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Fan speed control Number:7,151,349 from the United States Patent and Trademark Office (PTO) owispatent

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Title: Fan speed control

Abstract: A method of synchronizing a pulsed drive signal applied to a DC fan motor to the TACH output of the motor is described. DC motors include a rotor that rotates in a path defined by a plurality of magnetic poles and the method defines an ideal TACH output for a specific rotation speed of the rotor of the DC fan and changes the period of the drive signal if the monitored TACH output does not match the ideal TACH output such that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

Patent Number: 7,151,349 Issued on 12/19/2006 to Williamson,   et al.

Inventors: Williamson; Russell John (Patrickswell, IE), Lillis; Elizabeth Anne (Corbally, IE)
Assignee: Analog Devices, Inc. (Norwood, MA)
Appl. No.: 10/821,035
Filed: April 8, 2004

Current U.S. Class: 318/254 ; 318/138; 318/439; 318/599
Current International Class: H02P 7/06 (20060101)
Field of Search: 318/139,254,439,599 388/827

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Other References

"PWM Fan Speed Controller with Fault Detection", TelCom Semiconductor, Inc., Mar. 11, 1998, pp. 1-14. cited by other .
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Primary Examiner: Ip; Paul
Attorney, Agent or Firm: Wolf, Greenfield & Sacks, P.C.
Claims


What is claimed is:

1. A method of synchronizing a pulsed drive signal applied to a DC fan motor to the TACH output of the motor, the motor including a rotor that rotates in a path defined by a plurality of magnetic poles, the method comprising: a) defining an ideal TACH output for a specific rotation speed of the rotor of the DC fan, said ideal TACH output being related to the ideal rate passing of a magnetic pole of the fan by the rotor, b) monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, c) comparing the monitored TACH output to the ideal TACH output, and d) changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output such that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

2. A method of synchronizing a pulsed drive signal applied to a DC fan motor to the TACH output of the motor, the motor including a rotor that rotates in a path defined by a plurality of magnetic poles, the method comprising the steps of: a) defining a time period for the rotor to rotate between two adjacent poles, b) using the defined time period to define an ideal TACH output for a specific rotation speed of a rotor of the DC fan, c) monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, d) comparing the monitored TACH output to the ideal TACH output, and e) changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output such that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

3. The method according to claim 1 or claim 2 wherein the duty cycle of the drive signal is changed by changing the length of time, T.sub.OFF, for which the drive signal is turned off.

4. The method according to claim 3 wherein the drive signal is turned off each time, for the time T.sub.OFF, when the actual TACH signal indicates that the fan rotor has rotated to the start of the next pole.

5. The method according to claim 4 wherein the drive signal is turned on once the duration of T.sub.OFF is complete, the drive signal being turned off again once the actual TACH signal indicates that the fan rotor has rotated to the start of another pole of the fan.

6. The method as claimed in claim 1 or claim 2 the actual TACH signal is only monitored when the drive signal is on.

7. The method as claimed in claim 6 wherein the monitoring of the actual TACH signal provides an indication of whether the actual TACH signal is at a high level or at a low level, the actual TACH signal flipping between the high and low levels on passing of a magnetic pole.

8. The method as claimed in claim 7 wherein the monitoring of the level of the actual TACH signal provides a tracking of the rotation of the rotor of the fan.

9. The method as claimed in claim 8 further including the step of sampling the level of the actual TACH signal during a time T.sub.OFF when the drive signal is normally off.

10. The method as claimed in claim 9 wherein the step of sampling the level of the actual TACH signal is effected by turning the drive signal on for a time period sufficient to enable a detection of the level of the TACH signal.

11. The method as claimed in claim 10 wherein the level of the actual TACH signal is sampled more than once during the T.sub.OFF time.

12. The method as claimed in claim 11 wherein two sample signals are applied, a first sample signal at a time approximately equivalent to 1/4 T.sub.OFF and the second at a time 1/2 T.sub.OFF.

13. The method as claimed in claim 12 wherein the duration of each sample signal is greater than a predetermined electromechanical delay T.sub.EM, the electromechanical delay being determined by looking at a transition on the drive signal from off to on, and measuring the time it takes for the actual TACH signal to change state from high to low, if the true state is low.

14. The method as claimed in claim 9 wherein sampling the level of the actual TACH signal during a time T.sub.OFF when the drive signal is normally off provides an indication as to whether the actual fan speed is a multiple of the speed indicated by the monitored actual TACH level.

15. The method as claimed in claim 14 further including, on determining that the actual speed is a multiple of the speed indicated by the monitored actual TACH level, correcting the actual speed to reflect the monitored actual TACH level.

16. The method as claimed in claim 15 wherein the correction is effected by reducing the duration of the off time T.sub.OFF of the drive signal.

17. The method as claimed in claim 16 wherein the off time T.sub.OFF is reduced to a time period shorter than the time period for the fan rotor to pass adjacent poles, T.sub.POLE.

18. The method as claimed in claim 7 wherein the level of the TACH signal is monitored after a predetermined time delay, an electromechanical (EM) delay T.sub.EM, has passed since the switching of the drive signal from an off condition to an on condition.

19. The method as claimed in claim 18 wherein the EM delay T.sub.EM is determined by looking at a transition on the drive signal from off to on, and measuring the time it takes for the actual TACH signal to change state from high to low, if the true state is low.

20. The method as claimed in claim 6 further including the step of sampling the level of the TACH signal continuously while the drive signal is on.

21. The method as claimed in claim 3 wherein the off time of the drive signal, T.sub.OFF, is adjustable, an adjustment of T.sub.OFF effecting a change in the speed of the fan motor.

22. The method as claimed in claim 1 or claim 2 wherein the defined ideal TACH output is provided by a look up table.

23. The method as claimed in claim 22 wherein the look up table is a discrete mode look up table.

24. The method as claimed in claim 22 wherein the look up table is a linear mode look up table.

25. The method as claimed in claim 22 wherein the look up table is a ratio mode look up table.

26. The method as claimed in claim 1 or claim 2 wherein the defined ideal TACH output is provided by a plurality of user inputs.

27. The method as claimed in claim 26 wherein the plurality of user inputs include the number of poles of the fan and a rotation speed of the fan.

28. The method as claimed in claim 27 wherein the rotation speed of the fan is provided as a direct input by the user.

29. The method as claimed in claim 27 wherein the rotation speed of the fan is related to an operating temperature, the relationship being user defined.

30. The method as claimed in claim 29 wherein the user defined relationship is effected by defining a range of temperature values and relating this defined range to a range of rotation speeds.

31. The method as claimed in claim 1 or claim 2 further including determining the speed of the fan motor by: a) defining the number of poles of the fan, b) determining when the rotor has passed a first pole of the plurality of poles, c) determining when the number of poles passed by the rotor is equivalent to the defined number, and d) calculating the time period between step b) and step c) so as to define a total time for a single rotation, thereby providing an indication of the actual speed of the fan motor.

32. The method as claimed in claim 31 further including the step of defining a desired speed for the fan motor.

33. The method as claimed in claim 32 wherein the desired speed is related to an operating temperature of the location of the fan motor.

34. The method as claimed in claim 32 wherein the off time of the fan motor, T.sub.OFF, is adjustable until the desired speed substantially matches the actual speed.

35. The method as claimed in claim 34 wherein the amount by which the off time T.sub.OFF of the drive signal is adjusted may be varied.

36. The method as claimed in claim 35 wherein the variance is related to the difference between the desired speed and actual speed.

37. The method as claimed in claim 34 wherein the frequency at which the off time T.sub.OFF is adjusted is configurable.

38. The method as claimed in claim 37 wherein the frequency is related to the characteristics of a particular fan.

39. The method as claimed in claim 1 or claim 2 further including monitoring the change of the actual TACH levels so as to determine whether the fan has stalled, a stall being defined by an occurrence of multiple sequentially monitored actual TACH levels being of the same level.

40. The method as claimed in claim 39 wherein on determining the occurrence of a stall, the drive signal is pulsed on and off until two sequentially monitored actual TACH levels are of a different level.

41. The method as claimed in claim 1 or claim 2 further including forcing the drive signal to a continuous on or a continuous off condition.

42. A DC fan speed controller adapted to synchronize a pulsed drive signal applied to a DC fan motor to the TACH output of the motor, the motor including a rotor that rotates in a path defined by a plurality of magnetic poles, the controller having: a) means for defining a time period for the rotor to rotate between two adjacent poles, b) computation means for defining an ideal TACH output for a specific rotation speed of a rotor of the DC fan based on the defined time period, c) means for monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, d) means for comparing the monitored TACH output to the ideal TACH output, and e) means for changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output, so as to ensure that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

43. A DC fan speed controller adapted to synchronize a pulsed drive signal applied to a DC fan motor to the TACH output of the motor, the motor including a rotor that rotates in a path defined by a plurality of magnetic poles, the controller having: a) means for defining a time period for the rotor to rotate between two adjacent poles, b) computation means for defining an ideal TACH output for a specific rotation speed of a rotor of the DC fan based on the defined time period, c) means for monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, d) means for comparing the monitored TACH output to the ideal TACH output, and e) means for changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output, so as to ensure that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

44. The controller as claimed in claim 42 or claim 43 wherein the speed of the fan motor is defined by: a) a first register value defining the number of poles of the fan, b) a first sensor adapted to determine when the rotor has passed a first pole of the plurality of poles, the first sensor being adapted to effect an update of a second register so as to increment the number of poles passed by the rotor c) a comparator adapted to determine when the number of poles passed by the rotor and stored in the second register is equivalent to the defined number, and d) a timer adapted to calculate the time period between step b) and step c) so as to define a total time for a single rotation, thereby providing an indication of the actual speed of the fan motor.

45. The controller as claimed in claim 42 or claim 43 wherein the ideal TACH output is related to the operating temperature of the fan.

46. The controller as claimed in claim 42 or claim 43 wherein the means for monitoring the actual TACH output of the DC fan monitors the actual TACH output when the drive signal is on.
Description


FIELD OF THE INVENTION

The invention relates to DC fans and in particular to the control of the fan speed of such DC fans by a synchronization of the drive signal of the fan to a TACH output of the fan. In particular the invention relates to a DC fan controller adapted to enable detection and maintenance of the speed of the rotors of brushless DC fans. The invention particularly relates to a method and apparatus adapted to control the speed of the fan rotors based on user defined rotation values.

BACKGROUND TO THE INVENTION

DC brushless motor fans come in a variety of shapes and sizes. There are a number of fan types including radial, axial, or blower designs, and the fan type determines the airflow direction with respect to the fan body. Airflow rate is one of the most important specifications for a fan as this quantifies the fans ability to circulate air. This can vary in volume depending on fan blade design, fan construction, and speed of rotation. Most of these properties are set at the fan design stage, leaving only the speed of rotation available to the user to determine the airflow rate and hence the amount of cooling needed to maintain a safe operating environment. In order to control this fan speed parameter we need to look at the underlying controlling circuit in a fan.

A basic DC brushless motor consists of a number of key components and will be well known to those in the art. The motor includes a fixed part, the stator having a fan housing and an electronic assembly. The motor also includes moving parts including a rotor comprising a fan blade and a circular permanent magnetic strip attached to it. The electronic assembly contains a number of coils that can be electronically energised in sequence as the fan rotates. The energising sequence is synchronised to the fan rotation by a Hall effect sensor. This sensor is placed close to the circular permanent magnetic strip on the rotor and as the fan rotates the sensor detects the magnetic field of the passing magnet. The sensor output changes state every time a magnetic pole change is detected, i.e., going from a North Pole to a South Pole and visa versa. The permanent magnetic strip is made up of a number of magnetic pole pairs and thus it is possible to use the detection of the number of state changes of the sensor output to determine one full rotation of the fan.

The basic DC brushless fan has a 2-wire connection, one connects to a DC power source, and the other is the ground return. Becoming more common now is a third wire known as the `tachometer output` or `TACH`. This is a feedback signal from the fan representing the speed the fan is rotating at; in effect this is the Hall effect sensor output signal. An example of a TACH signal is shown in FIG. 1. As will be seen the signal is a square wave with a number of rising and falling edges separated by respective "high" and "low" levels. Each edge corresponds to a passage of a respective pole of the permanent magnet past the Hall sensor, so in effect a complete rotation of a fan having a four pole magnet will include two rising edges and two falling edges; a six pole magnet three rising edges and three falling edges and an eight pole magnet four rising edges and four falling edges. The fan construction determines the number of poles of the fan; this is the number of pole pairs that are passed in one complete rotation of the fan blade. To understand how this signal is generated we need to look at the basic fan drive circuit of a DC brushless motor, an example of which is given in FIG. 2.

The three leads from the fan are labelled:

The power lead, Vdd,

The ground lead, GND,

The Tachometer signal, TACH.

When Vdd is powered, the hall-effect sensor will initially be at a steady state, say GND. This is applied to the gate of transistor T1, keeping it in the off state, so coil 1 has no current flowing through it and therefore no voltage dropped across it. This in turn means the gate of T2 is pulled high, turning it on and therefore energizing coil 2. Energizing coil 2 has the effect of rotating the fan blade and once the hall-effect sensor detects the next pole change, its output changes state to high and the transistor states change, T2 turns off and T1 turns on, thus energizing coil 1, and causing the fan blade to rotate further until the next pole change is detected by the hall effect and the process repeats again.

Although a fan could be designed to operate either in a fully on or a full off position, it is more normal for the fan to operated at a number of different requirements depending on the conditions where the fan is operating. As such it is necessary to enable a control of the fan speed. There are several existing control schemes for controlling fan speed, including linear and pulse width modulation (PWM) control.

Linear control is also sometimes referred to as voltage control and relies on the principal that changing the voltage drive level to the fan will proportionally change the fan speed. Some fans will operate in this way, but not all. Typically, fans with internal electronic driver integrated circuits (ICs) require some minimum DC voltage to operate and therefore will stall below this level, sometimes this can be as high as 50% of the maximum allowed DC level.

In order to generate the variable DC level an interface circuit such as that shown in FIG. 3 is required. A digital-to-analogue converter supplies the variable voltage to this interface circuit and it then acts as a buffer to supply the current load demanded by the fan. At slower speeds the reduced voltage level across the fan means there is a larger voltage drop across the pass transistor Q1, therefore a lot of power is lost especially if the fan requires large current drive, and this in turn acts as a heat generator which stacks against the reason for needing a fan in the first place to cool the system. However, despite the drawbacks this control method has good acoustic performance as the reduced DC level helps to minimise the acoustic noise generated by the fan coil switching as it rotates. Therefore, although linear control is advantageous in that it is a proven technique endorsed by many fan vendors, has known reliable operation, there is extensive historical data available, has good acoustic performance and is easy to measure using the TACH signal it also suffers from a number of disadvantages. These include the fact that some fans do not operate at the lower voltage levels and this can reduce the range of speed of the fan. It is also difficult to determine the fan start and stall voltages and this requires calibration or characterization of every fan. Furthermore, such operation effects large power dissipation in power transistors (>5A fans) due to the voltage drop across transistor. This may require multiple external components such as for example amplifier/buffers, power transistors, gain setting resistors etc.

An alternative control technique is provided by pulse width modulation (PWM). PWM control is probably the simplest fan speed control technique available. The fan speed is changed by varying the Duty Cycle of a square-wave drive signal applied to the fan interface circuit, as shown in FIG. 4. The scheme is quite effective for most fan types but can present difficulties. The primary drawback is the fact that the PWM drive signal is not synchronized to the internal fan electronics, therefore the fan is being pulsed on and off regardless of where the fan is in its rotation cycle. The other drawback is the loss of the speed information. The PWM drive is asynchronous to the tachometer (TACH) output and there is therefore no way of knowing when successive TACH output changes occur. One way around this is to periodically keep the drive on for sufficient time to gather this speed information, thereby stretching the drive on-time.

FIG. 5 shows the PWM signal being held on for 1 TACH period. This stretching out of the drive on time can be for 1, 2 or more TACH periods depending on the accuracy required for the speed measurement. In order to maximize the speed accuracy, this stretching should last long enough to allow the fan complete one full rotation. However, stretching the drive on for this length will cause a very noticeable speeding up of the fan, particularly at lower speeds. This stretching can also cause a `hunting` noise effect where the fan appears to speed up momentarily while the speed information is gathered. Therefore, although there are a number of advantages associated with this technique including the fact that it is a relatively simple low side driver fan drive interface, is a low cost solution and has relatively good speed control over a range of values, these have to be compared with the associated disadvantages such as the fact that the TACH signal is destroyed by drive switching, a 30 Hz to 100 Hz PWM frequency can cause audible ticks and buzzes, and the pulse stretching technique to measure fan speed can be audible at lower speeds, and causes speed variation.

Although hereinbefore described with reference to two and three wire fan systems it is also known to have a four wire system, the fourth wire providing an external feed or sync signal to the fan. U.S. Pat. No. 6,381,406 assigned to the Hewlett Packard Corporation describes a method and apparatus for controlling the speed of such a four wire voltage-controlled fan by locking the pulse-width modulated speed control voltage to a tachometer signal of the fan. By triggering the off time of the PWM pulse to the detection of the tachometer signal and ensuring the off time is less than one tachometer period, no phase and frequency information is lost. The synchronization is achieved by providing the fan controller with a system-generated speed signal SYNC and a tachometer signal TACH from the fan. When a voltage is applied to the fan, the fan rotor begins to spin and generates a tachometer signal TACH one or more times per full revolution of the rotor, each TACH signal having a fixed TACH period. The controller generates a pulse-width modulated signal PWM OUT which is used to turn the fan motor on and off. The controller adjusts the width of the PWM OUT pulses to the fan such that the fan's speed will either decrease or increase until the TACH signal matches the frequency and phase of the control signal SYNC. In order to allow the controller to properly operate using low-frequency PWM signals, the controller synchronizes the off time PWM OUT signal with the detection of the TACH signal and guarantees that the off time is always less than one TACH period. This ensures that the power to the fan is always turned on by the time the TACH signal arrives, and is therefore detected by the controller. Accordingly, accurate TACH data is available for calculation of the next "off" period of the power to the fan, and pulse width modulation can be accomplished at a TACH frequency of less than 200 Hz without losing any TACH phase or frequency information. However, as the synchronization requires a monitoring and correlation between the speed signal and the TACH signal, it can only be implemented in a four-wire system. Such systems, although known, are not the most prevalent of available fans and it is therefore desirable to provide a fan controller that enables a synchronization of the PWM drive signal yet does not require an externally generated sync signal to achieve this synchronization.

SUMMARY OF THE INVENTION

These and other needs of the prior art are addressed by a fan controller in accordance with the present invention which provides for an adjustment of the off time of the fan drive signal to match the ideal TACH output of the fan for that desired speed.

According to a first embodiment of the invention a fan controller is provided which is adapted to enable a synchronization of the drive signal applied to a DC fan motor with a TACH output of the motor. The controller achieves this synchronization by enabling a definition of an ideal TACH output for a specific rotation speed of a rotor of the DC fan, monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, comparing the monitored TACH output to the ideal TACH output, and changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output such that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

As such, the invention provides a method of synchronizing a pulsed drive signal applied to a DC fan motor to the TACH output of the motor is provided, the method comprising the steps of: a) defining an ideal TACH output for a specific rotation speed of a rotor of the DC fan, b) monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, c) comparing the monitored TACH output to the ideal TACH output, and d) changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output such that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

The duty cycle of the drive signal may be changed by changing the length of time, T.sub.OFF, for which the drive signal is turned off. Typically, the drive signal is turned off each time, for the time T.sub.OFF, when the actual TACH signal indicates that the fan rotor has rotated to the start of the next pole. In operation, the drive signal is usually turned on once the duration of T.sub.OFF is complete, the drive signal being turned off again once the actual TACH signal indicates that the fan rotor has rotated to the start of another pole of the fan.

In preferred embodiments, the actual TACH signal is only monitored when the drive signal is on. The monitoring of the actual TACH signal provides an indication of whether the actual TACH signal is at a high level or at a low level, the actual TACH signal flipping between the high and low levels on passing of a magnetic pole. Such monitoring of the level of the actual TACH signal provides a tracking of the rotation of the rotor of the fan.

In order to ensure that an accurate assessment of the actual TACH signal is made, the level of the TACH signal is typically monitored after a predetermined time delay, an electromechanical (EM) delay T.sub.EM, has passed since the switching of the drive signal from an off condition to an on condition. The EM delay T.sub.EM may be determined by looking at a transition on the drive signal from off to on, and measuring the time it takes for the actual TACH signal to change state from high to low, if the true state is low.

The invention also provides for sampling the level of the TACH signal continuously while the drive signal is on. Such sampling may be done continuously when the drive signal is normally on, but what may additionally be provides is the step of sampling the level of the actual TACH signal during a time T.sub.OFF when the drive signal is normally off.

If such sampling is effected, it is desirably achieved by turning the drive signal on for a time period sufficient to enable a detection of the level of the TACH signal, and the level of the actual TACH signal is normally sampled more than once during the T.sub.OFF time. Typically, two sample signals are applied, a first sample signal at a time approximately equivalent to 1/4 T.sub.OFF and the second at a time 1/2 T.sub.OFF. The duration of each sample signal is desirably greater than the predetermined electromechanical delay T.sub.EM of the system.

Such sampling the level of the actual TACH signal during a time T.sub.OFF when the drive signal is normally off provides an indication as to whether the actual fan speed is a multiple of the speed indicated by the monitored actual TACH level. The invention may additionally provide the step, on determining that the actual speed is a multiple of the speed indicated by the monitored actual TACH level, of correcting the actual speed to reflect the monitored actual TACH level. Such a correction is desirably effected by reducing the duration of the off time T.sub.OFF of the drive signal, typically to a time period shorter than the time period for the fan rotor to pass adjacent poles, T.sub.POLE.

The defined ideal TACH output may be provided by a look up table, which can optionally be a discrete mode look up table, a linear mode look up table, a ratio mode look up table or the like.

The defined ideal TACH output may also be provided by a plurality of user inputs. Such plurality of user inputs may include the number of poles of the fan and a rotation speed of the fan. The rotation speed of the fan may be provided as a direct input by the user or may be related to an operating temperature, the relationship being user defined. Such a user defined relationship may be effected by defining a range of temperature values and relating this defined range to a range of rotation speeds.

In preferred embodiments, the off time of the drive signal, T.sub.OFF, is adjustable, an adjustment of T.sub.OFF effecting a change in the speed of the fan motor.

The invention additionally provides for a determination of the speed of the motor, the speed being determined by: a) defining the number of poles of the fan, b) determining when the rotor has passed a first pole of the plurality of poles, c) determining when the number of poles passed by the rotor is equivalent to the defined number, and d) calculating the time period between step b) and step c) so as to define a total time for a single rotation, thereby providing an indication of the actual speed of the fan motor.

The additional step of defining a desired speed for the fan motor may be included. Such a desired speed may be related to an operating temperature of the location of the fan motor. The off time of the fan motor, T.sub.OFF, is typically adjustable until the desired speed substantially matches the actual speed. Such adjustment may include the amount by which the off time T.sub.OFF of the drive signal is adjusted. If such a variance in the amount by which T.sub.OFF is adjusted is provided, typically the variance is related to the difference between the desired speed and actual speed. Such adjustment may also or alternatively include the frequency at which the off time T.sub.OFF is adjusted is configurable. The frequency adjustment rate is desirably related to the characteristics of a particular fan.

The step of monitoring the change of the actual TACH levels so as to determine whether the fan has stalled may also be provided. A stall is defined by an occurrence of multiple sequentially monitored actual TACH levels being of the same level. On determining the occurrence of a stall, the drive signal may pulsed on and off until two sequentially monitored actual TACH levels are of a different level, thereby overcoming the stall.

The invention may also include the step of forcing the drive signal to a continuous on or a continuous off condition.

A DC fan speed controller which is adapted to synchronize a pulsed drive signal applied to a DC fan motor to the TACH output of the motor, the motor including a rotor that rotates in a path defined by a plurality of magnetic poles is also provided. Such a controller, in accordance with the an embodiment of the invention includes: a) means for defining an ideal TACH output for a specific rotation speed of a rotor of the DC fan, b) means for monitoring the actual TACH output of the DC fan, the TACH output indicating the passing of the magnetic poles of the fan by the rotor, c) means for comparing the monitored TACH output to the ideal TACH output, and d) means for changing the period of the drive signal if the monitored TACH output does not match the ideal TACH output.

The means for changing the period of the drive signal is normally adapted to enable a change in the period so as to ensure that the period of the drive signal matches the time taken for the fan to rotate through one magnetic pole of the fan.

The speed of the fan motor may be defined by: a) a first register value defining the number of poles of the fan, b) a first sensor adapted to determine when the rotor has passed a first pole of the plurality of poles, the first sensor being adapted to effect an update of a second register so as to increment the number of poles passed by the rotor c) a comparator adapted to determine when the number of poles passed by the rotor and stored in the second register is equivalent to the defined number, and d) a timer adapted to calculate the time period between step b) and step c) so as to define a total time for a single rotation, thereby providing an indication of the actual speed of the fan motor.

The ideal TACH output is typically related to the operating temperature of the fan.

The means for monitoring the actual TACH output of the DC fan is configured so as to monitor the actual TACH output when the drive signal is on.

These and other features of the present invention will be better understood with reference to the following drawings, which illustrate preferred embodiments of the invention. It will however be understood that such preferred embodiments are provided for illustrative purposes only and it is not intended to limit the present invention to any one specific configuration or embodiment except as may be deemed necessary in the light of the appended claims, as many modifications may and can be implemented without departing from the spirit and scope of the present invention, as will be appreciated by those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a TACH signal.

FIG. 2 is a basic known fan drive circuit of a DC brushless motor.

FIG. 3 is an interface circuit used to generate a variable DC signal.

FIG. 4 is an example of a circuit that may be used to vary the duty cycle for PWM applications in accordance with the prior art.

FIG. 5 is a timing diagram showing a stretching of the PWM signal.

FIG. 6 is timing diagram showing the relationship between an ideal TACH signal, an actual TACH signal and a drive signal in accordance with the present invention.

FIG. 7 is a fan speed control circuit in accordance with the present invention.

FIG. 8 shows in block diagram form further detail of the control block of the SSC system according to the present invention

FIG. 9 is a timing diagram showing the effect of EM delay on the operation of a system in accordance with the present invention.

FIG. 10 is a timing diagram showing the effect of the fan slowing down on the operation of a system in accordance with the present invention.

FIG. 11 is a timing diagram showing the effect of the fan speeding up on the operation of a system in accordance with the present invention.

FIG. 12 is a timing diagram showing the effects of losing lock on the TACH signal, and not getting enough samples.

FIG. 13 is a timing diagram showing the effect of unequal pole spacings on the operation of a system in accordance with the present invention.

FIG. 14 is an example of how the rate of change of T.sub.OFF may be varied depending on the difference between the desired and actual speed of the fan.

FIG. 15 is an example of a discrete mode look-up table for use with the system and method of the invention.

FIG. 16 is an example of a linear mode look-up table for use with the system and method of the invention.

FIG. 17 is an example of a ratio mode look-up table for use with the system and method of the invention.

FIG. 18 shows an example flow sequence for use with the methodology of the present invention.

FIG. 19 shows a specific sequence that may be adopted to ensure that any stalling of the fan may be detected and rectified.

FIG. 20 shows how the system of the invention provides for reaching a desired fan speed.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention provides a synchronous speed control (SSC) system that is a variation on PWM control and synchronizes the


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