Abstract:
A circuit and method for assuring rapid initiation of resonant oscillation of an electromechanically oscillatory system driven by phase lock loop circuits. An open loop starting signal commences driving of the object at a starting frequency above the resonant frequency. The starting signal reduces the drive frequency until the resonant frequency of the system is reached and the starting signal is removed.

Description:
TECHNICAL FIELD 
     The present invention concerns the problem of improving the performance of a resonant circuit and, more particularly, the startup of a resonant tuning fork driven by an amplifier in a phase locked loop circuit. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric vibratory gyroscopes utilizing vibratory eletrostatically driven tuning forks are being used in a variety of angular velocity measuring applications. In such applications the tuning fork may be excited in a closed loop drive circuit which includes a phase locked loop. 
     Phase locked loop resonant drive circuits measure the phase difference between a sensor signal indicative of vibration of tuning fork and the drive signal applied to the amplifier providing the drive signal which drives the vibration of the tuning fork. A problem may arise in the starting of such circuits because upon the initial application of power, the tuning fork produces no output feedback signal to the phase comparator because it is not yet vibrating. At that time the output frequency of the voltage contolled oscillator, VCO, has no correlation to the tuning fork frequency because it is initially receiving a zero input signal. Such systems start up, eventually, when amplified white noise present in the system starts to be positively fed back through the system. 
     Where the tuning fork sensor has a resonant frequency f 0 , and the efficiency of the resonant system is Q, the starting time may be determined by dividing Q divided by f 0 . For vacuum tuning fork sensors with electrostatic drive and pickoffs, a typical Q of 100,000 and f 0  of 20,000 may require starting times of as long as five seconds. While this starting time could be reduced if the tuning fork were initially overdriven at the resonant frequency at the time that it was turned on, it has proven difficult to predict the resonant frequency of the fork in a particular circuit since it is common for them to have a resonant frequency that may vary in the ±20% range between similar tuning forks. Also, to be effective, the drive frequency must be within ±f 0  divided by Q cycles of the sensor resonant frequency. 
     SUMMARY 
     A phase locked loop circuit for driving an oscillatory mechanical object, which, when constructed according to the preferred embodiments of the present invention uses an amplifier for providing a drive signal to the mechanical object in response to an input signal, a phase detector for receiving a first signal and a second signal proportional to the movement of the mechanical object, the phase detector constructed and arranged for comparing the phase of the first and second signals and for providing an output signal having an average voltage proportional to the phase difference between the first and second signals, a voltage controlled oscillator receiving the output signal from the phase detector and producing an output signal which is the first signal received by the phase detector and an input of the amplifier, and a signal source for providing the voltage controlled oscillator with a starting signal at the time that power is applied to the system such that the frequency of the output of the voltage controlled oscillator commences at an initial frequency of the voltage controlled oscillator which is higher than a resonant frequency of the mechanical object, the signal source constructed and arranged for reducing the output frequency of the voltage controlled oscillator until the it corresponds to the resonant frequency of the mechanical object. 
     The apparatus and method may also use a voltage controlled oscillator, a driver circuit for providing a signal for driving the mechanical object at a frequency determined by the voltage controlled oscillator, a drive signal source for providing a starting drive signal to the voltage controlled oscillator when power is applied to the circuit prior to commencement of the oscillation of the mechanical object, the starting drive signal providing an output from the voltage controlled oscillator at a starting frequency predetermined to be above the range of expected resonant frequencies of the mechanical object, the starting drive signal varying with time to reduce the output frequency of the voltage controlled oscillator, a phase detector for comparing the phase difference between the output of the voltage controlled oscillator and a feedback signal indicative of the oscillatory motion of the mechanical object and providing an error signal to the input of the voltage controlled oscillator having an average voltage proportional to that phase difference, and a switch for removing the starting drive signal from the input of the voltage controlled oscillator when the frequency of the oscillatory motion of the mechanical object reaches a resonant frequency of the mechanical object. 
     A method of starting a closed loop resonant drive circuit for a mechanical object may involve providing an open loop starting signal to the circuit for initially driving the mechanical object at a frequency well above the range of frequencies expected for the resonant frequency of the mechanical object, varying the starting signal for sweeping the driving frequency to lower frequencies until the resonant frequency of the mechanical object is reached, and removing the starting signal while the mechanical object continues to oscillate at the resonant frequency. 
     A method of rapidly initiating oscillation of a resonant mechanical system driven by a phase lock loop circuit comprising an amplifier having a positive feedback path may also involve applying a decaying open loop starting voltage to the VCO (voltage controlled oscillator) input of a phase lock loop circuit so that the phase lock loop circuit output frequency starts at a maximum frequency which is above the resonant frequency of the mechanical system and sweeps downwardly toward a minimum frequency below the resonant frequency of mechanical system, allowing the phase locked loop circuit to lock at the resonant frequency of the mechanical element when the frequency of VCO reaches the resonant frequency of the mechanical system; and removing the starting voltage. 
     A method of rapidly initiating oscillation of a resonant mechanical system driven by a phase lock loop circuit comprising an amplifier having a positive feedback path may involve applying a decaying open loop starting voltage to the voltage controlled oscillator input of a phase lock loop circuit so that the phase lock loop circuit output frequency starts at a maximum frequency which is above the resonant frequency of the mechanical system and sweeps downwardly toward a minimum frequency below the resonant frequency of mechanical system, allowing the phase locked loop circuit to lock at the resonant frequency of the mechanical element when the frequency of the voltage controlled oscillator reaches the resonant frequency of the mechanical element, generating a lock signal to indicate that the phase locked loop circuit is locked, and removing the starting voltage in response to the lock signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a prior art phase locked loop circuit; 
     FIG. 2 shows a block diagram view of an embodiment of the present invention; and 
     FIG. 3 shows a more detailed, partially schematic view of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a block diagram of a prior art phase locked loop circuit  10  in which a voltage controlled oscillator (VCO)  12  is phase locked to a reference signal  13  received from an external source V in . A phase detector  14  continuously monitors the phase difference between the reference signal  13  and a feedback signal  16  which is an output signal of voltage controlled oscillator  12 . Feedback signal  16  is either provided by voltage controlled oscillator  12  directly or, it may alternatively be derived from the voltage controlled oscillator  12  as an intermediate frequency (IF) signal from a mixer driven by a local oscillator (not shown). Phase detector  14  generates an output error voltage  18  that is further processed by a low pass filter  20  and a loop amplifier gain stage  22  in the forward path of the phase locked loop. Output voltage  23  is fed back to the input of VCO  12  to control the frequency and phase of the output signal  16  of phase locked loop  10 . Phase locked loop  10  is maintained in the locked state when the voltage controlled oscillator  12  output error signal  16  is within a prescribed capture range. 
     FIG. 2 is a block diagram of a phase locked loop circuit  24  in accordance with one embodiment of the present invention for driving a high Q resonant mechanical object  26  at a resonant frequency of the mechanical object. In one embodiment, the resonant mechanical object is an electrostatically driven silicon micromachined tuning fork  26  which may find use in a number of rate and acceleration sensing applications. 
     The resonant mechanical object  26  is constructed and adapted to receive and be driven by a time varying drive signal  28 . Such a drive signal  28  is provided in FIG. 2 by an amplifier or driver circuit  30 . Where tuning fork  26  is electrostatically driven, driver circuit  30  may in one embodiment be an automatic gain controlled amplifier which provides a drive signal  28  at a fixed amplitude at the frequency of output signal  32  of voltage controlled oscillator  34 . 
     The forward loop of phase locked loop  24  comprises a phase detector  36  which receives a feedback signal  38  from mechanical object  26  which is indicative of the oscillatory motion of the mechanical object. Phase detector  36  compares feedback signal  38  to the output signal  32  of voltage controlled oscillator  34  and provides an output signal  40  which is coupled to filter  42  which, in one embodiment is a low pass filter. Low pass filter  42  delivers an output voltage  44  to an amplifier stage  46  which produces an output voltage  48 . The feedback loop is closed by connecting output voltage  48  to the input of VCO  34 . 
     At the time that the circuit of FIG. 2 initially receives power, tuning fork  26  is not vibrating so that there is no feedback signal provided to phase detector  38  from tuning fork  26 . Because VCO  34  is starting with no output voltage  48  being delivered from amplifier  46 , its output is at a minimum frequency as determined by the electronic component values of VCO  34 . Thus phase detector  36  initially has input signal  32  but signal  38  is at a near zero level. Only white noise from amplifier  46  is initially present in the loop. Eventually, the noise generates enough of a signal to provide an output signal  28  which, in turn, begins to apply a signal to start driving tuning fork  26 . 
     As the noise signal  28  increases, the vibration amplitude of tuning fork  26  also increases and the phase detector input  38  reaches a sufficient level to allow the phase lock loop to achieve lock at the resonant frequency of fork  26 . Since fork  26  has a very high Q or a narrow vibration/signal bandwidth, signal  38  is predominantly the resonant frequency of fork  26  despite the fact that signal  28  also contained white noise and signals that were displaced from the resonant frequency. 
     In order to more quickly commence operation of resonant drive circuit  24  of FIG. 2, a starting signal  50  is provided from a drive signal source  52 . In one embodiment, starting signal  50  is a high voltage which initially drives voltage controlled oscillator  34  at a frequency which is above the range of expected variation of a resonant frequency of tuning fork  26 . In one embodiment, starting signal  50  is reduced in amplitude to sweep the frequency of the output of voltage controlled oscillator downwardly. When the frequency of the voltage controlled oscillator reaches a resonant frequency of tuning fork  26 , phase locked loop  24  locks and drive signal source  50  removes starting signal  52  so that the phase locked loop continues to operate at the resonant frequency of tuning fork  26 . 
     FIG. 3 shows an embodiment of the resonant drive circuit  54 . A phase locked loop integrated circuit  56  is connected with its VCO output signal  58  coupled to driver circuit  60  and to the reference input  62  of phase locked loop circuit  56 . Driver circuit  60  drives tuning fork  64  with a drive signal  66 . As tuning fork  64  oscillates, a feedback signal  68 , indicative of the oscillatory motion of tuning fork  64 , is coupled to the input terminal  70  of phase lock loop circuit  56 . The signal at phase detector output terminal  72  is connected to a first order filter  74  comprised of a resistor  76 , a capacitor  78  and a resistor  80 . The output of filter  74  is coupled from output terminal  82  to the input  84  of voltage controlled oscillator on circuit  56 . A driver circuit  86  comprised of a resistor  88 , a capacitor  90  and a switch  92  is connected to a positive voltage source  94 . 
     In one embodiment, the resistance of resistor  88  is much larger than that of resistor  76  and the capacitance of capacitor  90  is much larger than the capacitance of capacitor  78 . Switch  92  is closed when power is applied to the circuit. The input  84  to the voltage controlled oscillator starts high and then sweeps down at a rate which is in accordance with the time constant of resistor  76  and capacitor  90 . The output  58  of the voltage controlled oscillator starts near the maximum frequency of the voltage controlled oscillator and is swept downwardly toward its minimum frequency. During the sweep of the frequency, driver circuit  60  is providing a maximum drive signal  66  at a decreasing frequency. The frequency sweep continues until the phase locked loop voltage controlled oscillator output  58  reaches the tuning fork resonant frequency. At this frequency the phase locked loop will lock, the lock signal  96  generated by the phase locked loop circuit  56  will be provided to open the contacts of switch  92 , removing the drive signal and allowing the phase detector output  72  to control the voltage controlled oscillator frequency at a resonant frequency of the tuning fork. Amplifier and driver  60  continues to drive tuning fork  64  at a maximum level until the desired amplitude of the tuning fork oscillation amplitude is reached. 
     CONCLUSION 
     Systems, devices, structures, and methods have been described to address situations relating to the rapid starting of drive circuits for resonant objects such as electrostatically driven tuning forks. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used. Accordingly, the scope of the invention should only be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.