Improving the system with Open-Loop

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In an open system, the measured values in some way to show the man (analogue, numbers ...). Such measurements can provide satisfactory results without the use of computers. But digital computer incorporated in the measurement system in different ways can improve this kind of measurement. Of course it is a microcontroller and is measured mostly just one measured, and is essentially a measurement in the narrowest sense, and not on the monitoring process.

Example III

Measurement of thermal energy using a digital computer to boost accuracy.

Hardware part of the thermal energy meter contains mostly all of the elements shown in Figure 7.3.1, with additional circuitry to allow removal of the temperature measurement errors that occur due to changes zeros amplifier and built-in amplifier gain. Can be used two sensors to comparing their readings might suggest there are large discrepancies in data readings which would then pointed to the failure of one of them.

Using computers at gauge conceived in this manner allows:

Improvements could be reduced by a large data processing power and speed of computers. Let them work on the scale of the transferred heat the liquid medium, such as a central heating system. In such a system, measure the temperature of the incoming water at the entrance and at the exit of the feedback system through which water flows. Temperature is measured thermal sensor such as a thermo-electric elements, and the flow can be measured by magneto-inductive sensor or measuring the position of a float or trowel. The very kind of sensors for analysis in this case is not of importance. From the known values of temperature and flow rate can be calculated thermal power according to the expression:

 Heat equation

Where is:

q - media flow

td , tp - water temperature of incoming line and returning line

k ( td, tp ) - coefficient of thermal properties of the medium

Thus, the measured thermal power and thermal energy is obtained by integrating thermal capacity. Measuring a systematic measurement error because it can not be considered an accurate value of the thermal coefficient expressing the change of media properties with temperature. Besides measuring the flow depends on the temperature and the density of the media being measured.

Thermal energy meter was realized with the computer compensates nonlinearity thermo-element, then the nonlinear dependence of the differential pressure flow and takes into account the change in k-factor and flow with the temperature line of incoming and returning line. With the removal of zeros changes and changes in the amplifier gain amplifier, to reduce measurement error in a measurement system are connected by two monitor test resistance with each sensor (very high quality resistors, which almost does not change the value depending on external factors). Readings of two sensors and four monitor test resistance in each measurement cycle, the linear interpolation is then decoded values thus achieving self-calibrating temperature input. So essentially reduces device error caused environmental temperature changes and aging of components. If, in addition to the A/D converter operates with a successive approximation error is even smaller. The circuit includes a D/A converter for diagnostic purposes, and its analog output can be used for something else.

Besides the built-in meter diagnostic elements that allow the detection of interference with A/D conversions. For this purpose, a special diagnostic channel analog output data obtained from the D/A converter returns to the input of the A/D converter to be used again given binary number. The program can determine the difference that can be tolerated between the original input number and the number obtained by returning the output. To all of these functions can be realized, there are different programs that must be completed in a certain time, as follows:

  • Sampling of temperature
  • Sampling of monitor test resistor
  • Sampling of differential pressure
  • Check D/A and A/D conversion
    with channel of diagnostic
  • Checking the reasonableness
    of the measured values
  • Calculation of output values
  • Updating data display
  • Indication of the minimum and maximum
  • Updating totals in the EPROM
  • Updating the analog outputs
  • Updating digital outputs
  • Indication of interference

So, a lot of different programs with the purpose of proper measurement and control and self-correction. The computer's memory shall be recorded and kept the measured values to be used just for correcting systemic errors.

Based on the above it can be concluded that the measurement of this type can also operate without using a computer, but it is undeniable that the use of computers brings a number of improvements, increasing accuracy and eliminating the effects of the environment, and offers the possibility of performing some other ancillary activities. It is indisputable that the computer and more expensive process of measurement, but with the price drop of microcontrollers and the appearance is very cheap universal instruments on the market with the A/D converters and a small computer, and these measurements are all present.

Little trip to the past can remind us how it used to measure a location in the task of making some maps or tracks using scale bars and expensive optical instruments. Today there are handheld trackers positions that are capable of very little money, thanks to the development of microcomputer technology and sensors, show the position where they are 'in the meter'.

Improving the system of Closed-Loop

In closed loop tends to be a size closer to a desired size, or set to automatic adjustment process (regulation). Thus, a parameter wants to keep as close to a desired value, or regulate what is usually implemented so-called PID algorithm (Proportional Integration and Derivative controller). Sometimes pneumatic and electronic-analog devices are now replaced this purpose digital computer and the appropriate software support. These PID controllers are already in standard microprocessor designs.

In order to adapt to the specific requirements of the regulator when building the first structure the meaning that it defines the mode for a particular job and the selection mode either as a fixed value regulator, the regulator ratio of the two input analog signal or something else. Structuring sets the type of PI or PID control, operating mode (normal or reverse), the size of the regulated area (for example from 10 V up to 10'000 V), the type of input signal, the type of external setpoint, the default mode operation and more.

Universal controllers work so that for example every 100 ms performed at a basic cycle regulation. Developed that is located in the controller allows to perform a number of necessary functions such as:

  • Collection of input signals
  • Sending output signals
  • Calculation of regulatory algorithm
  • Communication with intelligent control panel
  • Store data at the moment of power failure
  • Calculation of the required values
    in the selection structure
  • Setting the parameters of the structure
  • A series of self-diagnostic functions
    to detect errors
  • Checking the status of the alarm
  • Sending alerts on errors
  • Communication with the
    coordinating computer system

All these and even some other computer tasks done in the controller, while the intelligent control panel associated with a particular serial microprocessor that communicates with the main controller during each basic cycle regulators. In addition the control panel includes a number of elements to display and used for structuring and process parameters. By performing this work can be replaced control panel 'blind panel', which will have virtually no element tuning and only basic views. In addition to the above must be provided manually managing executive device in case of failure of the regulator from working.

A special advantage of the universal controller is the possibility that they are the standard way of connecting with various other elements in the whole decentralized systems for monitoring and control of complex processes, which is a property of microcontrollers more or less autonomously perform local functions within the entire complex system. The degree of association between individual microcontroller to complete independence and continuous connection to a computer on a higher level. All together constitute the global system as a whole.


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