Abstract:
An externally controlled fan drive includes a fluid clutch that alters torque delivered to a fan housing to rotate the fan housing at a target fan speed. An adaptive controller measures the actual fan speed and adaptively updates the control logic to compensate for variable physical characteristics of the fan device.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to fans for cooling engines and to the control of such fans for accurately matching the cooling needs of the engine.  
         [0003]     2. Description of the Related Art  
         [0004]     Engines, such as the engines of automotive vehicles, generate heat during use and are subject to broad ranges of environmental heat. Accordingly, engines must be cooled to prevent overheating. The cooling needs of the engine are dependent upon characteristics of the engine, ambient temperature and the operating speed of the engine. Most engines are cooled by a liquid coolant that flows through channels in proximity to the engine. Heat from the engine is transferred to the coolant, and the heated coolant then passes through a heat exchanger or radiator. A fan directs cool air through the radiator to effect the cooling of the liquid.  
         [0005]     The cooling rate is dependent partly upon the temperature of the cooling liquid, the temperature of the air and the rate of flow of the cooling air. The rate of flow of cooling air is dependent upon the rotational speed of the fan, and the speed of the vehicle.  
         [0006]     Some prior art engines are configured to have the fan rotate sufficiently fast to accommodate all anticipated cooling needs. However, the fan is driven by the engine, and hence the engine operates less efficiently when there is more energy diverted to the operation of the fan. Additionally the rotating fan creates noise that is roughly proportional to the rotating speed of the fan. Thus, the fans in these prior art systems often generate more fan noise than is required.  
         [0007]     Problems associated with fan noise and engine operating efficiencies are well known in the art, and there have been many efforts to match the fan speed to the cooling needs of the engine. The assignee of the subject invention has done considerable work in the field of fan fluid clutches that have proved to be very effective in matching the fan speed to the cooling needs of the engine.  
         [0008]     The typical fan fluid clutch has a driving disc fixedly mounted to a rotating shaft of an engine. Thus, the driving disc rotates at the speed of the shaft of the engine. A housing is mounted rotatably to the shaft by bearings. Thus, the rotating shaft does not rotate the housing directly. Fan blades are mounted to the exterior of the housing. The interior of the housing includes a torque transmission chamber that surrounds the drive disc and an oil reservoir spaced from the torque transmission chamber. However, one or more oil supply holes and an oil recirculation passage provide communication between the oil reservoir and the torque transmission chamber.  
         [0009]     A torque transmission fluid is placed in the oil reservoir and can flow into the torque transmission chamber via the oil supply hole between the oil reservoir and the torque transmission chamber. The torque transmission fluid transmits torque from the drive disc to the housing in proportion to the contact area of the torque transmission fluid with both the drive disc and the housing.  
         [0010]     A valve is mounted in the housing and controls the oil supply hole between the oil reservoir and the torque transmission chamber. The valve opens when cooling needs are high. Hence, more oil will flow into the torque transmission chamber at these times. The greater amount of oil in the torque transmission chamber results in greater torque transmission between the drive disc and the housing. Under these conditions, the housing and the fan blades thereon rotate faster to provide more cooling. The valve closes the oil supply hole between the oil reservoir and the torque transmission chamber when cooling needs are low. As a result, less oil will flow into the torque translation chamber and the drive disc will transmit less torque to the housing. Under these conditions, the housing rotates more slowly and the fan blades on the housing direct less cooling air towards the engine. Additionally less energy is diverted from the engine to the fan when cooling needs are low, and therefore the engine operates more efficiently.  
         [0011]     Fan fluid clutches are described extensively in the patent literature, including many U.S. patents assigned to the assignee of the subject invention. The simplest of these devices provides a temperature sensor, such as a bi-metal strip, at an exterior position on the housing. The temperature sensor communicates with the valve to open the oil supply hole during periods of high temperature and to close the oil supply hole during periods of lower temperatures.  
         [0012]     More sophisticated fan fluid clutches have been developed in recent years and provide more inputs for controlling the movement of the valve that opens and closes the oil supply hole between the oil reservoir and the torque transmission chamber. These more sophisticated systems measure conditions such as: ambient temperature, vehicle speed, engine speed, air conditioning operating parameters, transmission oil temperature and the like. A controller is programmed with logic to determine a target fan speed (TFS) based on an analysis of these inputs. The controller then operates an electromagnet to move the valve relative to the oil supply hole. More particularly, the controller is programmed and calibrated to turn the electromagnet on and off for opening and closing the valve at a “duty rate” (DR) that is intended to achieve the appropriate volume of oil in the torque transmission chamber, and hence to achieve the target fan speed TFS. A prior art system that provides for outside control of the fan coupling is shown, for example in U.S. Pat. No. 6,550,596 and U.S. Pat. No. 6,915,888, which are assigned to the assignee of the subject invention. The disclosures of U.S. Pat. No. 6,550,596 and U.S. Pat. No. 6,915,888 are incorporated herein by reference.  
         [0013]     Some recent efforts in the field of externally controlled fan drives employ “smart fluid” in a torque transmission chamber. These systems use a constant volume of fluid in the torque transmission chamber, and hence avoid the use of condition responsive valves to control the volume of fluid in the torque transmission chamber. Rather, these recent efforts attempt to control the torque transmission characteristics of a fixed volume of fluid. For example, a magnetic field can be applied in a controlled manner to alter the viscosity of the torque transmission fluid. Examples of these externally controlled fan drives are described in U.S. Pat. No. 5,960,918, U.S. Pat. No. 6,032,772, U.S. Pat. No. 6,102,177, U.S. Pat. No. 6,318,531 and U.S. Pat. No. 6,585,092, the disclosures of which are incorporated herein by references.  
         [0014]     Prior art externally controlled fan devices (ECFD&#39;s) are better at achieving the target fan speed when the engine and the fan coupling apparatus are new. However, the ability to achieve the target fan speed TFS deteriorates over time. This deterioration is due to gradual wear of components in the fan coupling device and/or changes in the viscosity of the torque transmission fluid. These externally controlled fan devices measure the actual fan speed AFS and compare the actual fan speed AFS to the target fan speed TFS. These devices then change the duty rate DR in an effort to narrow or eliminate the difference between the target fan speed TFS and the actual fan speed AFS. However, the control logic in the prior art controller is calibrated based on new fan coupling apparatus and hence may not consistently match the target fan speed TFS if the externally controlled fan drive undergoes deterioration over time. As a result, these recent prior art externally controlled fan drives tend to swing the actual fan speed AFS substantial amounts to one side or the other of the target fan speed TFS. Prior art externally controlled fan drives that focus directly on the duty rate DR often provide less cooling than is required or more cooling, and hence less efficient engine operation. U.S. Pat. No. 6,807,926 is one example of a prior art externally controlled fan drive that includes actual fan speed as an input to the controller. This system has plural oil supply holes and plural valves. The duty rate DR of the respective valves are changed independently of one another in an effort to bring the actual fan speed AFS closer to the target fan speed TFS. However, a plural valve system is more complex then a single valve system. Furthermore, externally controlled fan drives that focus primarily on the duty rate DR are known to swing widely on one side or the other of the target fan speed TFS, as explained above.  
         [0015]     Accordingly, an object of the subject invention is to provide an outside control-type fan coupling apparatus that provides optimum cooling for an engine substantially uniformly over a long period of time.  
       SUMMARY OF THE INVENTION  
       [0016]     The invention relates to an adaptive control system for an externally controlled fan drive. The mechanical components of the externally controlled fan drive may be similar to the fan drive developed by the assignee of the subject invention and disclosed in the above-referenced U.S. Pat. No. 6,550,596 or U.S. Pat. No. 6,915,888. In particular, the fan drive may include a drive disc mounted to a shaft that is rotatably driven by the engine. The fan drive further includes a housing rotatably mounted relative to the shaft by bearings. An array of fan blades may be mounted to the exterior of the housing.  
         [0017]     A fluid reservoir and a torque transmission chamber may be formed in the housing. The torque transmission chamber surrounds the drive disc with a small gap therebetween. An oil supply hole may be formed in the housing to provide communication between the fluid reservoir and the torque transmission chamber. A recirculation passage then may be provided at a radially outer position on the housing to provide a return flow path between the torque transmission chamber and the fluid reservoir.  
         [0018]     A torque transmission fluid is provided in the housing and may circulate from the fluid reservoir to the torque transmission chamber back to the fluid reservoir. The torque transmission fluid has a consistency and viscosity to transmit torque from the drive disc to the housing. Thus, the torque transmission fluid enables the housing to be rotated in response to rotation of the drive disc. The amount of torque transmitted from the drive disc to the housing may be a function of the amount of torque transmission fluid in the torque transmission chamber. However, the amount of torque transmitted also may be a function of the characteristics of the fluid (e.g. viscosity) for a system using a fluid with controllable torque transmission characteristic.  
         [0019]     A valve may be provided in the housing for selectively opening and closing the oil supply hole between the fluid reservoir and the torque transmission chamber. Thus, the amount of torque transmission fluid that can flow into the torque transmission chamber may be determined by the operation of the valve. The valve preferably is operated by an electromagnet that repeatedly opens and closes the valve at a duty rate for controlling the flow of torque transmission fluid through the oil supply hole. Alternatively, an electromagnet may be used to vary the viscosity of the torque transmission fluid.  
         [0020]     The fan drive of the subject invention further includes an external controller. The controller is operative to receive input data relating to parameters that affect cooling needs. The inputs to the controller may include the throttle position, vehicle speed, engine speed, air conditioner operating conditions, transmission oil temperature, ambient temperature, actual fan speed and the like. The output of the controller may be a specified duty rate intended to achieve a target fan speed.  
         [0021]     As noted above, the efficiency of the prior art external control apparatus for a fan fluid coupling changes over time. Even though the control apparatus works well initially, effectiveness and efficiency degrade over time. Accordingly, the externally controlled fan drive of the subject invention includes an adaptive controller that continuously adjusts the control logic based on the sensor inputs, including the past and existing actual fan speeds. The adaptive controller may use a deterministic autoregressive moving average (DARMA) model where the current output is represented as a linear combination of past outputs and past and present inputs. Alternatively, other adaptive models may be used. Furthermore, a recursive least square algorithm may be used to continuously estimate the coefficients of the model. As a result, the adaptive controller does not merely change the duty rate, but continually revises the control logic based on a continual analysis of past and present outputs and inputs. Furthermore, the assessment of trends performed by the adaptive controller permits the adaptive controller to predict performance trends. Thus, in certain instances, the control logic can be altered preemptively to avoid a time delay between an assessment of operating conditions and a response to those operating conditions. Accordingly, the adaptive controller provides the optimum control algorithm for the current operating condition of the fan drive. The adaptive controller also can be used to compensate for changes to original equipment in a vehicle (e.g., air conditioning vs. no air conditioning or automatic transmission vs. standard transmission) without a complex recalibration of the controller. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a longitudinal view of an external control fan coupling device in accordance with the subject invention.  
         [0023]      FIG. 2  is a schematic view of the adaptive controller and fan device of the subject invention.  
         [0024]      FIG. 3  is a schematic view of an alternate adaptive controller with means for fixed control during a short interval prior to startup of an engine.  
         [0025]      FIGS. 4   a - 4   d  show the performance of an adaptive externally controlled fan coupling device.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     An externally controlled fan device (ECFD) in accordance with the invention is identified generally by the numeral  10  in  FIG. 1 . The fan device  10  is used with an engine  12 , such as the internal combustion engine of an automotive vehicle. The engine  12  generates heat and must be cooled to ensure continued efficient performance. Accordingly, a cooling fluid is circulated in and near critical parts of the engine  12 . Heat from the engine is transferred to the coolant, thereby raising the temperature of the cooling fluid. The heated cooling fluid then is circulated through a radiator that is identified generally by the numeral  14 . The fan device  10  is operative to generate a flow of air adjacent to the various heat exchange channels in the radiator  14  for cooling the fluid that had been heated by the engine  12 . The cooler fluid then is recirculated back to the engine  12 . The cooling needs of the engine  12  vary in accordance with several parameters, including the throttle position, the speed of the engine  12 , ambient air temperature, air conditioning operating characteristics, coolant temperature and such. The externally controlled fan coupling device  10  functions to generate flow of cooling air sufficient to achieve a heat transfer in the cooling fluid so that the cooling fluid can cool the engine appropriately.  
         [0027]     The fan coupling device  10  is mounted to a shaft  16  that is driven by the engine  12 . A drive disc  18  is mounted fixedly to an end of the shaft  16 , and hence rotates at the input speed of the shaft  16  and the engine  12 .  
         [0028]     A sealed housing  20  is mounted rotatably on the shaft  16 . More particularly, the sealed housing  20  includes a casing  22  mounted around a portion of the shaft  16  by bearings  24 . Thus, the shaft  16  rotates substantially independently of the casing  22 . The casing  22  is formed to define an oil reservoir  26  that is enclosed by a partition  28 . The drive disc  18  is mounted to a portion of the shaft  16  that projects beyond the casing  22  and the partition  28 . The sealed housing  20  further includes a cover  30  mounted substantially rigidly to the casing  22 . Thus, the casing  22 , the partition  28  and the cover  30  are rotatable in unison and substantially independently of the shaft  16  and the drive disc  18 . Fan blades  32  are mounted to the exterior of the sealed housing  20  and rotate with the sealed housing  20  to generate a flow of cooling air across the radiator  14 .  
         [0029]     The casing  22 , the partition  28  and the cover  30  are configured to define a torque transmission chamber  34  that surrounds the drive disc  18 . An oil supply hole  36  extends through the partition  28  at a substantially radially outer position of the oil reservoir  26  to provide communication between the oil reservoir  26  and the torque transmission chamber  34 . An oil recirculation passage  38  is formed through the casing  22  and provides communication between a radially outer position in the torque transmission chamber  34  and the oil reservoir  26 . A dam  40  is disposed at a radially outer position in the torque transmission chamber  34  and in proximity to the oil recirculation passage  38 .  
         [0030]     A viscous oil is disposed initially in the oil reservoir  26 , but can flow through the oil supply hole  36  and into the torque transmission chamber  34 . Oil in the torque transmission chamber  34  will transfer torque from the drive disc  18  to the sealed housing  20 . As a result, the sealed housing  20  will rotate with the drive disc  18 , and the fan blades  32  on the sealed housing  20  will generate a flow of cooling air across the radiator  14 . Centrifugal forces will urge the oil into outer positions in the torque transmission chamber  34 . The dam  40  then will urge the oil into the oil recirculation passage  38  and back to the oil reservoir  26 .  
         [0031]     The amount of torque transferred from the drive disc  18  to the sealed housing  20  varies in accordance with the amount of oil in the torque transmission chamber  34 . More oil in the torque transmission chamber  34  transmits more torque from the drive disc  18  to the sealed housing  20 . This greater torque results in greater rotational speeds of the sealed housing  20  and hence a higher flow of cooling air generated by the fan blades  32 . Less oil in the torque transmission chamber  34  results in a lower torque transfer from the drive disc  18  to the sealed housing  20 . The lower torque results in lower rotational speeds of the sealed housing  20  and hence a lower flow of cooling air generated by the fan blades  32 .  
         [0032]     The amount of oil in the torque transmission chamber  34  is controlled by a valve  42  mounted in the oil reservoir  26  in a position for selectively opening and closing the oil supply hole  36 . More particularly, the valve  42  includes a leaf spring  44  with a fixed end  46  and a movable end  48 . The leaf spring  44  can be deflected from a position where the movable end  48  of the leaf spring  44  closes the oil supply hole  36  to a position where the movable end  48  of the leaf spring  44  opens the oil supply hole  36 . Movement of the leaf spring  44  into a position for closing the oil supply hole  36  will interrupt the flow of oil into the torque transmission chamber  34  without interrupting the flow of oil from the torque transmission chamber  34 , through the oil recirculation passage  38  and back into the reservoir  26 . Hence, movement of the leaf spring  44  into a position for blocking the oil supply hole  36  will result in a gradual reduction of the amount of oil in the torque transmission chamber  34 , thereby resulting in less torque transmitted from the drive disc  18  to the sealed housing  20 . As a result, the closing of the oil supply hole  36  by the leaf spring  44  lowers the rotational speed of the sealed housing  20  and decreases cooling effect achieved by the fan blades  32 . Conversely, movement of the leaf spring  44  into a position for opening the oil supply hole  36  results in a greater flow of oil from the oil reservoir  26  into the torque transmission chamber  34 . As a result, movement of the leaf spring  44  into position for opening the oil feed hole  36  permits a greater torque transfer from the drive disc  18  to the sealed housing  20 . Consequently, the sealed housing  20  will rotate faster and the fan blades  32  will generate a higher flow of cooling air.  
         [0033]     The opening and closing of the oil supply hole  36  is controlled by an electromagnet  50  mounted rotatably on the shaft  16  and opposed to a portion of the casing  22  near the valve  42 . In particular, activation of the electromagnet  50  will attract an armature  52  on the leaf spring  44  and will urge the leaf spring  44  away from the oil supply hole  36 . Conversely, the leaf spring  44  will return resiliently to a position for closing the oil feed hole  36  when the electromagnet  50  is deactivated. The electromagnet  50  is pulse width modulated PWM to sequentially open and close the oil supply hole  36 . The duty rate DR of the pulse width modulated signal controls the proportion of time that the valve  42  remains open. The pulse width modulated duty rate DR can vary between 0%, corresponding to a valve  42  that is not opened and 100%, corresponding to a valve  42  that is open continuously. During most operating conditions, the duty rate DR will be between these two extremes. The valve  42  will open more often and/or for longer times to generate greater flows of cooling air. Conversely, the valve  42  will close more often and/or for longer periods of time to achieve less cooling.  
         [0034]     The duty rate DR achieved by the electromagnet  50  is based on a control signal that is input to the electromagnet  50  from an adaptive controller  54   b , which in turn receives input from an external input analyzer  54   a . The external input analyzer  54   a  generates a target fan speed TFS required to achieve the necessary cooling based on information from different sensors, such as engine coolant temperature sensors, intake air temperature sensors, a vehicle speed sensor, an engine speed sensor, a throttle position sensor and a sensor for air conditioner operating conditions. The algorithm or logic utilized by the external input analyzer may vary from one vehicle to another and typically the logic or the algorithm will be developed by the vehicle manufacturer. The control signal will dictate a target fan speed TFS appropriate for achieving the necessary cooling. This differs from the prior systems where the control signal generates a precalibrated duty rate DR associated with a target fan speed TFS.  
         [0035]     As noted above, physical characteristics of the externally controlled fan device  10  may result in an actual fan speed AFS that differs from the target fan speed TFS that was intended to have been produced by the duty rate DR dictated by the controller. This difference between the actual fan speed AFS and the target fan speed TFS can be caused by dimensional differences attributable to manufacturing tolerances. Alternatively, the difference between the actual fan speed AFS and the target fan speed TFS can be attributable to physical changes in the externally controlled fan device  10  that develop over time. For example, the viscosity of the oil may change, wear debris may accumulate in the oil, the resiliency of the leaf spring  44  can change, the bearing  24  can wear or parts of the fan coupling device  10  may be damaged due to unintended contact. Any of these changes can cause the actual fan speed AFS to be less than the target fan speed TFS or greater than the target fan speed TFS. Additionally, any of these conditions can cause the actual fan speed AFS to be greater than the target fan speed TFS under certain operating conditions, but less than the target fan speed TFS under other operating conditions.  
         [0036]     In view of the above, the controller  54   b  of the externally controlled fan coupling device  10  is an adaptive controller. The adaptive controller  54   b  is operative for measuring the actual fan speed AFS and then using the measured actual fan speed AFS as an input. The duty rate DR also is an input. The adaptive controller  54   b  then uses the various inputs over time, including the duty rate DR and the actual fan speed AFS, and generates an updated control algorithm or logic to achieve a duty rate DR that will keep the actual fan speed AFS at or very near the target fan speed TFS.  
         [0037]     The application of the adaptive controller  54   b  to the fan device  10  is illustrated schematically in  FIG. 2 . More particularly, the target fan speed TFS illustrated in  FIG. 2  is determined based on sensors at various locations in the vehicle. The specification of the sensors may vary from one vehicle manufacturer to another. However, as noted above, sensors may be operative to identify ambient temperature, coolant temperature, engine speed, vehicle speed and the like. These data are inputs to the external input analyzer  54   a . The external input analyzer  54   a  includes certain logic, i.e. an algorithm, that relate these inputs to a target fan speed TFS. The target fan speed TFS is an input to the adaptive controller  54   b  and specifically to the valve control algorithm unit  56  thereof. The algorithm includes certain coefficients and produces a duty rate DR as an output to the externally controlled fan device  10 . The existing duty rate DR also is provided as an input to the control law update unit  58  of the adaptive controller  54   b . Torque is transmitted to the sealed housing  20  and rotates the housing  20  at an actual fan speed AFS based on the duty rate DR and physical characteristics of the fan device  10  existing at a particular point in time. The actual fan speed AFS and the duty rate DR then are inputs back to the control law update unit  58  of the controller  54   b . The control law update unit  58  may employ a deterministic autoregressive moving average (DARMA) model or other such model to represent the performance of the ECFD  10 . A DARMA model representation of the externally controlled fan device (ECFD)  10  input-output behavior may be given by the relationship:  
             a   0     ⁢     AFS   ⁡     (   t   )         =       -       ∑     j   =   1       n   1       ⁢       a   j     ⁢     AFS   ⁡     (     t   -   j     )             +       ∑     j   =   0       m   1       ⁢       b   j     ⁢     DR   ⁡     (     t   -   j   -   d     )               ;     t   ≥   0         
 
 More particularly, the control law update unit  58  uses certain algorithms, like the recursive least square algorithm, along with past values of AFS and present and past values of DR to estimate the model coefficients a j  and b j  that will best represent the present behavior of the ECFD  10 . After these model coefficients are identified, they are used in the control law update unit  58  for solving a series of equations to estimate new control coefficients (p 1 , p 2 , I 1 , I 2 , M, etc) to determine a new duty rate DR value. The valve control algorithm  56  to calculate the duty rate may be substantially as follows:  
         DR   ⁡     (   t   )       =       1     l   1       ⁡     [       M   ⁡     (     TFS   ⁡     (   t   )       )       -     (         p   1     ⁢     AFS   ⁡     (   t   )         +       p   2     ⁢     AFS   ⁡     (     t   -   1     )           )     -       l   2     ⁢     DR   ⁡     (     t   -   1     )           ]           
 
 As a result, the control law update unit  58  may use the model coefficients to update the control logic in order to dictate a new duty rate DR for more closely matching the actual fan speed AFS to the target fan speed TFS. This process is carried out continually to ensure that the actual fan speed AFS is at or very close to the target fan speed TFS throughout the operation. 
 
         [0038]      FIGS. 4   a - 4   d  demonstrates actual test data. The test was carried out using a temperature control test bed referred to herein as a “hot box”. The hot box is an enclosed system having a main drive motor with a speed control. The inside temperature is maintained constant at preset values with the help of a PLC controller. The fan clutch system was coupled to the shaft of the motor and positioned in a shroud of known diameter to simulate the in-vehicle system resistance. The electric motor use in the test is capable of running at speeds up to 4,500 RPM. A fiber-optic speed sensor was mounted in the hot box near the motor shaft to measure the in put shaft speed (IS).  
         [0039]     The experimental adaptive controller used a rapid prototyping system for converting each signal into signals that can be inputted into a computer with provisions for interfacing the clutch cable and the fiber-optic speed sensor. Frequency to-voltage converters were used to convert the input speed reading and the fan speed reading from frequency to voltage signals, which were then fed to two channels of an A/D board. Tests then were performed at various input speeds and various target fan speeds. The test results are illustrated in  FIGS. 4   a - 4   d . In each of these figures, the horizontal line IS represents the input speed, which correspondence to the rotational speed of the shaft  16  shown in  FIG. 1 . The stepped rectangular line shown in these figures represents the target fan speed entered as an input to the test apparatus. In actual practice, the target fan speed TFS would be developed based on inputs from various sensors on the vehicle. The actual fan speed AFS is represented by the non-rectangular and non-linear line on the grafts. These grafts show that the adaptive externally controlled fan drive  10  was able to have the actual fan speed AFS closely follow the target fan speed TFS at different input speeds and with frequent changes in the target fan speed TFS.  
         [0040]     The above-described procedure cannot be used directly when the engine is started because there is no history for assessing the ability of the duty rate DR to achieve the actual fan speed AFS. Accordingly,  FIG. 3  shows a controller that further includes a fixed gain PID controller that utilizes a known specified algorithm to produce a duty rate DR that ideally would achieve the target fan speed TFS. The apparatus of  FIG. 3  further includes a switch  60  that switches to the above-described adaptive controller  54   b  after a sufficient history of duty rates DR and actual fan speeds AFS can be developed following the engine startup. The switch  60  typically can be activated after 60-120 seconds of engine operation.  
         [0041]     The adaptive controller  54  can be used for other purposes as well. For example, the externally controlled fan coupling device  10  may be calibrated for a particular vehicle with a specific array of accessories. A vehicle manufacturer may provide different arrays of accessories for certain vehicles in a particular line of vehicles. For example, some vehicles from a manufacturer may have a unique air conditioning system that could significantly affect the externally controlled fan device  10 . In the prior art, such a change would require a complex, time consuming and costly recalibration of the controller for externally controlled fan coupling device  10 . However, the adaptive controller  54  can avoid such complex recalibration. In this regard, the adaptive controller  54  will generate signals to adjust the duty rate DR up or down so that the actual fan speed AFS conforms to the cooling requirements.  
         [0042]     The invention has been described with respect to a preferred embodiment of an externally controlled fan device  10 . However, it will be understood by those skilled in the art that the adaptive controller of the subject invention can be used with other types of fluid clutches. For example, the adaptive controller can be used with the above-identified fluid clutches that employ a “smart fluid” with a viscosity that can be varied for altering the torque applied to the casing of the fan device. More particularly, the adaptive controller may update the control logic that operates the electromagnet to change the viscosity of the liquid in much the same way that the duty rate of a valve is varied.