Patent Application
20120017856
Rotating components can excite the structural resonances of themselves or of nearby components due to an excitation force at some multiple of the rotation speed. These resonances cause unwanted vibration and noise resulting in discomfort for individuals nearby as well as reducing the durability and performance of the affected components.
Therefore, systems are characterized so the speeds that excite resonances are avoided. Avoiding resonance reduces noise and vibration and resulting machinery damage. However, because of manufacturing variances, the use of temperature-sensitive plastics in temperature-varying environments, or a lack of knowledge of what the rotating-component ultimately attaches to, the resonances of many systems cannot be fully characterized, and a preemptive approach to avoiding resonances cannot be achieved.
Many engine cooling fans are manufactured using plastics which have material properties that change with humidity and/or temperature changes. Thus, characterizing the resonances of these fans at a fixed temperature and humidity level results in a system characterization that is different than the actual system when the fans are exposed to different environmental conditions.
Engine cooling fan resonances can shift in frequency and acoustic significance as temperature, plastic moisture content, manufacturing variance, and/or automobile dynamic characteristics change. Resonances may be excited by imbalance forces, cogging torque, electromagnetic forces, commutation phenomena, and aerodynamic blade forces. When an engine cooling fan operates at resonance; vibration, noise, and wear increase often resulting in customer complaints and product redesigns.
The fan can only be designed to be structurally quiet under a very specific set of environmental conditions. Because of changing properties of the fan, the fan will often perform better or worse than the design target based on the stiffness and damping of the plastic under actual operating conditions. Accordingly, what is needed are methods of continuously determining and avoiding the resonant speeds of a rotating component to avoid resonances under normal operation and improve the noise, vibration and/or durability performance of the component.
The present invention relates to systems and methods of continuously determining and avoiding resonant frequencies excited by a rotating component in a situation (for example, the component's environment, e.g., temperature, humidity, etc., and relationships with other components, e.g., radiators, automobile chassis, etc.).
In one embodiment, the invention provides a system for operating a rotating component. The system includes a rotating component, a motor driving the rotating component, a sensor detecting a stimulus related to the rotating component, and a controller including a memory. The controller stores a representation of a magnitude of the stimulus for a plurality of operating speeds of the rotating component, and receives an indication of the magnitude of the stimulus from the sensor. The controller determines which operating speeds represent resonant frequencies based on the stored magnitudes, and adjusts a speed of the rotating component when the controller determines the rotating component is operating at a resonant frequency.
In another embodiment, the invention provides a method of operating a rotating component. The method includes rotating the rotating component, detecting a stimulus, the stimulus related to at least one of noise and vibration, determining an operating condition of the rotating component, recording a magnitude of the stimulus and the operating condition of the rotating component, determining the operating condition of the rotating component is exciting a resonance, and adjusting the operating condition of the rotating component to avoid the resonance.
In another embodiment, the invention provides a vehicle including an engine cooling fan. The vehicle includes a control module, an engine cooling fan, a motor driving the engine cooling fan, a sensor detecting a stimulus related to the engine cooling fan, and a motor controller coupled to the control module. The motor controller controls the motor to rotate the engine cooling fan based on an indication from the control module. The motor controller also receives an indication of the magnitude of the stimulus from the sensor and adjusts a speed of the engine cooling fan when the stimulus indicates the engine cooling fan is operating at a resonant frequency.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
In the embodiment shown, an engine controller 135 receives an indication of engine temperature from a temperature sensor 140. The engine controller 135 provides a signal to the motor controller 115 indicating that engine cooling is required (e.g., the temperature of the engine exceeds a threshold, an air conditioning system is turned on adding additional load to the engine, etc.). In some embodiments, the motor controller 115 is coupled directly to the temperature sensor 140 and operates the cooling fan 105 based on a signal received from the temperature sensor 140. In some embodiments, the signal from the engine controller 135 indicates an operating speed for operating the engine fan 105. The motor controller 115 ramps the motor 110 to an operating speed (e.g., from zero to the operating speed), rotating the engine cooling fan 105 and cooling the engine. The sensor 120 continuously detects a stimulus (e.g., vibration or pressure) and provides a signal indicative of the magnitude of the detected stimulus to the motor controller 115. The motor controller 115 records the detected stimulus and the corresponding speed (e.g., rotations per minute) of the motor 110/fan 105 in the memory 130. Using the recorded data, the motor controller 115 determines if the operating speed coincides with a resonant frequency of the fan 105. If the operating speed coincides with a resonant frequency of the fan 105, the motor controller 115 adjusts the operating speed faster or slower so that the fan 105 is operating at a speed that does not coincide with a resonant frequency of the fan 105, reducing noise, vibration, and wear of the fan 105 and other components (e.g., a radiator, etc.).
In some embodiments, the speed of the motor 110 is not known, e.g., for a brushed DC motor operating under pulse width modulation (PWM) control. In an embodiment using PWM control of the motor 110, filters are used to obtain relationships between the PWM duty cycle and response amplitudes for different critical frequency bands. The critical frequency bands can be weighted to account for human perception (e.g., providing more weight to a frequency band in a human auditory range). Thus, critical frequency bands that do not offend humans can have a reduced level of correction for resonance, still taking into account the effects on wear and performance of the component.
In one embodiment, imbalance forces (i.e., a first order excitation) on a fan 105 occur at low frequencies. The motor controller 115 detects a set of response amplitudes of the sensor 120 using a low pass or band pass filter having a cut-off frequency slightly higher than the maximum operating RPM of the fan 105. Tracking the response amplitudes versus the PWM duty cycle identifies the duty cycles for which resonance occurs. For example,
One or more noise sources can often exist due to resonances of the fan 105 excited by the motor 110. Often these noise sources fall in a range of 250 to 700 Hz.
Higher frequency noise sources can occur due to internal motor resonances. Again, a band-pass filter is used to identify resonances in this range. For example,
In embodiments where the speed of the fan 105 is known (e.g., electronically commutated motors), or when the signal processing capabilities of the motor controller can determine fan speed from the sensor 120 (e.g., using a model-based filter, Kalman filter, etc.), the actual speed of the motor 110 is adjusted to avoid resonances instead of adjusting a duty cycle of a PWM signal.
In some embodiments, resonant frequencies are determined by direct calculations such as Discrete Fourier Transforms (DFT). A vibration level can be determined for a single frequency by using DFT. In some embodiments, a digital filter (e.g., a finite impulse response filter) is used to obtain vibration levels for a certain frequency bandwidth.
Vibration levels are stored in a table for given speeds or duty cycles. The vibration levels are compared to a pre-determined threshold to determine if a vibration level corresponds to a resonant frequency. The pre-determined threshold can be determined based on a comparison to known scaling of excitation forces compared to the speed of the fan 105. Thus, a resonant frequency is identified when a vibration level that exceeds an expected response by a pre-determined amount and the speed at which the resonance occurs can be avoided. The table can be continuously updated or can be generated each time the system is started. The data used to update the table can be filtered, weighted, or raw. In addition, multiple tables can be maintained based on various operating conditions (e.g., temperature, humidity, etc.). In some embodiments, these tables can be model-based filters.
In some embodiments, a slope of a vibration level versus RPM or PWM duty cycle is used to identify resonant frequencies, and to determine whether to increase or decrease the speed of the rotating component 105 to avoid a resonant frequency. Referring back to
In some embodiments, lower resonant fan speeds result in noise, vibration, and wear that are deemed not significant. At these lower speeds, resonant frequencies are ignored and the fan 105 is allowed to operate at the resonant frequency.
In addition to noise, vibration, and wear, resonance can cause other psychoacoustic phenomena to occur. One such phenomenon is “beating” which can occur when the fan's speed is at or near the RPM or firing rate of the engine. Beating may also occur when critical fan orders are near a critical order of other rotating components. In some embodiments, beating is avoided by the motor controller 115 receiving a signal from the engine controller 120 indicative of the RPM of the engine. The motor controller 115 then ensures that the speed of the fan 105 does not come within a critical range of the RPM of the engine or a critical harmonic of the engine RPM. Where the speed of the engine is not available or the speed of the fan 105 is not known, signal processing of the sensor 120 signal is used to identify the beating phenomena and adjust the duty cycle of the motor 110 to avoid the frequency of the engine.
Excitation forces, due to imbalance, scale with the square of the speed of the rotating component 105. In some embodiments, a regression curve of the expected excitation forces is calculated. This regression curve is compared to the actual measured response, and when the measured response significantly exceeds the regression curve, the speed of the rotating component 105 is considered to be at a resonant frequency.
Thus, the invention provides, among other things, systems and methods for determining when a rotating component is exciting resonances and for adjusting the speed of a rotating component to avoid operating the rotating component at resonant frequencies. Various features and advantages of the invention are set forth in the following claims.
