Control Of Electronic Expansion Valves
EEVs have small stepper motors that open and close their valve port. They do this in response to signals sent to them by an electronic controller. Sensors like thermistors and pressure transducers are wired to an electronic controller and act as feedback devices to the controller telling it what is happening out in the actual refrigeration and air system in a refrigerated case. (See Figure 1.)
This gives the EEV very accurate control of refrigerant flow. Evaporator superheat and the refrigerated case's discharge air temperature can be controlled very precisely.
EEVs consist of many components often referred to as hardware. This hardware consists of the stepper motor itself, its wiring, and the controller. A controller with a built-in microprocessor controls the EEV.
There are also many transistors involved in the control of most EEVs. Transistors are nothing but solid-state switches. They receive a small electrical signal from the microprocessor to their base lead. This allows current flow from the emitter to the collector.
The microprocessor actually sequences signals to the base lead of the transistor. This sequencing turns the transistors on and off in pairs, which steps the EEV either open or shut.
Controllers and their software or algorithms have many different variations or schemes in which they control. Three types of control schemes are proportional, integral, and derivative.
In proportional control, actual superheat temperatures will try to approach the superheat set point but may not reach it. The difference between the actual superheat temperature and the set point is called the offset. Some means of predicting offset must be used because it changes with time and heat loads.
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With integral control, the control program or algorithm calculates the deviation or the amount of offset that is changing and is added to the set point. It does this by calculating the area under the curve of time versus temperature. This type of control scheme is often referred to as reset.
A derivative control scheme looks at the rate of change in temperature versus time graph or its slope. If the rate of change is great, the software of algorithm steps the valve faster in order to satisfy the set point.
Algorithms
In order for the hardware to operate, a set of instructions or software must be given to the microprocessor. As mentioned earlier, this set of instructions is often referred to as an algorithm. Below is an example of an algorithm for an EEV trying to control evaporator superheat.Notice that the last line of the algorithm closed the valve 1,000 steps. This is a process referred to as overdriving. Most EEVs have been made to handle overdriving without damage to the valve. Overdriving makes sure the EEV is closed so there will not be any damage to the compressor from flooding or slugging refrigerant. This algorithm, however, will only allow the EEV to control 5 degrees of superheat. Any higher superheat will open the valve and any lower superheat will close the valve.
A more advanced algorithm uses "let" statements and "input" statements. There are also variables like X and Y for numbers to be assigned. "If-then" statements are also used, and each line of the algorithm is numbered.
The line numbering allows for loops to be used in the programming. Below is an algorithm that will eventually allow the EEV to reach its superheat set point. It is referred to as a proportional algorithm because it will change the EEVs output (steps) directly in relation to the input (evaporator superheat).
20 Let ‘X' be evaporator superheat set point
Because proportional control will experience overshoot and undershoot of the set point, adding the integral feature can enhance proportional algorithms as is the case with the one above. Remember, integral control senses the actual deviation of actual evaporator superheat from the set point. An offset can then be applied to the valve. Below is an example of an integral algorithm enhancement. If average evaporator superheat for 40 seconds is 3 degrees high, then open the EEV 15 steps. Another enhancement that can be added to the proportional and integral control algorithm is a derivative feature. The rate of change of evaporator superheat is sensed by estimating the slope of the curve for the change in evaporator superheat. A greater slope means faster changes to the EEV steps. Below is an example of a derivative algorithm enhancement.
100 If the evaporator superheat had decreased 0.5 degrees in 10 seconds, then close the EEV 15 steps
Notice that the derivative enhancement to the software or algorithm dealt with rates of changes of evaporator superheat.
John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at tomczykj@tucker-usa.com.
Publication date: 09/06/2004
30 Input ‘X'
40 Let ‘Y' be actual evaporator superheat
50 If X=Y, then close the valve 0 steps
60 If X>Y, then close the EEV 1 step
70 If X
101 If the evaporator superheat had decreased 5 degrees in 3 seconds, then close the EEV 150 steps.
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