The present invention relates to audible alert devices, and more particularly to controlling the operation of such devices.
Audible alert devices are used in a variety of applications. In some instances, audible alert devices are configured as backup alarms that may be mounted to heavy equipment such as forklifts or tractor-trailers. Backup alarms are activated when a reverse gear is used and provide important warnings to those nearby. This promotes safety and helps to reduce accidents.
Audible alert devices such as backup alarms are typically calibrated at the factory to operate at a predetermined output level before they are deployed for use in the field. The alarm, for example, may be rated at a specified decibel level which may be established by governmental regulation or industry standards. Unfortunately, such factory calibration cannot take into account the effect of environmental factors. As a result, there may be uncontrolled variation in audible output when the device is operated. This variation reduces the effectiveness of the audible alert device. Thus, there is a need in the art for an audible alert device with improved performance that addresses the effect of environmental factors.
According to one embodiment of the present invention, a method of controlling an audible alert device includes receiving a supply voltage signal and determining a level of the supply voltage. The method also includes adjusting drive signal parameters based upon the level of the supply voltage and generating a drive signal according to the drive signal parameters. The method further includes delivering the drive signal to a transducer to control the audible output of the transducer.
In some embodiments, the drive signal comprises a pulse-width modulated signal and the drive signal parameters include a duty-cycle of the drive signal pulses. The method may include reducing the duty cycle of the drive signal pulses if the level of the supply voltage is determined to be higher than a reference value associated with the audible alert device and increasing the duty cycle of the drive signal pulses if the level of the supply voltage is determined to be lower than the reference value.
The method may also include determining a resonant frequency of the transducer and adjusting the drive signal parameters to operate the transducer at the frequency determined to be the resonant frequency. A frequency of the drive signal pulses may be adjusted according to the transducer resonant frequency. In some embodiments, the transducer resonant frequency is identified by generating a series of drive signal pulses corresponding to predetermined frequency values and detecting a feedback signal from the transducer in response to the series of drive signal pulses. An initial set of drive signal parameters may be established with reference to transducer data such as the transducer's physical characteristics.
According to another embodiment of the present invention, an audible alert device comprises an adapter configured to receive a supply voltage signal. The device also includes a voltage monitor configured to produce a first input signal representative of a level of the supply voltage signal. The device includes a control block configured to adjust drive signal parameters based upon the first input signal and a drive signal generator configured to generate the drive signal using the drive signal parameters. The apparatus further includes a transducer configured to receive the drive signal and to produce an audible output that varies according to the drive signal.
In some embodiments, the audible alert device includes a voltage divider configured to receive the supply voltage and to produce the first input signal. The audible alert device may also include an analog-to-digital converter configured to produce a digital signal representative of the first input signal. In other embodiments, the audible alert device includes an RC network that receives the supply voltage such that the first input signal corresponds to a voltage across the capacitor. The control block may be configured to measure a charging time required for the first input signal to reach a predetermined level and to determine a level of the supply voltage based upon the charging time. The audible alert device may include a thermistor configured to establish a voltage signal representative of the ambient temperature of the audible alert device. In some embodiments, the audible alert device includes a microcontroller.
An audible alert device in accordance with the present invention determines the effect of environmental factors upon the output sound pressure level of a transducer and establishes drive signal parameters adapted to the current operating environment. The drive signal parameters are based upon supply voltage, ambient temperature and/or resonant frequency. The drive signal is optionally a pulse width modulated signal for which the drive signal parameters represent the frequency and duty-cycle of the drive signal pulses. The audible alert device generates the drive signal using the drive signal parameters and delivers it to the transducer thereby controlling the output sound pressure level.
Power supply 110 provides the electrical power that is used by audible alert device 100. Power supply 110 generally includes one or more batteries configured to supply a specified operating voltage. In a backup alarm configuration, for example, a supply voltage of 12-36V may be received from a vehicle battery. Audible alert device 100 uses the supply voltage to drive transducer 150 at a predetermined audio output level.
Audible alert device 100 is configured to monitor the supply voltage and ambient temperature and to maintain the sound pressure level of transducer 150 at a substantially constant value. As shown, operating environment monitor 120 receives the supply voltage signal and determines a level of the supply voltage. For example, the supply voltage may increase as a result of charging the vehicle battery or it may decrease due to heavy loading of the vehicle's electrical system. Operating system monitor 120 provides information about the level of the supply voltage to control block 130.
Operating system monitor 120 also determines an ambient temperature of the audible alert device. It is known that physical properties of a transducer may change with temperature. Accordingly, operating system monitor 120 provides ambient temperature information to control block 130 so that the sound pressure level of transducer 150 can be adjusted to account for the effect of temperature.
Control block 130 sets and adjusts parameters used to produce the drive signal and to thereby control the output of transducer 150. The drive signal parameters are based upon inputs provided by operating system monitor 120 and may also be determined according to operating characteristics of transducer 150. For example, control block 130 may store data about the size and type of transducer 150 or about its resonant frequency or about other performance related characteristics. Transducer data may be based upon manufacturing specifications or it may be determined as the result of a testing process.
Drive signal generator 140 produces an output signal having the characteristics that are determined by the drive signal parameters. In some embodiments, drive signal generator 140 produces a pulse-width modulated output signal wherein the duty cycle of each pulse and the pulse frequency (or period) are determined by the drive signal parameters. The output of drive signal generator 140 is delivered to transducer 150 and controls the frequency and amplitude of the audible alert. By monitoring its operating environment and adjusting for environmental factors, audible alert device 100 maintains a precise control of the transducer output level.
With reference to
Operating environment monitor 120 also includes temperature monitor 220. Temperature monitor 220 determines an ambient temperature of audible alert device 100 through any known temperature sensing means.
Control block 130 produces parameters that are used by drive signal generator 140 to control the output of transducer 150. In some embodiments, control block 130 stores transducer data 430 in a memory and retrieves it at the beginning of an operating cycle. Transducer data 430 may supply an initial set of drive signal parameters which are determined according to characteristics of transducer 150. For example, transducer data 430 may reflect initial parameters suitable for driving a 3″ speaker to an output level of 107 dB at a specified operating voltage if the audible alert device is configured in this manner.
Voltage compensation block 410 adjusts the drive signal parameters based upon the current level of the supply voltage. By way of illustration, if the audible alert device is configured for use with a 12V source and the supply voltage is detected as being only 11V, then voltage compensation block 410 adjusts the drive signal parameters to compensate for the voltage difference and to thereby maintain the sound pressure level at a substantially constant level. In some embodiments, voltage compensation block 130 determines an adjustment value with reference to a table or other data structure specifying a relationship between the supply voltage level and the drive signal parameters. In other embodiments, control block 130 determines the adjustment value by performing a calculation using one or more correlation coefficients.
Temperature compensation block 420 adjusts the drive signal parameters according to the ambient temperature. For example, transducer 150 may operate more efficiently in cold temperatures and its output may therefore increase as temperature decreases. Similarly, transducer 150 may operate less efficiently when the ambient temperature rises. Temperature compensation block 420 adjusts the drive signal parameters to compensate for differences in transducer sound pressure level due to ambient temperature. In some embodiments, temperature compensation block 420 determines an adjustment value with reference to a table or other data structure specifying a relationship between ambient temperature and the drive signal parameters. Temperature compensation block 420 may alternatively determine the adjustment value by performing a calculation using one or more correlation coefficients.
In this manner, control block 130 produces drive signal parameters that are based upon transducer characteristics, supply voltage level, and ambient temperature. Thus, for example, a low supply voltage and a relatively high ambient temperature may indicate that the drive signal parameters need to be adjusted to drive the transducer harder in order to maintain the specified output sound pressure level. On the other hand, if the supply voltage exceeds the specified value while at the same time ambient temperature is relatively high, drive signal parameters may represent a net adjustment based upon the relative magnitude of these factors. In other words, the drive signal parameters reflect conditions prevailing in the operating environment.
In some embodiments, drive signal generator 140 produces a pulse-width modulated (PWM) drive signal. The drive signal parameters for the PWM signal are set or adjusted by control block 130 and include a duty-cycle (“on-time”) of the drive signal pulses. By adjusting the pulse duty-cycle, environmental factors may be compensated for by driving transducer 150 as required to maintain a steady output sound pressure level. For example, based upon supply voltage and ambient temperature, control block 130 may determine that a net increase of 1 dB is required to offset environmental factors and maintain transducer output at a specified level (e.g., 96 dB). If it is known that an 11% increase in pulse duty-cycle corresponds to a 1 dB increase in sound pressure level, then control block 130 would increase the pulse duty-cycle parameter by 11% without changing the pulse frequency. In other words, the frequency of the generated pulses would be held constant but the “on-time” of each pulse would be increased by 11%.
Control block 130 also includes resonant frequency detector 440. Driving transducer 150 at its resonant frequency is efficient and produces a maximum sound pressure level output at a given source voltage. However, the resonant frequency of a transducer may change over time and may change based upon operating conditions. For example, resonant frequency may be altered if the transducer becomes wet. Similarly, mud or debris may accumulate on the transducer, changing the mass of its diaphragm and thus changing its resonant frequency.
Resonant frequency detector 440 adjusts drive signal parameters to operate transducer 150 at its resonant frequency. In some embodiments, a frequency sweep is performed when the audible alert device is activated. For example, control block 130 may generate a series of drive pulses corresponding to different transducer operating frequencies. Resonant frequency detector 440 monitors a feedback signal from transducer 150 at each of the operating frequencies and detects a level of the feedback signal.
In various embodiments, audible alert device 100 includes a high-impedance feedback network coupled with transducer 150. The high-impedance feedback network is configured to detect a back-emf signal from the transducer. Resonant frequency detector 440 monitors the transducer back-emf signal and, in one embodiment, determines the operating frequency that maximizes its amplitude. This value represents the resonant frequency of the transducer under current operating conditions.
Referring to
At step 630, the audible alert device determines the resonant frequency of the transducer under current operating conditions. This may include performing a frequency sweep. In some embodiments, the audible alert device is configured to generate a series of warning beeps and it determines the resonant frequency by performing the frequency sweep at the start of a beep tone. For example, the audible alert device may prepend a series of frequency sweep pulses to the first beep tone that is generated. At step 640, the audible alert device sets drive signal parameters based upon the transducer resonant frequency. This may be accomplished by setting a timer value to produce drive signal pulses at the resonant frequency.
The audible alert device compensates for operating environment factors. At step 650, the supply voltage is determined in relation to its specified input voltage. Ambient temperature is also determined at step 660. Using information about the supply voltage and ambient temperature, the audible alert device adjusts drive signal parameters so that variation in sound pressure level due to these factors is avoided. In some embodiments, for example, audible alert device performs a lookup operation and retrieves values from a table or matrix based upon the current voltage and ambient temperature. The audible alert device may also perform a calculation to determine the net effect of the operating environment.
At step 670, audible alert device adjusts drive signal parameters based upon the supply voltage and ambient temperature by setting the duty-cycle of the drive signal pulses. At step 680, the audible alert device generates a drive signal adapted for the current operating environment using the drive signal parameters. The drive signal is applied to a transducer to generate audible output at a predetermined sound pressure level.
The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.
The present application claims benefit under 35 USC 119(e) of U.S. Provisional Application No. 60/778,822, filed on Mar. 2, 2006, entitled “Adjusting Alarm Drive Pulse For Changes In Temperature And Supply Voltage Via Microcontroller” the content of which is incorporated herein by reference.
Number | Date | Country | |
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60778822 | Mar 2006 | US |