Electricity: motive power systems – Automatic and/or with time-delay means – Responsive to thermal conditions
Reexamination Certificate
1999-12-23
2001-02-13
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Automatic and/or with time-delay means
Responsive to thermal conditions
C318S268000
Reexamination Certificate
active
06188189
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to fan speed control and in particular to a fan speed control system for electronic equipment enclosures, and is more particularly directed toward a fan speed control system that communicates with an external processor resource while retaining some autonomy, and utilizes pulse width modulation to maintain a predetermined relationship between fan speed and equipment temperature.
BACKGROUND OF THE INVENTION
Electronic equipment always generates heat, largely as a consequence of the fact that no electronic system is one hundred percent efficient. Some of the input power of the system must, of necessity, be dissipated as heat.
With the advent of the semiconductor, it became possible to construct electronic systems that operate at low power consumption. These early solid-state electronic systems generally exhibited low overall power consumption, and, consequently, even at low efficiency, there was little heat. Only applications requiring high power to be generated somewhere within the equipment, such as radio transmitter implementations, had hot spots within the equipment requiring the use of heat sinks and/or cooling fans.
Early computers were virtually room-size because of the need for massive numbers of switching circuits that could only be provided through the use of vacuum tubes. Since vacuum tubes were inherently inefficient, much of the size and expense of early computer systems is attributable to power supplies and cooling systems. As the transistor, and eventually the integrated circuit, became more ubiquitous, the size and power requirements of computer systems decreased dramatically.
Because microprocessor systems are so small and use so little power, availability of portable, battery-powered systems has grown by leaps and bounds. But many new application programs require large amounts of processing power, and high-speed operation of new, sub-micron geometries requires the expenditure of considerable amounts of power.
This has not discouraged the development of faster processors or portable systems, however. Fixed equipment that is not dependent upon batteries for power can tolerate the additional power consumption that cooling fans require, and, because of the recent development of batteries with very high capacities, even in small packages, portable computing equipment can take advantage of new, more powerful processing technologies by conceding the need for cooling fans and budgeting power accordingly.
Of course, even the best of the modern battery packs do not have unlimited power, and there are environmental standards associated with acoustic noise that is produced by fans running at high speed. In many forms of high performance equipment, such as high-speed, high-capacity file servers, multiple processors generate sufficient heat that banks of cooling fans, as many as eight or sixteen, for example, can be required to achieve acceptable cooling. Acoustic concerns make it desirable to run the fans at low speed in order to reduce the noise level, but it may be impossible to provide adequate cooling at low fan speeds, even though environmental requirements related to noise levels may best be met through low fan speed operation. It should not be necessary to compromise equipment cooling for the sake of compliance with noise-emission standards. After all, lack of proper cooling can shorten component life, and the cost of system maintenance continues to mount.
It has long been recognized that temperature-proportional speed control can be accomplished through the use of pulse width modulation (PWM). There are a number of devices known in the art that provide PWM fan speed control in response to a temperature signal from an external temperature sensor.
Even though the devices currently available are capable of providing fan speed control in response to a temperature signal, these devices do not permit operational parameters to be reprogrammed easily to accommodate the thermal peculiarities of a particular chassis, nor do they allow an external controller to supervise fan management without taking over fan operation completely. These devices universally fall short of providing an adequate interface to an external control element for maximum flexibility in a wide range of applications.
SUMMARY OF THE INVENTION
These shortcomings of the prior art, and others, are addressed by the fan control system of the present invention. Computer systems typically have multiple heat sources, including the processor, power supply module, etc., which generate significant amounts of heat while the computer system is operating. Temperature rise within the computer system enclosure is significant, so fans are used to keep temperatures at an acceptable level.
Having the fans operate at maximum speed at all times is hardly an optimum solution, however. It would be desirable for the fans to run at the minimum speed appropriate for the particular temperature to minimize acoustic noise and power consumption, as well as to prolong the life of the fans used for cooling. Of course, the power consumption considerations apply primarily to portable equipment using battery power.
One or more thermal diodes may be used for temperature sensing. Use of a thermal diode is less expensive than thermistor solutions, and also potentially more accurate. The invention also includes a technique for communication with system software that increases the flexibility of the invention and renders it useful across a broad range of applications and environments.
The invention as a whole describes a PWM fan speed control circuit in which the PWM pulse width, and consequently the fan speed, varies linearly over a predefined temperature range T
min
to T
max
. There are options to set a number of different values for T
min
, as well as options to set the temperature range (T
max
-T
min
) by specifying the number of degrees C corresponding to each increment in pulse width.
By way of example, one can assume that fan speed is roughly linearly related to PWM duty cycle, and that the scheme provides for a linear increase in PWM duty cycle from T
min
to T
max
in a predetermined number of increments. It has been observed that fans do not operate particularly well at very low speeds, and empirical determinations support the notion that a practical minimum fan speed is about ⅓ of maximum. The proposed system accommodates about 240 speed increments, with each increment corresponding to a fraction of a degree C. For ease of implementation, only a limited number of increments would normally be permitted, say {fraction (1/16)} of a degree C, ⅛, ¼, ½, and 1 degree. In fact, under normal circumstances, only about 160 of the speed increments are generally useful, since the first one-third (80 levels out of 240) are not used in most applications. Of course, it is always possible that a particular fan might operate satisfactorily below one-third of full speed, so there may be instances in which more of the available 240 speed increments may be used.
The range of values for T
min
can be set through an external resistive divider network. The system is capable of distinguishing among eight voltage levels. Seven of these levels correspond to discrete values of T
min
, while the eighth value acts to disable automatic fan speed control entirely. The temperature increment can also be preprogrammed into the system. In the interest of simplicity, it is probably best that the increment values be restricted to power-of-two multiples of 1 degree C.
This constraint makes the mathematical manipulations very simple, since multiplication by two or powers of two can be accomplished by a simple shift operation rather than a considerably more complex floating point arithmetic operation.
Since only about 160 of the 240 discrete levels available are generally used, it is at least theoretically possible to select an increment temperature value of 1° C. with a minimum temperature of 20° C. This would mean that the max temperature would be 180° C. There is also a provision for a cri
Analog Devices Inc.
Ro Bentsu
Wolf Greenfield & Sacks P.C.
LandOfFree
Fan speed control system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fan speed control system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fan speed control system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2573861