Electricity: motive power systems – Positional servo systems – With particular 'error-detecting' means
Reexamination Certificate
2003-03-06
2004-12-28
Leykin, Rita (Department: 2837)
Electricity: motive power systems
Positional servo systems
With particular 'error-detecting' means
C318S459000, C318S500000, C388S928100, C323S234000
Reexamination Certificate
active
06836092
ABSTRACT:
TECHNICAL FIELD
The present invention is related to direct current (“DC”) fans used in various devices. More particularly, the present invention is related to the detection of operational information of the DC fans such as the presence and/or rotational speed of the DC fan within a device.
BACKGROUND
Various devices utilize DC fans for purposes such as cooling components to prevent overheating. Computers are an example of a device that may utilize one or more DC fans to cool the components of the computer, such as a bank of hard drives. LCD projectors are another example, wherein the light bulb of the LCD must be cooled to prevent failure. The cooling from the DC fan is often critical to the continued operation of the device being cooled, so the DC fan must often be monitored for proper operation to prevent DC fan failure which would otherwise cause failure of the device itself.
To extend the life of the DC fan, a pulse width modulated voltage is typically applied rather than a continuous DC voltage. The pulse width modulated voltage provides a peak voltage for each pulse that is great enough to maintain an adequate rotational speed of the fan, but the pulses reduce the increased strain on the fan that otherwise results from operating with a constant voltage. A controller is often employed to monitor the rotational speed of the DC fan resulting from the pulse width modulation and adjust the duty cycle as necessary to ensure that the DC fan continues to adequately perform.
The controller includes an input that is pulled to a high voltage via a pull up resistor linked to a power supply voltage. In DC fans where the pull up resistor is internal to the fan, the pulse width modulated input to the DC fan is applied to the pull up resistor to pull up the voltage at the input of the controller. Where the pull up resistor is external to the DC fan, the pulse width modulated input to the DC fan or a separate voltage source may be applied to the pull up resistor to pull up the voltage at the input. The input is further connected to a transistor, typically internal to the fan, that is switched on and off by rotation of the fan, such as through the output pulse from a Hall effect sensor powered by the pulse width modulated input voltage to the fan. This on and off switching results in temporary reductions of the pulse width modulated voltage being received at the input and these temporary reductions are counted to detect the rotational speed of the fan.
The DC fans may be removable from the device. For example, a DC fan may fail and may need to be replaced or repaired. However, it is important to know when a DC fan is not present in a device so that harm due to overheating can be prevented. Where multiple DC fans are present, it is also important to know that one or more fans are not present since the remaining fans may not be able to provide sufficient cooling and because the duty cycle of the pulse width modulated voltage for the remaining fans will increase or even result in a constant voltage to allow the remaining fans to compensate for the missing fan(s). Thus, it is important that missing fans be replaced before a device is used or as soon as possible after the fan is removed.
Because for some DC fans the input to the control circuit is pulled up by the presence of the pull up resistor that is integral to the fan, when the fan is removed from the device then the input is no longer pulled up. This results in the input to the control circuit floating, or having no definite logical high or low value. This prevents the control circuit from detecting whether the fan is missing or present. Therefore, other cumbersome methods of detecting whether the fan is missing or not must be used, such as manual inspections of the devices prior to the devices being used.
In addition to detecting the presence of a fan, it is important to detect the rotational speed of the fan so that its performance can be evaluated to determine whether maintenance or replacement is necessary. The speed of the fan is detected from the temporary reductions in the voltage pulled up at the input that are proportional to speed, but these temporary reductions can only be detected during the period of time that the pulse width modulated voltage is at the high state. This limited time of detection of temporary reductions is due to the rotation sensor integral to the DC fan requiring power from the power supply for the fan to accurately produce a pulse that results in the temporary reduction at the input to the control circuit, but no power is provided during the low state of the pulse width modulated voltage thereby limiting the time of accurate detection. For DC fans where the pull up is powered by the pulse width modulated voltage, then this limited time of accurate detection of temporary reductions is also due to no pull up voltage being present during the period when the pulse width modulated voltage is at the low state.
The output pulse train resulting in the temporary reductions that enables the control circuit to detect rotational speed is asynchronous relative to the pulse train of the pulse width modulated power supply. Due to this asynchronous relationship, erratic measurements result because during one high period of the pulse width modulation, there may be few if any output pulses while during another high period there may be several output pulses. These erratic measurements are not effective in monitoring the rotational speed of the fan since they arc inaccurate. Additionally, during the low periods of the pulse width modulation occurring during an output pulse, the rotational sensor may produce residual noise that appears as temporary reductions at the input to the control circuit since the sensor may be producing an output pulse which is briefly terminated by the low state of the pulse width modulation and which results in multiple temporary reductions rather than only one. This causes the input to the control circuit to detect a speed reading that is too high.
Thus, other methods of attempting to accurately detect fan speed are used. One technique is to apply a continuous voltage from the pulse width modulated power supply for a period of time when speed will be measured, so that the input is continuously pulled up, the rotation sensor is continuously powered, and the temporary reductions resulting from the output pulse may be measured consistently. However, such periodic application of continuous voltage from the pulse width modulated power source has significant drawbacks as well. This method is likely to be harmful to the fan, it results in audible noise that is distracting due to the variation in fan speed, and it also results in inaccuracy because the DC fan speed is higher and less constant for the measurement period than it is during the normal operating period.
SUMMARY
Embodiments of the present invention address these and other issues by providing methods and circuits that detect whether the fan is present through the input of the control circuit and that measure rotational speed at the input of the control circuit during both the high and low states of the pulse width modulated voltage. In certain embodiments, a pull down resistor may be provided at the input of the control circuit to provide a continuously low voltage indicative of a missing fan. In certain embodiments, a secondary voltage may be provided to the fan to power the rotation sensor, and pull up the input if applicable, during the low state of the pulse width modulated voltage to allow temporary reductions indicative of rotational speed to be detected at all times without altering the duty cycle of the pulse width modulated voltage.
One embodiment is a method of detecting operational information about a DC fan of a device where the DC fan is powered by pulse width modulation and produces an output pulse in proportion to rotational speed. The method involves providing a pulse width modulated voltage to the DC fan and providing a voltage to an input of a control circuit when the DC fan is present in the device. A temporary reduction in th
American Megatrends Inc.
Leykin Rita
Merchant & Gould
LandOfFree
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