation of the program. In automatic mode, the freezing time is calculated by the tabular method based on the air temperature in a dry well. In manual mode, the freezing time is determined by the operator.
Thus, a “soft start” of the system is carried out, providing a smooth (stepwise) increase in the load on the supply network. At the end of the time delay, the automation switches off all active consumers. If for some reason the operator needs to interrupt the process before the specified time has elapsed, then he must put the system into manual mode, while all the loads will take on the values specified by the buttons on the panel. #Process, #Layer-by-layer, #frost, #ice, #course, #work