Abstract:
A frequency monitor provides high resolution frequency monitoring over a range of input signal frequencies by measuring the time period of the signal to produce a raw frequency value and then filtering the raw frequency value with a low pass filter to remove an error component caused by quantization effects at high input signal frequencies. The system automatically accommodates changes in the input frequency eliminating a need for manual adjustment of the system based on anticipated measured frequencies.

Description:
FIELD OF THE INVENTION 
     This application generally relates to frequency monitors for measuring the frequency of a periodic signal and particularly to a frequency monitor that may automatically accommodate a wide range of input frequencies with high resolution. 
     BACKGROUND 
     Frequency monitors may be employed to measure periodic signals, for example, electrical pulses from a tachometer or the like. Low-frequency signals are typically measuring the time between signal “features” of two successive cycles of the signal, for example, the rising edge of a square-wave pulse or zero crossing of a sine wave. The measured time may be obtained by counting “ticks” of a high-speed clock between the occurrence of these signal features and this count may be inverted to obtain a measure of frequency. 
     As the frequency rises, the number of measured clock ticks falls decreasing the effective resolution of the measurement. This decrease in resolution may be avoided by increasing the resolution of the clock (for example, using a higher clock frequency) or by converting to a second measurement method of counting the number of signal features for a given unit of time, for example, the number of rising edges or zero crossings per second. This second method provides increased resolution as the frequency rises and more clock ticks are counted in that given unit of time. This approach yields a direct measurement of frequency without the need for inversion. 
     In order to accommodate a wide variety of frequencies to be measured, frequency monitors may have manual settings allowing the user to set the clock frequency (effectively the resolution of the measurement) or to select between these two above-described methods of frequency measurement: (1) timing the duration between pulses or (2) counting a number of pulses for a given time duration. 
     This need to manually set the frequency monitor or to determine the likely frequency of the input signal can be problematic in an automatic control environment where knowledge about the frequency of the input signal may not be well known and there is a desire to avoid the need for human intervention. 
     SUMMARY OF THE INVENTION 
     The present invention provides a monitoring system that employs the first method described above, of timing an interval between successive signal features, and then applying a time domain filter to a period or frequency value derived from that interval. As the frequency of the signal to be measured rises, loss in resolution is manifest as a high frequency error component to the measurement and this error component is removed by the filter. The effectiveness of the filter in removing the error component increases as the frequency of the signal to be measured increases, providing a higher sampling rate that is increasingly filtered, in effect, compensating for the decreased resolution that would normally be associated with higher frequencies. 
     Specifically then, the present invention, in one embodiment, provides a frequency monitor having an input for receiving a signal to be measured and a signal feature extractor identifying a regular feature of the signal related to the signal&#39;s period. A timer system communicates with the signal feature extractor to provide an output being a function of period of the signal, and a low pass filter receives the output to reduce high frequency fluctuations in the output value to provide a measurement output for monitoring frequency of the signal. 
     It is thus a feature of at least one embodiment of the invention to provide a frequency-measuring device that may automatically accommodate a wide range of input signal frequencies without the need for manual or semiautomatic adjustment. The filter smoothly compensates for quantization error that would otherwise decrease the resolution of frequency measurement using the period monitoring technique for high frequency input signals. 
     The signal may be a square wave and the feature extractor may be an edge detector. 
     It is thus a feature of at least one embodiment of the invention to provide a system working with common square wave inputs using a robust feature extractor. 
     The timer system may include a timer measuring the period of the signal and an inverter converting the period to frequency. 
     It is thus a feature of at least one embodiment of the invention to apply the filter directly to a frequency signal for more intuitive filtered output when frequency is desired. More generally, the filtering can occur before the inversion process to act directly on the deduced signal period. 
     The timer may be a clock providing a periodic clock signal communicating with a counter counting the periodic clock signal between detection signals from the signal feature extractor identifying a regular feature of the signal. The output of the timer may be a digital word and the low pass filter is a digital low pass filter. 
     It is thus a feature of at least one embodiment of the invention to provide a digital domain system that may be readily implemented with digital logic. 
     Alternatively the output of the timer may be an analog voltage and a low pass filter is an analog low pass filter. 
     It is thus a feature of at least one embodiment of the invention to provide a versatile system that may be implemented in analog or a combination of analog and digital circuitry. 
     The frequency monitor may further include input to the low pass filter adjusting a corner frequency of the low pass filter. 
     It is thus a feature of at least one embodiment of the invention to permit adjustment by the user based on anticipated rates of change of frequency to effect a flexible trade-off between resolution and measurement response. It is a further feature of some embodiments of the invention to providing a filtering of noise that may cause loss of pulses in the primary signal. 
     Numerous other features, objects and advantages of the present invention will be made apparent from the following detailed description taken together with the drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified perspective view of an industrial control, point I/O unit providing a housing that may hold multiple input/output modules for communication between an industrial process and an industrial control system, one input/output module providing frequency monitoring according to the present invention, shown as monitoring a frequency of a tachometer; 
         FIG. 2  is a block diagram of the functional components of the frequency-monitoring module of  FIG. 1  showing a feature extractor, a timer, an inverter, and a low pass filter; 
         FIG. 3  is a graph of an input signal to the feature extractor aligned with a graph of the resulting output of the clock and the inverter for an input signal having a relatively low frequency relative to an internal clock; 
         FIG. 4  is a figure similar to that of  FIG. 3  for an input signal having a relatively high frequency relative to the internal clock; and 
         FIG. 5  is a fragmentary view of the block diagram of  FIG. 2  showing adjustment of the filter corner frequency and its effect on output responsiveness. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , an industrial control I/O unit  10  may provide for a housing  12  holding multiple modules  14  including those supporting terminal blocks, input and output circuits, power supplies, and network communication devices. Each of these modules  14  may communicate on a common backplane  15  distributing power to the modules  14  and providing a communication path for data between the modules  14  and between the I/O unit  10  and a remote industrial control system (not shown) via a network communication device. 
     A frequency monitoring module  14 ′ may receive an input signal  16 , for example a square wave or other periodic signal from sensors on controlled equipment, for example a tachometer,  18  which provides signal  16  whose frequency changes in proportion to a rotational rate of the tachometer  18  when attached to a rotating machine or the like. Other signal sources include, for example, optical switches and the like. 
     Referring now to  FIG. 2 , the input signal  16  may be received by a feature extractor  20  in the module  14 ′ that may extract a regular feature from the input signal  16  related to the period of the input signal  16 . In the case of a square wave input signal  16 , the feature extractor is preferably a rising edge detector detecting a rising edge  22  of the square wave of the input signals  16 . It will be appreciated that other feature extraction systems may be provided, for example those detecting zero crossing of the signal  16  or particular levels of voltage or current of the signal  16  that can be used for other signal types including sine waves. The feature extractor  20  may include preprocessing circuitry of a type known in the art including buffer amplifiers for amplifying the input signal  16 , automatic gain control circuits for adjusting the amplitude of the input signal  16 , filters for removing noise from the input signal  16 , and the like. 
     An output of the feature extractor  20  may be a series of pulses  24  coincident with the detective feature and that pass to a timer  26  and to an inverter  28 . The timer  26  may comprise a clock signal generator  30  producing a regular train of pulses  32  at a predefined and substantially constant frequency that are applied to the clock input of a standard digital counter  34  counting those pulses  32 . 
     The pulses  24  of the feature extractor  20  may be applied to the reset input of the counter  34  so that it starts counting from zero at the occurrence of each extracted feature. The pulses  24  may also go to the inverter which may serve to capture and invert the output  36  of the counter  34  immediately before it is reset. Thus, the time between the features extracted by the feature extractor  20  (and hence the time period of the input signal  16 ) may be captured and received by the inverter  28 . Generally, the output of the counter  34 , in this embodiment, will be a digital word expressed by binary bits each represented by a voltage level. 
     Inverter  28  performs an arithmetical inversion (as opposed to a Boolean inversion) on the output  36  to provide a measured frequency output  38  which may also be a digital word. Inverter  28  thus converts period to frequency according to a well-understood formula. 
     The output of the inverter  28  representing frequency is provided to a low pass filter  40  which performs a time domain low pass filtering of the measured frequency output  38 . The measured frequency output  38  will be sampled by the low pass filter  40  at the same rate as the clock  30  as synchronized by clock line  41 . As will be understood to those of ordinary skill in the art, low pass filtering preferentially passes low frequency components of a signal and thus will tend to pass slow changes of the measured frequency output  38  and suppress rapid changes of the measured frequency output  38 . The low pass filter  40  may be, for example, a four-pole, infinite impulse response filter. 
     Referring now to  FIG. 3 , when the frequency of the input signal  16  is relatively slow (e.g. relative to the pulses  32 ) the output  36  of the counter  34  will accumulate to a relatively high value output  36   a  before the counter  34  is reset. This will correspond to a reduced value of the measured frequency output  38   a . Further, the calculation of frequency (for example indicated by a measured frequency output  38   a ) will occur at a relatively low rate to provide a set of frequency samples  42  describing change in a measured frequency output  38  of the signal  16 . This slowly changing measured frequency output  38  when received by the low pass filter  40  will pass generally unaffected to produce a corrected frequency output  47  providing high resolution measurement of the frequency of the input signal  16 . Generally any error  46  in the measured frequency will be very low and is exaggerated in  FIG. 3  for illustrative purposes. 
     Referring now to  FIG. 4 , in contrast to the situation depicted in  FIG. 3 , relatively high frequency of input signal  16  will produce relatively low maximum count output  36   a  and correspondingly higher measured frequency output  38   a . The short counting time of the counter  34  will result in higher frequency of samples  42  and also the imposition of an error level  46  jitter caused by the high quantization error of the counting process in proportion to the total count when the count is truncated at a low value. The error level  46  will be higher in amplitude and at a higher frequency than the error level  46  of  FIG. 3 . This can be understood intuitively, for example, by considering when the clock frequency of the clock signal generator  30  represented by pulses  32  is close to the frequency of the input signal  16  and the count value will fluctuate between zero and one representing an error level  46  of 100 percent. This error level  46 , however, will be at a high frequency because of the relatively frequent resetting of the counter  34  such as produces a sample  42 , and thus this error level  46  will be removed by the low pass filter  40  to produce a smooth corrected frequency output  47  with a substantially reduced error level  46  and thus having implicitly a higher resolution. 
     Importantly, the action of the filter  40  is substantially automatic without the need to anticipate the frequency of the input signal  16  or to make adjustments in the measuring technique. 
     Referring now to  FIG. 5 , the corner frequency of the low pass filter  40  will generally be set many decades below the frequency of the pulses  32  depending on the responsiveness desired from the frequency monitoring module  14 ′ (for example, there may be as many as four decades between the corner frequency and the frequency of the clock  30 ). In addition, a user input  50  may be provided to the filter  40  to adjust the corner frequency  52  of the filter between corner frequency  52  and corner frequency  52 ′. Alternatively, digital filter constants may be adjusted while leaving the filter sample rate  50  the same. This adjustment can affect the filter output dither, but will primarily affect the speed at which the corrected frequency output  47  may change and is an important consideration when the frequency of the input signal  16  is expected to change rapidly. Generally a higher corner frequency  52 ′ will provide a reduced adjustment time  54  in the corrected frequency output  47  when there is a step increase (or decrease) in the frequency of the measured signal  16 . In contrast, a lower corner frequency  52  will provide a lengthened adjustment time  56  in the corrected frequency output  47  when there is a step increase (or decrease) in the frequency of the measured signal  16 . 
     Referring still to  FIG. 5 , it will be appreciated that the counter  34  and inverter  28  may be implemented by analog circuitry (for example, using an integrator and a logarithmic inverter) or the hybrid arrangement using digital and analog components and/or can produce analog counter output  36 ′ and analog measured frequency output  38 ′ permitting the filter  40  to be implemented using various analog rather than digital techniques. For example, the counter output  36  or inverted output  38  could be fed to a DAC and subsequently provided to an analog filter. 
     Various other embodiments of the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.