Patent Publication Number: US-7219507-B1

Title: Configurable, nonlinear fan control for system-optimized autonomous cooling

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to the fields of system cooling and nonlinear control, and, more particularly, the present invention relates to a configurable, nonlinear fan control for system-optimized autonomous cooling and method of operating the same. 
   BACKGROUND OF THE INVENTION 
   Cooling issues associated with the design of modern computer and communications equipment include the dissipation of high-power levels from temperature sensitive electronic devices. This requires system designers to pay special attention to protecting against temperature hazards while obtaining optimal noise performance. 
   Conventional cooling tasks in computerized systems commonly include monitoring and controlling the temperature of various temperature-sensitive devices and zones. Speed controlled fans are used for the actual cooling task. Commonly, each monitored zone or device may have a different operating temperature range/limit, which requires that each zone or device be controlled separately to provide the required operating temperature ranges. 
   Fan speed is controlled commonly through modification of the duty cycle (“DC”) of a pulse-width modulated (“PWM”) signal (hereinafter, broadly defined as a “control signal”). For purposes of illustration, it is assumed that the control signal is “off” when an associated fan is stopped (i.e., providing no cooling function and generating no noise), and is “on” when the fan is rotating to provide cooling power and generating noise (when the fan is at maximum power, the fan is rotating at “full” speed to provide maximum cooling power and generating maximum noise). 
   Turning initially to  FIG. 1 , illustrated is a temperature control graph of an exemplary PRIOR ART fan-cooling temperature control scheme. This scheme is commonly known as a linear control scheme. Conventional exemplary linear control schemes (e.g., NSC LM85, SMsC EMC6D100/EMC6D101, ADI ADM1027) may suitably consist of:
         a Minimal Temperature limit (“Tmin”) that defines a low temperature limit for activating cooling means in minimal cooling power (“DC min ”),   a Maximal Temperature limit (“Tmax”) that defines a high temperature limit for activating cooling means in maximum cooling power (“DC max ”), and   a Proportional Range in which the DC of the control signal is linearly changed in proportions with the changes of the measured temperature.
 
Under this scenario, when the fan is activated, the fan control performs a “spin-up cycle” in which the control signal may be provided with maximum DC to overcome fan inertia. Additionally, a hysteresis range (“H1”) may be defined to prevent sequential spin-ups that can occur due to temperature fluctuations near Tmin.
       

   A common problem with linear control schemes occurs from various disadvantages due primarily to the nonlinear nature of the cooling function and control components. For instance, fan speed and DC of the control signal may be represented in quadratic relation, V≅K×√{square root over (PWM DC)}. Such schemes cause abrupt PWM DC changes of the control signal that result in noisy fan speed oscillations. 
   The efficient cooling function shown in  FIG. 1  generates minimal acoustic noise, however, it is nonlinear, and therefore the linear control schemes of the PRIOR ART are not efficient, commonly suffering from significant acoustic noise. Further, additional conditions in controlled cooling schemes may arise due to uncertain factors related to the physical conditions, such as airflow, packaging, available space or the like, in each zone and in the system packaging. It often difficult to approximate an efficient cooling function as a result thereof, particularly since these conditions may vary from one system to the next. 
   In one example, a modified linear control scheme is implemented that improves temperature control efficiency by dynamically adapting the value of Tmin. By dynamically adapting Tmin as a function of the measured temperature, the slope of the proportional range may continuously be adapted to obtain more efficient cooling, however, this solution commonly requires complex algorithms including different updating cycles to update the Tmin limit value. It is therefore difficult to stabilize the temperature control loop in a specific system as it involves controlling the repetition rate of the two kinds of updating cycles. 
   More recently, efficiency problems involved in fan speed control schemes were minimized using nonlinear control schemes that are closer to the efficient cooling function of the system. In one example, the fan speed may be controlled by:
         gradually increasing the DC of the control signal whenever the measured temperature is above a predefined high limit,   gradually decreasing it whenever the measured temperature is below a predefined low limit, and   keeping the control signal constant/unchanging whenever the measured temperature is within said limits.
 
This control scheme minimizes some acoustic noise by preventing noisy fan oscillations outside limits, however cooling efficiency is not improved since no adjustments of the control signal are performed within the operating range defined by temperature limits.
       

   An alternate nonlinear control scheme utilizes a multi-step look-up table to program a non-linear transfer function for fan control. This solution provides improved efficiency and reduction in the acoustic noise by allowing rough approximation of a desired nonlinear transfer function. Nonetheless, performing control on the basis of a multi-step look-up table is costly in terms of memory resources and, if the number of steps is low, the scheme may cause abrupt changes of the control signal that result in increased acoustic noise. 
   In summary, all of the above-described methods have provided solutions to the fan controlled cooling problems that are less than optimal relative to the need in the art. An important goal in cooling system optimization is to generate minimal noise (by setting the control signal at the minimal effective DC) for a given power dissipated by the system. A need therefore exists in the art for efficiently cooling various devices in the system individually or in zones utilizing a nonlinear control scheme. A further need exists in the art for reducing the acoustic noise of fan cooling systems by utilizing a nonlinear cooling control scheme. A yet further need exists in the art for an improved temperature control scheme that allows accurate approximation of desired nonlinear control functions. A still further need exists in the art for an improved temperature control scheme that minimizes the stability problems of the temperature control loop by using a control process that is based a single-loop. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
       FIG. 1  illustrates a temperature control graph of an exemplary PRIOR ART fan-cooling temperature control scheme; 
       FIG. 2  illustrates a temperature control graph of an exemplary nonlinear fan-cooling temperature control scheme according to principles of the present invention; 
       FIG. 3  illustrates a system flow diagram of an exemplary method of operating the nonlinear fan-cooling temperature control scheme according to the principles of the present invention; and 
       FIG. 4  illustrates a system flow diagram of an exemplary method of controlling fan speed according to temperatures measured from various zones having different ranges according to the principles of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use with modern computer and communications equipment, a configurable, nonlinear fan control for system-optimized autonomous cooling. The features and technical advantages of the present invention are discussed in this DETAILED DESCRIPTION so that those skilled in the art may better understand the principles of the present invention. Various features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. 
   Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     FIGS. 2 through 4 , and the various embodiments used to describe the principles of the present invention in this patent document, are by way of illustration only. The present invention is directed to a method and system for controlling fan speed by accurately approximating a nonlinear temperature control function to be activated within a given temperature control range. According to an advantageous embodiment, a temperature control scheme allows accurate approximation of a cooling function. Efficient cooling functions are usually nonlinear and hence they are difficult to approximate. The temperature control scheme of the invention results in improved cooling efficiency and thus reduction in the acoustic noise by accurately approximating efficient cooling functions. 
   Turning to  FIG. 2 , illustrated is a temperature control graph of an exemplary nonlinear fan-cooling temperature control scheme according to principles of the present invention. According to the illustrated embodiment the nonlinear cooling function curve is divided into numerous linear segments, S 0 , S 1 , S 2 , . . . , S n . 
   For each segment (S i ), a temperature range (T i =T i+1 ) and a duty cycle range (DC i =DC i+1 ) are defined for deriving a slope (a i ) to be used according to a measured temperature (Temp). By adjusting the temperature range and the slope of each segment, an accurate approximation of a desired cooling function can be obtained. This cooling function may be used to optimize cooling system performance in terms of fan response, acoustical noise, and cooling efficiency. 
   According to the control scheme hereof, the measured temperature (Temp) is used to determine the control parameters (a i , b i ) that should be used at any given time to derive the DC value of the control signal (DC=b i +a i ·Temp). The control parameters of each segment are determined as follows: 
               a   i     =         DC     i   +   1       -     DC   i           T     i   +   1       -     T   i           ;       b   i     =           DC   i     ·     T     i   +   1         -       DC     i   +   1       ·     T   i             T     i   +   1       -     T   i                 
for i=0, 1, 2, 3, . . . , n, where T 0 =Tmin, T n+1 =Tmax, and DC n+1 =100%.
 
   A “high” hysteresis range (“H2”) may suitably be defined in which the control signal of all the fans is set to a fixed value for activating all the fans in “full” power (e.g., at least approaching 100% DC), if the measured temperature in one zone rises above a critical temperature limit (“Tabs”). 
   The control signal of all the fans remains set to the full power until the measured temperature decreases beyond H2. In this way, the control signal of all the fans is set back to the normal operation value only if the measured temperature has decreased beyond H2, which prevents sequential activations of the full power operation of all the fans, which may be caused due to temperature fluctuations near the Tabs. 
   Before undertaking the DETAILED DESCRIPTION of  FIGS. 3 and 4  below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller might be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. 
   Turning to  FIG. 3 , illustrated is a system flow diagram of an exemplary method of operating the nonlinear fan-cooling temperature control scheme (generally designated  300 ) according to the principles of the present invention. Nonlinear fan-cooling temperature control scheme  300  is illustratively associated with controller  10 . Exemplary controller  10  operates to divide the temperature control range into numerous linear segments each of which is associated with a sub-range of temperatures and approximates a segment of the control function. 
   Controller  10  determines control coefficients for each of the linear segments. The controlled temperature is measured and used for determining the corresponding sub-range, and the linear segment. A control signal is then calculated according to the control coefficients of the segment determined by the measured temperature, and it is continuously determined for new measured temperature and used for the activation of one or more fans. 
   Controller  10  is illustratively associated with exemplary computer and communications equipment  100 , such as any of the same found in industry or developed there from in the future for use in fields using computer processing, communications or the like employing system cooling and nonlinear control. 
   Nonlinear fan-cooling temperature control scheme  300  initially operates to measure the temperature (“Temp”) of the controlled device (or zone; process step  30 ). Scheme  300  determines whether the measured temperature is within the range defined by the minimal (“Tmin”) and maximal (“Tmax”) temperature limits of the control scheme (Tmax&gt;Temp&gt;Tmin; process step  31 ). 
   If the measured temperature is not within this range (“NO” branch of process step  31 ), the operation of the fan is determined according to the hysteresis ranges (H1 and H2), the currently measured temperature, and current state of fan operation (process step  37 ), as follows:
         If DC of CS=DC 0  and Temp is beyond the H1 range then set DC=0%;   If DC of CS=“Full Power” and Temp is beyond the H2 range then set DC=100%.   If DC of CS=100% and Temp is above Tabs then set DC=“Full Power” (DC=100% for all fans).
 
Upon completion thereof, control is passed to process step  30 , which reinitiates scheme  300 .
       

   If the measured temperature is within the range defined by the temperature limits (Tmax&gt;Temp&gt;Tmin; “YES” branch of process step  31 ), scheme  300  determines if fan operation was previously activated, or whether DC=0% (process step  32 ). 
   If DC=0% (“YES” branch of process step  32 ), then a spin-up cycle is activated (process step  33 ) and scheme  300  is reinitiated by passing the control to process step  30 . 
   If DC&gt;0% (“NO” branch of process step  32 ), then the measured temperature (Temp) is used to determine the operating segment S i  to be used with the control scheme (process step  34 ). Scheme  300  may, for instance, check if:
 
 T   i   &lt;Temp&lt;T   i+1  (for  i= 0, 1, 2, . . . ,  n ).
 
After determining the operating segment, scheme  300  determines the control parameters (a i  and b i ; process step  35 ), and DC is calculated and the DC of the control signal is set accordingly (process step  36 ). Scheme  300  then reinitiates by passing the control to process step  30 .
 
   Stated broadly, fan speed control may comprise defining a hysteresis range for setting the control signal to a value of minimal-operation whenever the measured temperature is reduced beyond a low-limit defined by the temperature control range, and setting the control signal to a no-operation (e.g., 0% DC) value whenever the measured temperature is reduced beyond the hysteresis range. An additional hysteresis range may be also defined above a high-limit defined by the temperature control range for setting the control signal to a full-power value whenever the measured temperature reaches this hysteresis range, and setting the control signal to the maximum-operation value whenever the measured temperature is reduced there beyond. 
   Further, as will be discussed in greater detail here below, “zeroing” one or more segments may reduce a number of linear segments. Optionally, all the segments may be “zeroed” except for one segment for performing linear control. Alternatively, the value of the control signal may be changed gradually from a current value to a newly calculated value, such values being dynamic or not relative to the related process. 
   Turning lastly to  FIG. 4 , illustrated is a system flow diagram of an exemplary method (generally designated  400 ) of controlling fan speed according to temperatures measured from various zones having different ranges according to the principles of the present invention. Scheme  400  for controlling fan speed according to such measured temperatures is illustratively associated with controller  10 . Exemplary controller  10  is again illustratively associated with exemplary computer and communications equipment  100 . 
   Exemplary scheme  400  may suitably be used to control cooling of a zone based on temperatures measured from various zones having different temperature ranges of operation (T min   (k) =T max   (k) , k=0, 1, 2, . . . ). According to such an exemplary embodiment, the temperature measured in each zone (Temp (k) ) may be normalized respective to a temperature range (T min   (k) =T max   (k) ) of the respective zone 
   
     
       
         
           
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   Scheme  400  initially operates to measure the temperature (“Temp (k) ”) of each zone (process step  40 ) and computes a normalized temperature (“Temp (k) norm”) of each zone (process step  41 ). 
   Scheme  400  determines a highest normalized measured temperature (process step  42 ), and the normalized value of Temp used for controlling the fan speed according to the control scheme hereof is set to the highest normalized measured temperature (Temp norm =max[Temp norm   (k) ]; process step  42 ). 
   According to an advantageous embodiment hereof, the operating range (Tmin=Tmax) is divided into multiple segments (i.e., advantageously, n=5) to approximate a nonlinear control function. In such an embodiment, the number of segments may suitably be reduced by “zeroing” the temperature and the DC (or duty cycle) range of one or more segments. 
   For instance, the control scheme can be reduced to one segment, S 0 , by zeroing the segments S 1 , S 2 , . . . , S n  as follows, T 1 =T 2 = . . . =T n =Tmax and DC 1 =DC 2 = . . . =DC n =100%. In this way, control scheme  400  (and, more globally, scheme  300 ), may suitably be reduced where appropriate to the conventional linear control by defining a single segment within the temperature range (Tmin=Tmax) required for the control, and zeroing all other segments. 
   Acoustic noise can be further reduced by changing PWM DC of the control signal in constant steps (ΔDC), which will prevent abrupt changes of DC in response to abrupt changes of measured temperature. A temperature filter, such as a two- or three-pole IIR low-pass filter, for instance, may also be used in an advantageous embodiment in order to remove interfering noise and possible spikes from the measured temperature. In this way interfering components are not affecting the temperature control operation. 
   Thus, stated broadly, fan speed control may be carried out by accurately approximating a nonlinear temperature control function to be activated within a given temperature control range by dividing the control range into numerous linear segments each of which is associated with a sub-range of temperature and approximates a segment of the control function and determining control coefficients for each of the linear segments. 
   The measured temperatures of one or more zones, each having a specific operating temperature range, may suitably be used for normalizing each measured temperature with respect to the specific range of the respective zone. The highest normalized measured temperature is used for calculating a Control Signal according to the control coefficients and the highest normalized temperature. The Control Signal is used for the activation of one or more fans, and is continuously determined according to new measured temperature. 
   Further, the fan speed control may further comprise setting the control signal to a maximum-operation value (e.g., 100% DC) whenever the measured temperature exceeds a high-limit (Tmax) defined by the temperature control range, or setting it to a minimum-operation value (e.g., DCmin) whenever the measured temperature falls beyond a low-limit (Tmin) defined by the temperature control range. 
   While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.