Method for controlling a compressor installation

Abstract

A method for controlling a compressor system comprising a plurality of compressors, wherein the compressor system is intended to maintain a predefined excess pressure in a pressurized fluid system, wherein decisions are met at fixed or variable intervals as to switching operations for adapting the system to current conditions, wherein—in a pre-selecting step, switching alternatives are excluded from the plurality of combinatorially available switching alternatives, —in a main selecting step, remaining switching alternatives are weighed against one another while referring to one or more optimization criterion (criteria) and optimum switching alternatives are selected from among the given criteria, and—in a control step, the selected switching alternative is output for implementation in the compressor system.

Claims

1. A method for controlling a compressor system comprising a plurality of compressors, the compressor system maintaining a predefined excess pressure in a pressurized fluid system despite a withdrawal of pressurized fluid from the pressurized fluid system, the method comprising: during operation of the compressor system, calculating a variable switch-off pressure; increasing generation of compressed pressurized fluid when the predefined excess pressure in the pressurized fluid system reaches a switch-on pressure, wherein the switch-on pressure is higher than an adaptation pressure which is not to be undercut by the predefined excess pressure in the pressurized fluid system; and after calculating the variable switch-off pressure, reducing generation of compressed pressurized fluid when the predefined excess pressure in the pressurized fluid system reaches the variable switch-off pressure, wherein the adaptation pressure and the variable switch-off pressure define a pressure band, and wherein the variable switch-off pressure is calculated by periodically and computationally minimizing a quotient of a total work loss in a predefined periodic time interval concerning one switching alternative, and the periodic time interval itself, the periodic time interval being based on virtual switching cycles, the periodic time interval representing a time for a pressure profile to rise from a minimum pressure value to a maximum pressure value and subsequently decrease to the minimum pressure value.

2. The method according to claim 1, wherein the switch-off pressure is assessed or calculated on a case-by-case energy optimization basis.

3. The method according to claim 1, wherein an optimum switch-off pressure is calculated based on the following formula:
Δp.sub.switch,opt=√{[Σ(P.sub.no-load⋅T.sub.no-load)+ΣW.sub.switch]/[0.5⋅r.sub.load⋅(P.sub.load1/ldp/dtl.sub.average1+P.sub.load2/ldp/dtl.sub.average2)]} with W.sub.switch as switching work loss, P.sub.no-load as no-load performance of individual compressors to be switched, T.sub.no-load as after-running time at no-load of individual compressors to be switched, r.sub.load as relative increase of load performance of load-running compressor per pressure unit, P.sub.load1 as load performance of the compressors to be switched included, ldp/dtl.sub.average1 as amount of the expected average pressure increase during a real pressure profile toward the switch-off pressure, P.sub.load2 as load performance of the compressors to be switched excluded, and ldp/dtl.sub.average2 as amount of an expected average pressure increase during the pressure profile toward the switch-on pressure from ldp/dtl.sub.average1 and the pressure compensating effect of the compressors to be switched.

4. The method according to claim 1, wherein the switch-off pressure is calculated based on an energy demand of one or more of load running compressors optionally when supplying against a continuously increasing pressure; no-load running losses of the load running compressors switched to no-load running or stopped status; no-load running losses of no-load running compressors; or switching loss energy of the compressors switched based on the selected switching alternative.

5. The method according to claim 1, wherein the switch-on pressure is calculated such that an actual pressure profile reaches a calculated adaptation pressure, which is below the switch-on pressure at a deviation of less than 5%.

6. The method according to claim 1, wherein determining of the switch-off pressure or switch-on pressure is performed in real-time.

7. The method according to claim 1, wherein control of the compressor system is based on empirical parameters from past switching operations.

8. The method according to claim 7, wherein the empirical parameters include one or more of: a level of energy demand of each of the compressors, switch-on response times of the compressors, consumption behavior of consumers of the pressurized fluid, a size of a pressure accumulator, or a pressure compensation degree of one or more compressors.

9. The method according to claim 7, wherein the empirical parameters include one or more of: a pressure compensation degree of a compressor depending on a storage volume and an installation scheme of the pressurized fluid system; a level of energy demand of a compressor depending on a previous mode of operation of the compressor, environmental temperature, maintenance, wear and contaminating states; or a switch-on response time and pressure compensation degree of a compressor depending on typical patterns of change of the withdrawal of pressurized fluid.

10. The method according to claim 1, wherein under predefined conditions, switch-on or switch-off commands triggered upon a real pressure profile reaching the switch-on pressure or the switch-off pressure are suppressed or additional switch-on commands or switch-off commands are triggered independent of the real pressure profile reaching the switch-on pressure or the switch-off pressure.

11. A non-transitory computer readable storage medium having stored thereon computer executable instructions which, when executed on a computer, configure the computer to perform a method for controlling a compressor system according to claim 1.

12. The method according to claim 1, wherein the switch-on pressure is calculated such that an actual pressure profile reaches a calculated adaptation pressure, which is below the switch-on pressure at a deviation of less than 2%.

13. The method of claim 1, wherein the switch-off pressure is a sum of the adaptation pressure and the calculated energetically optimum switching cycle pressure difference Δp.sub.switch,opt.

14. The method according to claim 1, wherein determining a current value of the withdrawal of pressurized fluid is either determined by a measuring device or calculated from a past real pressure profile, an operating state of the compressors, or an adaptively adjusted accumulator size of the compressor system.

15. The method according to claim 1, wherein a determined switching off is only triggered by the control when a necessary switch-on operation can be performed in time in consideration of a startup behavior of a possible switching combination.

16. The method according to claim 1, wherein a switching on of one or more compressors is performed such that in consideration of the startup behavior of the one or more compressors and in consideration of preferably adaptively learnt switch-on response times of each of the compressors, a real pressure profile reaches the adaptation pressure at a deviation of less than 2%.

17. The method according to claim 1, wherein each compressor has a different design or performance.

18. The method according to claim 1, wherein the switch-on pressure is variable.

Description

(1) The invention will be described in more detail below on the basis of exemplary embodiments which will be explained by means of the figures.

(2) Shown are in:

(3) FIG. 1 a schematic representation of a compressor system comprising a plurality of compressors;

(4) FIG. 2 a schematic representation of one embodiment of the control means according to the invention for controlling the compressor system shown in FIG. 1;

(5) FIG. 3 a schematic representation of one embodiment of the control according to the invention in a flowchart representation;

(6) FIG. 4 a representation of a real pressure profile in a compressor system indicating specific control parameters according to one embodiment of the control method according to the invention.

(7) FIG. 1 shows a schematic representation of a compressor system 1 which comprises six compressors 2 in total each connected to a communications bus 5. Via appropriate pressure lines, each of the compressors 1 is connected to processing elements 21 which can be realized as dryers or filters, for example. The six compressors 2 fluidically supply a central pressurized fluid reservoir 3 which additionally has a measuring means 20 which is communicatively coupled to the communications bus 5. The measuring means 20 allows in this case, for example, the pressure state within the pressurized fluid reservoir 3 to be measured continuously and is capable of forwarding measured parameters to the control of the compressor system 1 via the communications bus 5 which are available in the control method 41 (presently not shown) in terms of control engineering.

(8) The pressurized fluid supplied by the compressors 2 in the pressurized fluid reservoir 3 is forwarded to a user for withdrawal of pressurized fluid via an appropriate pressure line which can alternatively comprise further functional elements 22 (in the present case e.g. a control valve). The control or regulating of the overpressure maintained within the pressurized fluid reservoir 3 is performed by means of a central control means 4 which is presently not shown but is communicatively coupled to the communications bus. In this case, the communication between the compressors 2 and the communications bus 5 can take place via conventional wired signal lines or else via wireless communication paths.

(9) In accordance with the embodiment, the selected communication protocol can ensure the control method explained below in more detail to be executed in real-time. The pressure prevailing within the pressurized fluid reservoir 3 is detected by the measuring means 20 preferably likewise in real-time. Practically, a sampling in time intervals of less than one second, preferably less than a tenth of a second is suitable for this purpose. In typical pressurized fluid applications, the measuring means 20 will measure an overpressure within the pressure reservoir 3. In also possible vacuum applications, the measuring means 20 will measure, as described above, a corresponding negative pressure which can likewise be provided in the pressurized fluid reservoir 3. As will be clear to the skilled person, the compressors 2 are for this purpose replaced by appropriate vacuum pumps. The pressure value detected by the measuring means 20 can be more or less smoothed, evaluated on an absolute or time-differential or combinatory basis depending on the purpose of use, in order to being introduced in the control or regulating method. The thus conditioned pressure value can be used inter alia for calculating an energetically optimum switch-off pressure 103 (presently not shown), for calculating a pressure compensation degree of the compressors, and for calculating the compressors' switch-on response times at stopped status or in the no-load state.

(10) Moreover, a further measuring means can be provided supportingly which is likewise connected to the central control means and ascertains the measured pressurized fluid consumption, respectively the withdrawal of pressurized fluid in order to determine e.g. switch-on response times with higher accuracy.

(11) The compressors' operating data exchanged with the central control means via the communications bus 5 inter alia concern the current operating state of each compressor. This information is required by the controlling or regulating method among other things for selecting the compressors to be switched to load. Furthermore, this information comprise the motor speed based on which the control method can ascertain the energy consumption of a compressor or a compressor group. Same can further include information on compressor-internal pressure sensors for assessing after-running times, when the compressor e.g. is at no-load running, respectively expected after-running times, when the compressor is at load running, as well as information on whether the compressor is in load operation at all or not. Alternatively, some or all of the mentioned operating data of the compressors can also be remodeled or approximated by way of data processing so that same do not need to be exchanged via the communications bus 5 and are all the same available to the central control means in sufficient approximation.

(12) For application-technological reasons, the compressor system 1 can moreover comprise editing elements 21 which give cause to a characteristic change of the system-internal fluid pressures. The influence of the editing elements 21 within the compressor system 1, however, can be appropriately compensated for by a suitable adaptive learning behavior of the control or regulation. An increasing time delay in the pressurized fluid conveying between a compressor and the central pressurized fluid reservoir due to a filter being increasingly contaminated, in the form of an increasing switch-on response time of the compressor from the off-state as well as the no-load running state can, for instance, be adaptively compensated. Such an increasing switch-on response time can be simply compensated by the control so that the increasing filter contamination does not affect the maintaining of the predefined overpressure within the pressurized fluid reservoir 3.

(13) Based on application-technological considerations, the compressor system 1 can moreover comprise one or more pressure regulating valves for pressure stabilization.

(14) FIG. 2 shows a schematic representation of the control method of control means 4. The control means 4 is in this case in communicative contact with the communications bus 5 and can both read in and out data. The control means 4 can optionally transmit switching commands to single compressors 2 via the communications bus 5. For the feeding of control parameters, respectively inputting of data for characterizing the compressors 2, the control means 4 includes a feeding interface 40. Said data is transmitted to the control method 41 which can be implemented as a software application in terms of an adaptive control method. The control method 41 generates suitable control commands, respectively switching commands for controlling the compressors 2 which are transmitted to the compressors 2 via the communications bus 5. The control method 41 includes a control algorithm 42 for this purpose which optimizes the energy demand of the compressor system within a pressure tolerance range between adaptation pressure 101 and upper pressure limit 104. It is to be noted here that the control algorithm 42 can also be understood as a regulating algorithm. The control means 4 comprises further a presently not shown system clock having an appropriate timer which is capable of provide an appropriate timing to the control method 41.

(15) In accordance with the embodiment, the control algorithm 42 permits an energy-guided adaptive regulating and assesses a switch-off pressure 103 for the compressors to be switched off load within the available pressure tolerance range in an energetically targeted manner. For this purpose, the control algorithm 42 calculates the energetically optimum switch-off pressure 103 in a mathematically analytical form. This optimum switch-off pressure 103 is defined in accordance with the embodiment by the minimum value of a function which describes the total power loss of all of the compressors 2 of the compressor system 1 during one switching cycle depending on the switch-off pressure 103. In accordance with the embodiment, then assumption enters here into the calculation that the withdrawal of pressurized fluid remains on average constant and that the switching cycle therefore repeats uniformly between two successive minimum, respectively maximum pressure values. The assumption of a pressure fluid withdrawal being on average constant allows for pressure profile fluctuations to be taken into account in the real pressure profile as well.

(16) In doing so, the energy-guided control algorithm 42 makes use of present regulation-technological degrees of freedom in that these degrees of freedom are not occupied or limited by fixedly predetermined control parameters or else a too small or strictly predetermined pressure regulating range, but optimizes same with respect to energy. Both the selecting of the compressors 2 to be switched and the points in time, respectively pressures for the switching operations to be implemented are not parameterized but are calculated by the control method 41 on a case-by-case basis in en energetically optimized manner.

(17) Apart from the energy-guiding of the control method 41, same is also characterized by an adaptive behavior with respect to adapting adaptive parameters during running operation. In doing so, the adaptive behavior supports the optimizing of the energy demand of compressor system 1 decisively. The adaptive behavior is based on an adaptation algorithm 43 comprised by the control method 41 which adjusts all adaptive parameters during the compressor system's operation and makes them available to the control algorithm 42. The adaptive behavior also allows the selecting of the compressors to be switched to being automatically adapted to regulation-relevant, fixed and variable characteristics respectively conditions of the compressor system as well as the use thereof in the running operation. Examples of such adaptive parameters can be the energy demand per conveyed fluid amount of a compressor 2, as well as the pressure-technologically active storage volume of the pressurized fluid system and the temporal switching behavior of the compressors 2.

(18) FIG. 3 shows a schematic representation of the sequence of single steps in accordance with one embodiment of the inventive method for controlling a compressor system 1 in a flowchart representation. In this case, determined switching alternatives 13 are excluded in a pre-selecting step 10 in an excluding means 6 of a control means 4 not shown in greater detail from the multitude of combinatorially available switching alternatives 13, preferably while taking into account the current conditions. The pre-selecting e.g. can be performed on the basis of selection criteria taking into account the technical feasibility of the predetermined switching alternative 13. In the present case, for instance, eight combinatorially possible switching alternatives 13 are available in total, from which four switching alternatives 13 (deselected by crossing out) have shown to be inappropriate for the present operating conditions and hence are deselected in advance. From the remaining four switching alternatives 13, one switching alternative 13 is selected in a main selecting step 11 in a selecting means 7 of the control means 4 not shown in greater detail while referring to one or more optimizing criteria by weighing up all of the switching alternatives 13 against one another which had not been deselected in the pre-selecting step 10. The selected switching alternative 13 assessed in the main selecting step 11 is output in a control step in an output means 8 of the control means 4 not shown in greater detail for being implemented in the compressor system 1. In the present case, the outputting has been illustrated symbolically as a forwarding of information from the output means 8 to the communications bus 5 which, however, should not be understood here as being limiting.

(19) FIG. 4 shows a representation of the real pressure profile 105 in the pressurized fluid system during a periodic time interval T.sub.switch. The length of the periodic time interval T.sub.switch here is related to just the length of one switching cycle. In accordance with one embodiment of the inventive control method, the control assesses an individual switch-off pressure 103 for the compressor 2 to be switched from load within the available pressure tolerance range according to principles of energetic optimization. The pressure tolerance range is in this case the pressure range between an adaptation pressure 103 not to be undercut and an upper pressure limit 104 not to be exceeded. In accordance with the embodiment, the energetically optimum switching cycle pressure difference and the energetically optimum switch-off pressure 103 for the compressors 2 to be switched to load is mathematically-analytically calculated as an energetic optimum. For this calculation will be assumed that the withdrawal of pressurized fluid is on average constant. The pressure drop can consequently be represented as a rise of a linearly falling straight line which approximately describes the real pressure profile. Analogously, the increase of pressurized fluid within the pressurized fluid system can be described by a largely similar mathematical averaging of the really rising pressure profile as a monotonously rising straight line.

(20) Under these presuppositions of on average constant withdrawal of pressurized fluid, the switching cycle including the next switch-on operation can be energetically described by a simple mathematical representation. Due to this simple mathematical representation it is possible to calculate the energetic optimum respectively the maximum efficiency of the compressor system during such a switching cycle. For this purpose, the control method 41 regulates the switch-off pressure 103 of the compressors 2 to be switched such that the entire total power loss depending on the switching cycle (total work loss per periodic time interval T.sub.switch) becomes minimal.

(21) Both the load-running as well as the switching and idle running compressors 2 contribute to this power loss depending on the switching cycle. The energy demand of the load-running compressors (load work) increases with the switching cycle pressure difference since the internal working pressure difference thereof increases on average. In contrast hereto, the switching work loss as well as the no-load work loss of the compressors to be switched decreases with an increasing switching cycle pressure difference since the number (frequency 9 of switching cycles decreases. The sum of loss components in an energetic optimization occupies a minimum in the calculated switching cycle pressure difference. The expression to be minimized result in accordance with the following equation (1):
P.sub.V=(ΔW.sub.load+ΔW.sub.no-load+ΔW.sub.switch)T.sub.switch  (1)
In this case, ΔW.sub.load is the work loss of the load-running compressors per switching cycle due to the pressure elevation as compared to the switch-on pressure, ΔW.sub.no-load is the no-load work loss of the compressors to be switcher per switching cycle due to the no-load performances and after-running time thereof, ΔW.sub.switch is the switching work loss per switching cycle of the compressors 2 to be switched due to the slow internal pressure compensation process during switching in no-load running, probably of a motor restart, and the internal pressure adaptation when switching to load, T.sub.switch is the duration of a switching cycle which temporally extends over a periodic pressure increase and the subsequent pressure drop.

(22) The individual components of the total work loss are in this case calculated in accordance with equation (2):
ΔW.sub.load=0.5⋅r.sub.load⋅Δp.sub.switch.sup.2.Math.⋅(P.sub.load1/ldp/dtl.sub.average1+P.sub.load2/ldp/dtl.sub.average2)  (2)

(23) In this case, r.sub.load is the relative increase of the load performance of load-running compressor 2 per pressure unit, Δp.sub.switch is the switching cycle difference, P.sub.load1 is the load performance of the compressors, the compressors 2 to be switched included, which are load-running toward the switch-off pressure 103 in the course of the pressure profile at the switch-on pressure 102, ldp/dtl.sub.average1 is the amount of the expected average pressure increase during the real pressure profile toward the switch-off pressure 103, calculated on the basis of a commensurate period of time, P.sub.load2 is the load performance of the compressors, the compressors 2 to be switched excluded, which are load-running toward the switch-off pressure 103 in the course of the pressure profile at the switch-on pressure 102, ldp/dtl.sub.average2 is the amount of the expected average pressure increase during the pressure profile toward the switch-on pressure 102 from ldp/dtl.sub.average1 and the pressure compensating effect of the compressors 2 to be switched.

(24) The no-load work loss ΔW.sub.no-load is calculated on the basis of the following equation (3):
ΔW.sub.no-load=Σ(P.sub.no-load⋅T.sub.no-load)  (3)

(25) In this case, P.sub.no-load is the no-load performance of the individual compressors to be switched, and T.sub.no-load, the after-running time at no-load of the individual compressors to be switched, is restricted to a time between the switching on and off.

(26) The switching work loss ΔW.sub.switch is calculated as a sum of the switching work losses W.sub.switch per switching cycle of the compressors 2 to be switched in accordance with the following equation (4):
ΔW.sub.switch=ΣW.sub.switch  (4)

(27) Furthermore, the periodic time interval T.sub.switch of a switching cycle can be easily calculated in accordance with equation (5) based on the following correlation which results from simple geometric considerations as per FIG. 4:
T.sub.switch=ΔP.sub.switch.Math.⋅(1/dtl.sub.average1+1/ldp/dtl.sub.average2)  (5)

(28) The calculation of the energetically optimum switching cycle pressure difference Δp.sub.switch,opt can be calculated using equation 81) by simply inserting the terms for the individual work losses ΔW.sub.load, ΔW.sub.no-load, ΔW.sub.switch as well as the length of the periodic time interval (switching cycle duration) T.sub.switch into the formula as per equation 1 for the power loss P.sub.V which depends on the switching cycle, by subsequently deriving according to the switching cycle pressure difference Δp.sub.switch and correspondingly zero-setting of the derivation. Consequently, the energetically optimum switching cycle pressure difference Δp.sub.switch,opt can be represented as a mathematically easy to handle expression in accordance with equation (6):
Δp.sub.switch,opt=√{[Σ(P.sub.no-load⋅T.sub.no-load)+ΣW.sub.switch]/[0.5⋅r.sub.load⋅(P.sub.load1/ldp/dtl.sub.average1+P.sub.load2/ldp/dtl.sub.average2)]}  (6)

(29) The energetically optimum switch-off pressure in applications of pressurized fluid results as a sum of the adaptation pressure 101 and the calculated energetically optimum switching cycle pressure difference Δp.sub.switch,opt. In corresponding vacuum applications, for example, the energetically optimum switch-off pressure 103 results as the difference of the two previously mentioned values as will be clear to the person skilled in the art.

(30) It should moreover be pointed out that the control method in accordance with the embodiment takes into account the delay times of the individual compressors 2 or combinations of compressors 2 which are determined from the times between the switching on or off of a compressor 2 and the points of time of the actual implementation of the change of state. Accordingly, the switch-on times T.sub.on just as the switch-off times T.sub.off are temporally advanced in comparison to the minimum pressure values of the real pressure profile 105 respectively the maximum pressure values.

(31) Furthermore, FIG. 4 shows a partly idealized switching cycle for illustrative purposes. An upper pressure limit 104 is defined by system-contingency, e.g. by the components' pressure resistance. The lowermost line in the diagram represents the adaptation pressure 101 which already had been discussed several times. The pressure profile in the switching cycle illustrated here moves between a (local) minimum value P.sub.min and a (local) maximum value P.sub.max. At a point of time T.sub.AB, namely upon reaching the switch-off pressure 103 at a rising pressure profile, measures are taken for reducing the generating of compressed pressurized fluid, which have the effect that the pressure shortly rises above the switch-off pressure 103 to the (local) maximum value P.sub.max but then the pressure increase reverses into a pressure drop. Once the switch-on pressure 102 is reached at a falling pressure profile, measures are taken for increasing the generating of compressed pressurized fluid so that the pressure further decreases to a (local) minimum value P.sub.min but the pressure drop then reverses into a new pressure increase.

(32) It is to be noted at this point that all of the above described components, whether alone or in any combination, are claimed as being essential to the invention, optionally the details depicted in the drawings. Variations thereof will be familiar to those skilled in the art.

LIST OF REFERENCE NUMERALS

(33) 1 compressor system 2 compressor 3 pressurized fluid reservoir 4 control means 5 communications bus 6 excluding means 7 selecting means 8 output means 9 switch-off pressure determining means 10 pre-selecting step 11 main selecting step 12 control step 13 switching alternative 20 measuring means 21 processing element 22 functional element 30 data record 40 feeding interface 41 control method 42 control algorithm 43 adaptation algorithm 101 adaptation pressure 102 switch-on pressure 103 switch-off pressure 104 upper pressure limit 105 real pressure profile T.sub.on switch-on time T.sub.off switch-off time