This article describes the functional structure, human-machine interface as well as software and hardware means of a computerized system for remote control of liquid level with discrete self-testing implementation. The system has a hierarchical structure in which information processing is decentralized, and software and hardware components are removed from each other. The proposed by authors method of self-testing of hydrostatic sensor correctness, that generally increases system reliability, is considered in detail. Resulted system is then tested on computer simulation and experimental setup.
Tasks requiring level measurement of liquid products are extremely diverse and are found in various areas of technology. Level measuring is required in most production processes; in the systems of environmental monitoring and safety; for mass or flow rate of liquid products accounting during their storage and transportation. The urgency of liquids level measurement increases with increasing degree of automation of production processes, monitoring and registration systems.
There are many different methods and devices of measuring the level of liquid
products [
ultrasonic local; radar; local laser; optical.
Non-wave methods include: capacitance; hydrostatic; buoyancy; mechanical float.
Combined include: magnetostrictive float; float radar.
Effects associated with the spread of electromagnetic or acoustic waves in the liquid, vapor or a mixture of structural elements (waveguides, sound guide pipes) in contact with the environment are used in the wave level measurement methods.
Other principles of measurement based on the change in capacitance of constructive capacitor compiled by pressure liquid column, buoyancy force acting on a body immersed in fluid are used in non-wave level measurement methods.
The combined level meters unite elements of wave and non-wave ones. Magnetostrictive level gauges are fixed by float, which position determination is performed by mechanical vibrations in a sound conductor.
All methods of measurement have inaccuracies and errors limiting the scope of their use. Errors can be partially compensated by various technical means, but as a consequence this obstacle leads for rise in price. The impossibility of full compensation of errors is caused by physical, economic and operational constraints.
The implementation of the digital level control system based on the principles of
remote monitoring using the SCADA (Supervisory Control And Data Acquisition)
software [
Computerized system for remote level control with discrete self-testing is designed
for the measurement of liquid level, volume, temperature and pressure in five tanks as
well as automatic alarm signalization in case of the acceptable limits exceeding of the
above-mentioned values [
The main technical parameters of the measurement and control system are: liquid operating temperature is −50..+120°C; the range of the measured liquid level is in range of 0..30 m; operating pressure in the tank is 0..0.3 MPa; the distance from the tanks to the control station is more than 50 m. Relative error should not exceed 0.5%.
The functional diagram of the proposed remote system of level measurement and control is shown in Fig. 1, where the following abbreviations are accepted: OTS – OTS
Human-Machine Interface
PLC
DOM Signal Bus
TDAM
M
Signal
Bus HPS1 PS1
P1 P2
T2 Tank 1 TS1 T1 TS2 TS3 T3
Signal
Bus L1 Q1
FLS1
V1 operator touchscreen; PLC – programmed logical controller; TDAM – temperature data acquisition module; CIM – current input module; DIM – discrete signals input module; DOM – discrete output module; TS, PS – the sensors of temperature and pressure respectively; HPS – hydrostatic pressure sensor; FLS – float level switch; L, T, P – the values of liquid level, temperature and pressure in the relevant tanks; V – drain valves; Q – the value of liquid flow at the opening drain valve.
Three-tiered SCADA-system for remote level control with discrete self-testing is developed by the authors on the basis of TRACE MODE 6 software package.
It is advisable to use PLC and I/O devices of the ICP DAS company as hardware means in the developed computerized liquid level control system. The advantages of this hardware include industrial design, versatility of application and easy integration into modern widespread SCADA systems. Also, products of the ICP DAS company possess big life cycle and high reliability as well as the optimal combination of cost and quality.
The TRACE MODE is used in this computerized system for remote level control as a software platform. The TRACE MODE software has its own integrated development environment with more than 10 editors arranged in it. Moreover, it includes the built-in drivers for more than 2572 PLCs and I/O devices (including the products of the ICP DAS company) and has its own industrial real time database management system. The perfect 3D graphics and at the same time easiness of the graphical editor can be also considered as the benefits of the TRACE MODE software.
CIM
Pn Pn+1
Tn+1 Tank n TSn Tn TSn+1 TSn+2 Tn+2 Ln Qn
DIM
Signal Bus
The lower level of the developed system include the following sensors and
actuators: Dwyer Instruments 673-type pressure sensor, thermocouples of the L-type,
discrete float level sensors and normally closed Jaksa D224 valves. The average level
(level of controllers) consists of programmable logic controllers and the peripheral
input/output (I/O) modules: PLC ICP DAS WP-8131 [
To measure the level and temperature values of liquids each tank of the remote level measurement system according to Fig. 1 is equipped with: two analog pressure sensors (PS and HPS), three temperature sensors (TS) and one discrete level sensor (FLS). The values of level, temperature and pressure are measured with specified sensors in each tank. The according data acquisition module then converts the analog signals from sensors into the proper digital ones, which are transmitted to the PLC.
PLC receives the data of process parameters and gives control commands to the actuators. PLC-based control is performed by previously developed algorithm that is executed cyclically (receiving data ‒ processing ‒ control commands delivery (forming)). The information on the current values of level, temperature and pressure in each tank are displayed on the OTS with the help of the specialized human-machine interface developed in TRACE MODE 6 software.
In each tank the level of liquid is measured using the hydrostatic pressure sensor, installed in the lower point of the tank, and pressure sensor, placed in the upper point of the tank. The float level switch is used for self-testing operations implementation and accuracy increasing of the proposed system. The current values of the pressure from the PS and HPS are transferred to the current input module (I7017C). Then this module converts the value of the measured pressure (range 4…20 mA) into the digital signal. Then the digitalized signals are transferred to the PLC by RS485 bus with DCON protocol where level value is calculated taking into account average temperature in the tank and liquid density.
The average temperature values of the liquid are calculated for each tank separately as the arithmetic mean on the basis of the data received from the 3 temperature sensors (TS1…TS3) installed at different levels of tank height (in the lower, middle and upper points). In this case, the thermocouples are used as the temperature sensors which signals are transmitted to the data acquisition module (I-7018P).
Besides liquid level measurements, the developed system is able to calculate liquid
volume VL in each tank based on the polynomial formula [
VL a1L4 a2 L3 a3L2 a4 L a5 , (1) where L – measured value of the liquid level in the tank; ai − experimentally obtained polynomial coefficients, i = 1…5.
The pressures in the tank as well as the discrete readings of the average value of the level are measured by the PS, HPS and FLS. The signals from the sensor’s data are, in turn, transmitted to the current and discrete data input modules.
Designed system also has the function of output valves control. The normally closed valve is programmatically opened if the level in the tank exceeds the userspecified level limit and remains open until the level of liquid in the tank returns to the specified boundaries.
Thus, the authors used two thermocouples data acquisition modules with 8 inputs each, 1 current input module with 10 inputs, 2 discrete input modules with 8 inputs and 1 discrete output module as the data input/output modules for the designed system.
To improve reliability and accuracy of the proposed system the authors developed
discrete self-testing method for liquid level control systems [
For realization of the proposed method of automatic liquid level control with discrete self-testing two gauges are installed within the tank workspace (area). The first gauge is made as a hydrostatic pressure sensor, and the second measuring device made as a discrete fixed level sensor and installed on the side of the tank. The essence of the proposed method of control of liquid level in tanks with discrete self-testing is as follows.
Initialization of system components (PLC and I/O modules) is performed at the
beginning (block "Start" in Fig. 2). Then hydrostatic pressure sensor, located at the
bottom of the tank, measures the current value of the liquid hydrostatic pressure РL. After
that, the values of pressure (Pa) recalculated in level value of liquid (m) by the
equation for hydrostatic pressure [
At the next step the difference ΔL between the fixed value level LS, where discrete sensor FLS installed, and the level value L, measured using HPS in the previous step of the algorithm, is calculated.
Then the absolute value ΔLA of the obtained difference ΔL, which corresponds to the level measurement error of hydrostatic pressure sensor relative to the fixed level value (height) of the discrete sensor FLS installation, is determined. During the tank filling/emptying the current liquid level L approaches to a fixed level value LS and module of their difference ΔLA gradually decreases.
The main stage of the self-testing method for level control system in the tank occurs at discrete sensor FLS triggering. PLC observe this time moment as the moment when the derivative value changes from the signal at the output of the discrete level sensor. When filling the tank, the liquid level increases and reaches a fixed value (where the FLS is installed), the electric contacts of the FLS are locked. At the FLS contact closure signal differentiating a certain positive value D is formed. In turn, if the current liquid level value in the tank decreases relatively fixed level LS electrical contacts of fixed FLS are unlocked. At the FLS contact unlocking signal differentiating a certain negative value D is formed. In either case (at the tank filling and emptying) the change in the absolute value DA D of the signal from the FLS output after differentiation is monitored. When the obtained value DA becomes greater than zero DA 0 , the transition to the next step of the algorithm is performed.
L PL ρL g
L LS L
LA L
operation DA>0 Yes
1, at LA Lmax E
0, at LA Lmax
On this step the current value of ΔLA is recorded in the measurements database that can be subsequently used for correcting of liquid level values in the tank measured by hydrostatic sensor ("Result of calculation ΔLA" step in Fig. 2).
Then PLC checks whether the obtained level measurement error value ΔLA exceeds some maximum permissible value, which is the maximum acceptable value of liquid level measurement error ΔLmax for hydrostatic pressure sensor HPS (set by the operator based on the required accuracy of level measurement and value of a fixed level for installed FLS).
Thus, self-testing process of analog measuring device HPS is implemented in the system: logical one pulse signal appearance in the variable E (E = 1) indicates hydrostatic sensor fault at FLS triggering. Consequently, the situation, where E = 0 at FLS triggering, corresponds to hydrostatic sensor operability and its functioning with a given accuracy (ΔLA ≤ ΔLmax).
The diagnostics results of the analog sensor (operability/fault) are displayed on the operator panel ("Diagnostic result" step in Fig. 2).
The diagnostics results of the analog sensor (operability/fault) are displayed on the operator panel. At the same time, the current value of ΔLA is recorded in the measurement’s database that can be subsequently used for correcting of liquid level values in the tank measured by hydrostatic sensor ("Diagnostic result" step in Fig. 2).
Self-testing procedure can be finished ("End" step in Fig. 2) depending on usergenerated conditions and other factors, such as a periodic operability checking of analog pressure sensor, low computing capacity of controller and so on.
Thus, the operability state of the hydrostatic pressure sensor HPS is performed in the proposed liquid level automatic control system every time at the filling and emptying of the tank, especially at the liquid level transition through fixed value LS and discrete sensor FLS triggering (locking / unlocking of electric contacts), by selftesting method.
The described operations with physical quantities are implemented in practice in
the algorithm with digital signals in the PLC. There other approaches to the proposed
self-testing method implementation are also possible using separate electronic units,
FPGA-based systems [
SCADA is the main and remains the most prospective method for automatic
control of complex dynamic systems (processes) in the vitally important and critical (in
terms of safety and security) situations [
In the computerized system for remote level control with discrete self-testing the
human-machine interface (HMI) is realized using the tools which are provided by the
basic version of the SCADA-system TRACE MODE 6 [
The proposed in section III algorithm is implemented in the FBD language program (Fig. 4) as well as other required data processing algorithms (for range checking, values conversion, etc.) are executed directly in the PLC.
The developed HMI of the proposed liquid level control system with self-testing based on SCADA Trace Mode 6 is reconfigured directly in the PLC using program possibilities of Micro Trace Mode 6. 5
The results of computer simulation are shown in the time diagrams in Fig. 5. The liquid level L in the tank is in three states separated by time frame: the process of the level increasing in the tank from 0 m to 10 m (from 0 s to 2 s); constant liquid level in the tank (L = 10 m, from 2 s to 6 s); the process of the level decreasing in the tank from 10 m to 0 m (from 6 s to10 s).
Throughout the time interval from 0 to 10 s the registered electrical signal, which is coming from the HPS and corresponds to the current value of the liquid level in the tank L, is compared with an electrical signal that corresponds to a fixed level value LS, on which the FLS is placed. The electrical signal, that corresponds to hydrostatic pressure sensor measurement error ΔLA, is formed based on the mentioned above signals comparison. Diagnosing process occurs at transition of liquid through the fixed level value LS and fixed float level switch activation (the 1 s and 8 s time moments in Fig. 5). Increasing of the FLS signal F from 0 V to 5 V (1 s) and F signal drop from 5 V to 0 V (8 s) form a certain positive and negative values of differentiated signal D. The absolute value DA of the signal from the FLS output after differentiation operation is formed as DA D . At time instants (1 s and 8 s) when DA is greater than zero, the level measurement error ΔLA is compared with the maximum limit of the level measurement error ΔLmax.
When data signal ΔLA doesn’t excess maximum threshold ΔLmax (ΔLA ≤ ΔLmax), for example, at time 1 s (Fig. 5), the signal E (indicator of the hydrostatical pressure sensor fault) remains zero (E = 0). The result of self-testing procedure is the correct operation (the serviceability) indication of the installed HPS.
When data signal ΔLA excess maximum acceptable value of the liquid level measurement error in the tank ΔLmax (ΔLA > ΔLmax), for example, at time 8 s (Fig. 5), the indicator of the hydrostatical pressure sensor fault E signalizes about HPS fault (E = 1), that means incorrect operation. The result of self-testing procedure is the incorrect operation (malfunction) indication of the installed HPS.
Thus, the discrete self-testing procedures of the hydrostatic pressure sensor occur
at every moment of discrete fixed level sensor activation (on/off) during the entire
exploitation time of the corresponding tank. This gives the opportunity to perform the
ongoing monitoring of the specified accuracy of the liquid level measuring as well as
serviceable or faulty state of the liquid level control system HPS without conducting
of periodic diagnostic checks and specialized maintenance procedures. The given
quality reduces the probability of occurrence of undesirable situations or emergencies
on the technological objects caused by incorrect liquid level measurements. This
allows increasing safety and reliability of the technological objects [
Also, timely detection of low accuracy or malfunction of the hydrostatic pressure
sensor allows carrying out its repair or replacement on time, that, in turn, reduces
maintenance costs as well as increases safety and durability of the developed
computerized liquid level control system itself [
Therefore, the proposed discrete self-testing method gives the opportunity to increase the reliability and safety of the given computerized liquid level control system itself as well as of technological objects in which it is applied. 6
In this work, the authors described the structure and main components of developed computerized system for remote control of liquid level in tanks with discrete selftesting method implementation. The self-testing procedures provide increasing of reliability and accuracy of the developed computerized system for remote level control as well as reduction of periodic diagnostic checks of hydrostatic pressure sensors serviceability. The proposed system structure can be easily adopted for bigger systems and successfully implemented into existing control and monitoring systems of plants, ships, floating docks, etc.
The designed human-machine interface of the proposed system for remote control, measurement and monitoring of the liquid’s level allows the necessary information displaying on the main operator screen. Also, it provides the indication of liquid level error presence in real time mode for each hydrostatic pressure sensor placed in the tank. The information both on current indications of the system and the dynamics of their change through the graphic panels on each of the controllable parameters is available to the operator.
Temperature and pressure readings control inside the tank together with the discrete self-testing algorithm based on float level switches improve the reliability and accuracy of measurement and control, as well as reduce maintenance costs under conditions of prolonged exploitation.