Single temperature sensor based evaporator filling control using excitation signal harmonics

Abstract

Abstract-An important aspect of efficient and safe operation of refrigeration and air conditioning systems is superheat control for evaporators. This is conventionally controlled with a pressure sensor, a temperature sensor, an expansion valve and Proportional-Integral (PI) controllers or more advanced model based control. In this paper we show that superheat can be controlled without a pressure sensor and without a model of the system. This is achieved by continuous excitation of the system and by applying Fourier analysis, which gives an error signal that can be used together with standard PI control. The proposed method works for systems with "S" shaped input/output maps that satisfy sigmoid function properties and such behavior has been identified in both an air conditioning and a refrigeration system. Tests on these systems show that the superheat can be controlled to a low level over a large operating range with only one sensor. It is believed that the method in general is applicable to a wide variety of nonlinear systems for which the desired operating points are close to points of zero mean curvature of system nonlinearities.

Utilization of excitation signal harmonics for control of nonlinear systems

“…The promising simulation results have lead to the implementation of the control method on two different test refrigeration system in our laboratory and the results are presented in [13].…”

Single temperature sensor superheat control using a novel maximum slope-seeking method

“…Additionally, if the I/O map has a non-positive first derivative (as in the cases shown in Section 4) then the integral gain K should be positive and negative in the other case. Previously, in Vinther et al (2012b) and Vinther et al (2012c), we used PI control instead of just integral control, which made it more difficult to tune the control and the P term can also make the output look less periodic due to jumps in i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 1 1 8 e1 1 2 9 the input signal, which results in a poorer estimation of the first and second harmonic. A method to tune the MSS controller for control of the evaporator filling in a refrigeration system will be given in the following, since the above guidelines are general in their statements.…”

“…We previously introduced the idea of searching for a maximum in the derivative or slope of an I/O map in Vinther et al (2012b) and Vinther et al (2012c). The novel solution named maximum slope-seeking (MSS) control is closely related to extremum and slope-seeking control, since they all rely on continuous excitation of the system.…”

“…This is particularly useful in systems with large delays. For a more detailed description of maximum slope-seeking see Vinther et al (2012b) and Vinther et al (2012c).…”

“…This gives large disturbances in the evaporation temperature and the heat load was additionally changed between approximately 2.8 kW and 3.6 kW with a constant water mass flow of 0.31 l s À1 . The chosen disturbance pattern is based on the test description in Izadi-Zamanabadi et al (2012) and equal to the test conducted in Vinther et al (2012b). This makes it possible to compare the proposed MSS control setup and safety logic with previous results and the TXV result obtained in Vinther et al (2012b), which is also shown in Fig.…”