Weak current College】 【ice storage air conditioning control in common sensors and actuators-Power By】 【China power house network.

On ice storage air conditioning room automation, we need to flow, pressure, level, temperature, and other physical quantities for sampling and detection.

Flow collection in qualitative and quantitative acquisition, acquisition of the former may use the flow switch, which is commonly used with electric remote rotor flowmeter, electromagnetic flowmeter and ultrasonic flowmeter. Rotor flowmeter for low cost, but because of its structural causes are not very suitable for use in the engine room control. Ultrasonic flow meter high metering accuracy, but the cost is too high or less. Insert-type Electromagnetic Flowmeter is divided into two classes, and flange type measurement principles and measurement accuracy of the same, but the slightly higher cost to the latter, the installation is the latter more trouble, but the former can only measure the velocity, to your computer after joining pipe diameter calculated flow. Electromagnetic Flowmeter installed on the pipe is based, in order to avoid the presence of air sampling sites and vortex.

Water pressure of the sample is divided into pressure and pressure. The pressure differential is samples in common with the pressure differential switch do qualitative sampling, i.e. the pressure reaches a certain value, contact closure. Pressure differential is also quantitative sampling, but because of the pressure sensor prices are often higher than the pressure sensor is more than double, so unless you have a specific reason (such as needed in the site constitute a control circuit), generally with two pressure sensors sampling and calculation by computer into differential pressure.

Liquid level detection is also divided into qualitative detection and quantification. Qualitative test using liquid level switch, the ceiling applicable to liquid level alarm or in liquid level lower startup fluid replacement device. Quantitative detection using liquid level do you need to accurately reflect, for example through level reflect the amount of expansion to calculate the amount of ice. Quantitative detection of commonly used tools are float type and pressure type two, whose precision is high, but to go through complex calculations.

Temperature sampling is the ice conditioning machine controlled by most. Different sampling principle of temperature sensors in a wide range of air conditioning room automation in comparative more resistive temperature sensor. From a strict sense, if the sensor is defined as the physical changes into electrical signals, so this type of sensor to temperature changes into resistance value changes, has not achieved from physical signals to electrical signals. This kind of sensor must be constant use resistance plus at both ends of the voltage change to reflect the change in resistance to enable the temperature of the sample. Therefore, this kind of sensor accuracy not only depends on the sensor itself, it also depends on the precision of a constant current source. At the same time as the transmission of wire-line on the sensor, so if measures are not taken, the conductor resistance with temperature changes or wiring resistance, long after the changes will affect the sampling accuracy.

Digital sensors is a prerequisite for fieldbus, and there are no links from the signal transmission and signal conversion link error. At this stage in the digital sensor because the cost is very difficult in small scale control system, but as manufacturing technology matures, most digital sensor price should be and the same accuracy of analog sensors balanced or even lower. In how to compare digital sensor and analog sensors, there is a misunderstanding that we should not let the system and device confusion. To temperature, for example, resistance temperature sensor needs with constant current source to use, but also through A/D conversion equipment to receive digital signals, in calculating the cost of these devices is required when the factors taken into consideration. Comparison of accuracy. Digital sensor precision already include constant current source of the error and A/D conversion device error, should belong to the system for precision and not accuracy, it is not the device simple in both comparisons between accuracy, design time cannot be used on equipment of precision to request digital sensor.

Now in more than one ice storage air conditioning works with digital temperature sensor and achieved good results. In addition to field bus lines technology advantages, digital temperature sensor and another feature is that it is not good or bad, there will not be allowed to the third State, or in the transmission and conversion occurs during damped and drift, which enables the automatic control system can safely use it, not because the sampling allowed by control of confusion.

On ice storage air conditioning room automation, in addition to the start-up and shut-down of the control device, the more commonly used control actuators are frequency converters and electric valve.

Inverter is used to adjust the pump speed, enabling flow, pressure, and other physical controls. Inverter is a simulation controller, automatic control system can achieve quantitative control. Now there are many industrial inverter fitted with RS485 bus interface or other bus interface for signal transmission of digital and bus.

The water temperature is controlled by the other circuit constant link, if we want to control the temperature of the water, you can use to adjust the frequency converter and then the flow pump speed, according to the energy equal to the flow and temperature difference between top-product relations can return water temperature control.

In the end for the variable flow system engineering, backwater differential pressure control, to avoid undervoltage or proximal distal over-voltage condition occurs. Control for water pressure difference there are a lot of ways, using the circulating pump inverter for speed regulation is a common method, the principle is the tuning cycle water flow so that it automatically keeps track of the amount at the end of the distribution, in order to stabilize the backwater of differential pressure.

The design uses inverter on the pump speed control system, the system of minimum flow calculations, generally if there is a minimum flow may be less than the design value of 20%, for consideration at the pump in the back of the loading and unloading valve to maintain the minimum flow pump. At the same time, the control system software designed to pump set lower limits of the regulation, when the speed limit values should stop responding to the regulation.

Electric valve into the switch valve and valve two categories, on-off valve more is used in the operating mode switch, to change the pipe system the logical relationship between devices, its design and use are relatively simple. Control valve is qualitative control devices, automatic control system through it to some of the physical quantities for precise control. Common control valve hasBall valves and butterfly valves in two, the structure of the caliber are restricted ball valve is small, so in large diameter project are often do control valve, butterfly valve or, using multiple ball valve of parallel use.

The design of the automatic control system, control valve door using a common common misconception is that the valve is linear devices. It is this mistake led to a number of engineering controls can not reach the desired result. From the actuator's angle, control valve is linear, that is, it's the valve to open degree is and automatic control system for analog control signals into a linear relationship. Also, if the valves at both ends of differential pressure is constant, then flow through the valve's flow should be and the valve to open degree into a linear relationship. Can the fact that the valve on the ends of differential pressure is distributed with the valve to open degree and change, which in the system of control signals and flow through the valve flow between a nonlinear distortion. Calculation and practice shows that this kind of nonlinear distortion if not to be in control of the model, the effect on the control of the compensation will have greater impact. To this kind of distortion in the control of mathematical model for compensation, the need for large-capacity and high-precision arithmetic, but on the valve control and real-time control, must be within the prescribed period is complete control of all the operations required, otherwise the next cycle will begin to form in the control cycle on nesting. Therefore, this automatic control system operation accuracy and speed make high demands.

Nonlinearity compensation for valves, current-controlled systems usually take three practices: the simplest approach is ignored. Such an approach more in the full control in the lower machine and host machine easy system. Because the slave's specialty is the control instead of the operation, so that the next bit machines achieve and non-linear compensation appropriate computing capacity will enable cost greatly increases the system loss of competitiveness, so that such systems are often not valve control characteristics of nonlinear clockwise to compensate, the price is relatively high running costs and poor comfort. The second approach was compensating all nonlinear operation that consists of a slave. This is standard practice, because in the DCS system, control and management has a clear division of responsibilities, all control-related operations should be in the next machine. But it is necessary to spend a lot of investment in improving the slave of computing capacity, so that it can meet the requirements. This is not the size under control, but according to the scale of operations on the lower selection of practices that will make the device functionality greatly increases the amount of redundancy, so a lot of investment just to meet the standards in the form of DCS structure. The third approach is to integrate complex operations refers to the host computer. This lower computer realization of information collection, the host computer for information processing Division of labour can make their own specialties are underutilized, thus the purpose is achieved without too much investment in improving equipment. When a computer fails, you can take control by the slave and the complex operations bypass in order to maintain the system's basic operation, which also can achieve decentralized DCS system failure risk purposes, only the top control effect appears unsatisfactory, but the resulting benefit is its investment than the second method to reduce the number.

More on the host computer and slave function distribution not only for valve nonlinear distortion compensation on pipe temperature control, etc need massive mathematical model of control loop can be used as a reference.

Ice cold air-conditioning automatic control system of basic functions

Ice storage air conditioning due to their characteristics and the control system has a dependency, and it determines the basic functionality of automatic control system. Generally speaking, ice storage air conditioning on the automation system has the following four basic requirements:

1. operating mode switch and the start-stop control device.

Ice storage air conditioning is on the same pipe system through the pumps and valves, and other devices with different combinations and different conditions and different condition combinations also reflect the different operational policy. Therefore, select ice storage air conditioning is to reduce the cost of running on the device and provides a possible, but really needs to reduce the cost of running all of the devices in the system to combine, and enable the operator to quickly and easily switch between the various operating conditions.

On specific projects, the different conditions on participation in the operation of pumps and valves open and close all have different requirements, at the same time, the device boot order and devices start interval has specific requirements. This requires the automatic control system for operating switch provides convenient, secure means of action. Ideally, the operator wished to pass the mouse on the screen, click on or through the menu selection you can switch condition. However, the automatic control system provides the ease of operation and to be able to prevent personnel misoperation, it is recommended that the operating mode switch, and system startup is a two-step operation, i.e. the switch condition is only for system start up conditions of choice, rather than directly after the switching condition of the system boots.

2. melt rate control.

In order to truly move peak, ice storage system is the pursuit of higher melting rate, with a view to be in peak power hours completely free cooling capacity. But the resulting problem is that if you do not melt rate control the ice plant will be the fastest speed of melting, the amount of waste caused by cold. Therefore, ice storage air conditioning system can request-rate control, so that it can track the load condition and meet system on cooling capacity requirements.

Control-rate methods vary, but generally can be grouped into two categories: changes in water temperature and changes in water flow. If the heat exchanger for ice unit load to describe that changed is the heat exchanger chilled water temperature of the side into the water, which changes the heat exchanger chilled water side into the water flow. Typically, a way to better take into account heat transfer efficiency, pursuit of lower heat temperature difference.

Control-rate of the ultimate goal is to control the temperature of the water. Because the pipeline in the great inertia, once established the trend toward natural after a temperature change and does not immediately on control system of regulation to respond, this makes the circuit of control properties to be soft, and there was a lot of lag. TubeRoad temperature these characteristics make regular PID often do not produce the desired effect. Therefore, in a more sophisticated application in the control model to measure the darker feedback, conditions permitting, you can control model introduced in certain warning measures so that the controllers of the actions before the temperature changes arise.

3, air conditioning, water supply and return water differential pressure control.

When a terminal with variable flow system, air conditioning water for water mains pressure difference between the end of use. Although variable flow Terminal system has many advantages, but if not for the difference between the return duct pressure control, its harm is evident. First, the differential pressure fluctuations will make the entire pipeline system control valve valve power of change, this will undermine the general stability of the PID control loop, when the valve to reduce the degree to a certain extent also causes the control valves. Second, when the pressure is low, will enable the distal end of the energy supply is insufficient, use effects; on the contrary, the differential is too large and will affect the security of Terminal system. Therefore, this requires the control system can make to the differential pressure, real-time monitoring and to take appropriate means to enable the adjustment of the differential pressure stability within a reasonable range.

Control air conditioning water for the difference between the return duct pressure, simple and proven method is in the air conditioning water for water mains on retrofitting the bypass valve, control system under real pressure to adjust the valve to open degree. In the heat exchanger system, this approach can guarantee through a heat exchanger secondary side flow constant values in the design, to take into account heat transfer efficiency and lower the temperature of the heat exchanger. The drawback of this method is the pump costs cannot be reduced with the load. At the same time, the bypass valve of the differential pressure vary greatly, this results in large degree the right to change the valve, the bypass valve is often the actual work in the on/off status, cannot achieve the desired effect of the control.

Therefore, another common approach is to use the pump speed is adjustable. Air-conditioning automatic control system under the water for the difference between the return duct pressure to adjust variable speed pump speed, so as to achieve the aim of stabilizing differential pressure. In order to be in the worst case still has enough water to ensure the safety of the water pump, many cases adopt speed pump after installation of the by-pass valve.

4. hourly load forecasting.

Strict sense hourly load forecast is not a control function, but because it on ice storage air conditioning system is important, therefore, included in the basic functions.

To minimize expensive peak electricity use, operators need to be on the same day there is an hourly load, thus develop operating strategies that seek to peak power hours rarely used cooler as far as possible, to make ice flux in peak power hours fully released. Lack of strict set action policies often yields this: or set aside too much cooling capacity resulting in a peak at the end of the electric window still more the amount of remaining cold, or prematurely will chill completely free but have to peak electricity use cooler periods. Develop action policy significance could reasonable arrangements in ice volume of use, in addition to the peak electric period to be set aside in the refrigerator because load lower operating efficiency decline use-melt cooling.

Develop action policies based on hourly, so, hourly load forecasts on ice storage air conditioning lower operating costs is important. However, to achieve the hourly load forecasts have considerable difficulty, this one is due to the impact factors of hourly load too much, mostly run on historical data. Current prediction method of hourly load of mostly historical data on the system running. Therefore, obtaining and maintaining a complete historical databases become realized hourly load forecast prerequisites, it is clearly in the operation have a certain degree of difficulty.

To enable the prediction of hourly load functionality feasible, this connection to be available without the aid of prediction method of historical databases, but also to be more simple and practical maintenance and means of establishing the historical database.