Abstract:
The present invention is embodied in a hand held caliper with a hand movable slide, whose motion is transformed into rotation of an apertured multi-row encoder disk, a radiation source and a radiation detector. The present invention accurately measures a linear dimension using a simple, efficient, and durable digital measuring system. Accuracy of 0.0001″ is attained, greater than the 0.0005″ of the prior art, without encountering greater manufacturing costs, by utilizing well known fabrication techniques when producing the multi-row encoder disk, and otherwise using well known and inexpensive technology. The present invention does not require a motion dampening device, as errors may be determined and signal jitter are compensated for in the invention electronically or through the use of software, which adds little or no additional cost to the system and does not add mechanical complexity. Further, the present invention only requires a single rotatable disk to determine the direction of rotation, and no phase offset circuitry, through the use of dual rows of light emitting slots in the disk, the slots being offset between rows by 90° (in quadrature), but may be offset by as little as 10°.

Description:
TECHNICAL FIELD 
     The present invention relates to digital linear measuring systems and more particularly to devices which measure a linear dimension using conversion of rotational motion to linear motion. 
     BACKGROUND OF THE INVENTION 
     Calipers are used to measure linear dimensions in various portable or hand held uses. Distances measured may be on the order of 0.001″ or smaller. This order of resolution gives rise to difficulty in practical representation to a human operator. Traditionally, a “Vernier” type caliper used a gear reduction system which translated rotational motion of a gear to be represented as a linear distance displayed on a mechanical dial. However, such systems have inherent problems such as slop in the gears (“backlash”)and limitations in accuracy of the machining of parts for calipers measuring dimensions less than 0.001″. Although these limitations can be overcome through the use of anti-backlash devices and modern machining methods, these add cost, complexity and frailty. 
     More recently, calipers have incorporated optical sensing systems which use a sensor to detect radiation from a source, and use a signal generated by an optical sensor to generate an analog electrical signal. Currently, calipers using optical sensing systems are accurate to 0.0005″. These devices (U.S. Pat. Nos. 4,008,523 and 4,684,257) typically use a glass plate in the shape of a rectangular strip, etched with lines that inhibit the passage of light or depositing a layer of light absorbing material in which lines of the absorbing material have been etched away. 
     Other calipers utilizing optical sensors (U.S. Pat. Nos. 4,034,477, 4,035,922 and 5,430,954) use a disk with a single row of etched lines which rotationally pass between a photocell that emits a light and an optical sensor, but otherwise operate similarly to the glass strip above. The rotational movement of the disk causes a continuous signal to be generated by the sensor relative to the amount of light it receives. 
     One problem with such a device is that if a user measures a dimension that is, say 0.001″, the sensor moves, or the glass strip/wheel moves, 0.001″, typically from one etch point to the next. This limits the accuracy the caliper because of the limitations imposed by costs of manufacturing. Typically, the accuracy of the prior art is no greater than 0.0005″. 
     Because of the 1:1 measuring mentioned above, calipers require a dampening system to inhibit rapid movement of the measuring jaws. The 1:1 measuring requires the CPU to sample the sensor-generated signal at a sufficient rate to accurately detect when the signal reaches a level signifying a valid increment/decrement of a measuring count. If the user moves the jaws of the calipers too rapidly, the CPU sampling rate may not be high enough to detect when a signal level crosses a threshold. This may result in missed counts, producing erroneous output to the user. This result manifests itself when signal threshold crossings are too close in time, such that more than one signal threshold crossing occurs in a single sampling time of the CPU. A motion dampening device is typically employed to restrict the speed of movement of the measuring jaws to comply with the CPU sampling rate. These devices are well known in the art. The addition of the dampening device adds cost and mechanical complexity to the measuring device, and thus it is desirable to remove the need for dampening, as well as increasing the sampling rate. 
     An additional disadvantage of calipers which utilize a disk is that only a single row of slots are used. In order to determine the direction of rotation of the disk (which is also indicative of the direction of travel of the jaws of the calipers), two light signals must be sensed through the same slot. This is carried out with at least one photocell and two sensors. Once the signals are sensed, one of the signals must be offset by 90°. This offset is done through a phase offset circuit, adding additional cost and complexity to the measuring device. In an alternative embodiment, prior art calipers utilizing a disk system require the use of two disks in order to determine direction of travel. This method also gives rise to additional cost and complexity. It would be desirable to use a single disk without additional phase offset circuitry and still be able to easily determine direction of rotation of the disk. 
     Another disadvantage of prior art calipers incorporating optical sensing systems is that each measuring device must be built as a single unit. The physical measuring jaws, the body of the entire device, and the optical sensing systems must be built as a whole. This precludes modularity between devices, restricting the use of the optical sensing system to a single caliper. It would be desirable to have a modular optical sensing system usable by a number of calipers or which can be retrofitted onto a mechanical caliper, as this would cut costs for these types of tools. Additionally, this would allow for ease of learning when the same display and modes of operation would be applicable to a range of different calipers, instead of a single device. 
     Further, prior art calipers which utilize optical sensing systems typically do not detect errors. An error can manifest itself when the signal input rate exceeds the CPU sampling rate, due to manufacturing defects in the etching of the glass strip or the wheel, resulting in varying etch widths. The prior art calipers use dampening systems to avoid exceeding the CPU sampling rate, but cannot compensate for defects in the etching process. Errors may also arise when the CPU sampling rate is exceeded, as discussed above. It would be desirable to incorporate an error detection system so that a motion dampening system would not be necessary and so that etch defects could be known to the system and compensated for, without a significant increase in manufacturing costs. 
     One problem with prior art calipers is signal jitter. Signal jitter is an effect seen when the user is not moving the jaws of the calipers. Due to vibrations, which may come from a variety of sources, a signal from a sensor oscillates about a point. This effect causes significant problems when the point at which the jitter occurs is at, or close to, a signal threshold. If a jittering signal crosses a signal threshold, the CPU may interpret each threshold crossing to be actual movement of the glass strip, or rotational movement of the wheel, and thus generate increment/decrement pulses. Prior art systems utilize motion dampening systems, as well as other mechanical devices, to overcome the effects of signal jitter, but these additional components increase cost and complexity of the measuring device, and may not be completely effective. It would be desirable to remove the need for motion dampening systems, or other mechanical components to compensate for signal jitter, while at the same time removing any adverse effects of signal jitter upon the measuring device, without adding mechanical complexity or cost to the device. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art as described above and other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a hand held caliper with a hand movable slide, whose motion is transformed into rotation of an apertured multi-row encoder disk, a radiation source and a radiation detector. The present invention accurately measures a linear dimension using a simple, efficient, and durable digital measuring system. Accuracy of 0.0001″ is attained, greater than the 0.0005″ of the prior art, without encountering greater manufacturing costs, by utilizing well known fabrication techniques when producing the multi-row encoder disk, and otherwise using well known and inexpensive technology. The present invention does not require a motion dampening device, as errors may be determined and signal jitter are compensated for in the invention electronically or through the use of software, which adds little or no additional cost to the system and does not add mechanical complexity. Further, the present invention only requires a single rotatable disk to determine the direction of rotation, and no phase offset circuitry, through the use of dual rows of light emitting slots in the disk, the slots being offset between rows by 90° (in quadrature), but may be offset by as little as 10°. 
     The present invention can be designed to be modular. A single unit can replace older mechanical and electrical measuring systems on calipers. In order to further remove the need of a motion dampening device typically required in a caliper, the present invention includes apparatus which detects errors in the CPU samples as well as compensates for consistently detected errors, such as those that would occur in a defective etch slot on the encoder disk. 
     The present invention is embodied in a system and a method for measuring a linear dimension using a multi-row encoder disk. The present invention transforms the linear dimension into a rotational displacement of an encoder disk. By determining the displacement of the encoder disk the linear dimension can be accurately determined. Rotational displacement is determined using the encoder disk, a radiation source and a radiation detector. The encoder disk includes at least two concentric encoder rows having a multiple equally-spaced slots. These slots are apertures in the encoder disk. The openings in one row are offset from the openings in the other row by a certain amount to aid in determining the direction of the encoder disk and allow for greater resolution and accuracy. 
     Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not by the preceding summary or the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a digital measuring system of the present invention incorporated into a digital caliper. 
     FIG. 2A shows an exploded view of an encoder assembly of the digital measuring system of FIG.  1 . 
     FIG. 2B is a simplified depiction of a linear to rotational converter within the assembly of FIG. 2A 
     FIG. 3 shows a plan view of an encoder disk according to the present invention. 
     FIG. 4 illustrates the encoder disk, a radiation source and a radiation detector of the encoder assembly shown in FIG.  2 . 
     FIG. 5 illustrates a plan view of a stationary aperture according to the present invention. 
     FIG. 6 is block diagram of the signal processing modules of the present invention. 
     FIG. 7 is a block diagram of the decode logic module  602  of FIG. 6 
     FIG. 8 shows a waveform pattern for the present invention and relative high and low threshold levels. 
     FIG. 9 shows a time domain waveform of signals generated as the encoder disk rotates between emitter/sensor pairs. 
     FIG. 10 shows a flow diagram of the operation of a comparator in the present invention. 
     FIG. 11 shows a flow diagram of the operation of a state machine in the present invention. 
     FIG. 12 shows a flow diagram of the operation of error detection in the present invention. 
     FIG. 13 shows a table corresponding to expected states in FIG. 12, when the encoder disk is moving in a clockwise direction. 
     FIG. 14 shows a table corresponding to expected states in FIG. 12 when the encoder disk is moving in a counter-clockwise direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the invention, reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration a specific example whereby the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     I. Introduction 
     As shown in FIGS. 2-6 for the purposes of illustration, the invention is embodied in a system and a method for measuring a linear or rotational magnitude using a multi-row encoder disk. The invention uses an encoder disk with multiple concentric rows, a radiation source on one side of the disk over some of the openings and a radiation sensor opposite the source on the other side of the disk. Each row contains a plurality of alternating openings and closures. As the encoder disk turns the radiation source emits radiation through some of the openings where the radiation is detected by the sensor. 
     When the radiation is detected by the sensor, the sensor emits a signal to a signal processor in the form of a continuous pseudo-sinusoid corresponding with the strengths of detected radiation. The signal processor uses this signal to determine direction of rotation of the disk, increment/decrement a counter and detect errors. 
     II. Structural Overview 
     FIG. 1 illustrates a digital measuring system of the present invention incorporated into a digital caliper. Although calipers are preferred, the present invention may be incorporated into several other types of portable measuring devices such as, for example, micrometers or height gauges, which require linear measurement as part of their operation. In general, the digital caliper  100  includes a main body  102  that includes a first jaw  105  and a second jaw  110  in a sliding relationship with each other that are used to measure an outside dimension (d) of an object to be measured  115 . The first jaw  105  and second jaw  110  are capable of being moved toward each other (closed) and away from each other (opened) over a certain distance to ideally permit the distance between the jaws to equal the outside dimension (d). At the top of the first jaw  105  is a first protrusion  120  that projects upward from the main body  102 . Likewise, a second protrusion  125  is located at the top of the second jaw  110  and projects upward in the same manner as the first protrusion  120 . The two protrusions are used, for example, when measuring an inner dimension of an object (not shown). 
     The digital caliper  100  further includes an encoder assembly  127  attached to the main body  102  of the caliper  100 . As discussed in detail below, the encoder assembly  127  includes elements that translate the linear motion of the jaws into rotational motion and determine a distance between the jaws by measuring the rotational displacement of an encoder disk. At the front of the encoder assembly  127  is a digital display  130  that displays the numerical output of a measured dimension in the desired units. For example, in a preferred embodiment the digital display  130  is capable of displaying measurements in both millimeters and inches. Further, although several variations are possible, in a preferred embodiment the digital calipers  100  includes three buttons on the face of the encoder assembly  127  that correspond to different functions. In particular, when depressed the zero button  135  resets the digital display  130  to zero, the on/off button  140  turns the caliper  100  on and off and the units button  145  toggles between a choice of available units (e.g. millimeters and inches). Power for the caliper  100  is provided by a battery (not shown) contained within a battery case  150  located on the encoder assembly  127 . 
     FIG. 2A shows an exploded view corresponding to FIG.  1 . The encoder assembly  127  is positioned on a base  205  that is located on the main body  102  of the caliper  100 . A shaft  210 , which projects from the base  205 , is attached to a linear-to-rotational converter  1500  (FIG. 2B) consisting of a series of conventional multiple-pass gears (not shown) that translate the linear motion of the jaws into rotational motion. In particular, as the jaws are opened and closed the shaft  210  rotates either counterclockwise or clockwise. Preferably, the multiple-pass gears have a conventional anti-backlash mechanism (such as a spring-loaded mechanism) that prevents backlash between the gears and provides more accurate measurements. In addition, the multiple-pass gears provide a way to amplify the linear movement of the jaws in relation to the rotation of the shaft  210 . For example, a certain gear ratio may be provided such that for every millimeter that jaws are apart from each other the rotational displacement of the shaft is much greater than one millimeter, thus amplifying the linear movement of the jaws and providing greater resolution. These types of multiple-pass gears and anti-backlash mechanisms are known in the art and will not be discussed further. The linear-to-rotational converter may be any suitable type of measuring device capable of converting linear displacement into rotational displacement. 
     In a working example the caliper  100  is a dial caliper that has been converted into a digital caliper. The dial, which was previously attached to the shaft  210 , was removed and the adapter  215  fastened onto the shaft  210 . It should be noted that this particular caliper includes the type of linear-to-rotational converter described above and preferred for the present invention. The encoder disk  220  was fastened to the adapter  215  and the remainder of the encoder assembly positioned on the caliper base  205 . Further, the cover  235  seals the unit from the outside environment and therefore protects the inside of the encoder assembly  127  from external contaminants. Thus, in the working example, a mechanical dial caliper was easily converted into a digital caliper having an encoder assembly protected from outside contaminants. 
     An adapter hub  215  is secured to the shaft  210  and facilitates the attachment of an encoder disk  220  to the shaft  210 . A printed circuit board  225  containing the electronics and logic modules of the present invention are positioned over the encoder disk  220  and include the digital display  130 . A cover  235  is located over the entire encoder assembly  127  so that the internal components of the encoder assembly  127  are sealed from the outside environment. A clear window  240  is positioned within the cover  235  for viewing the digital display  130 , and forms part of the seal against the outside environment. The cover  235  and window  240  are securely fastened to seal the encoder assembly  127  from liquids, dust, dirt and other harmful contaminants that may interfere with the operation of the encoder assembly  127 . 
     FIG. 3 shows a plan view of the encoder disk  220  according to the present invention. The encoder disk  220  is preferably chemically or photolithographically etched to provide concentric multiple rows of slots with the slots of each row being offset with each other. Other methods of etching the slots in the encoder disk may be utilized, as will be known by one skilled in the art. In a preferred embodiment shown in FIG. 3, the encoder disk  220  contains two rows of slots, which are apertures in the encoder disk  220 . A first row of slots  310  is located near the circumference of the encoder disk  220  and the slots are uniformly spaced from one another. A second row of slots  320  is concentric with the first row  310  and has a smaller radius than the first row  310 . As shown by the slots  300  and  302 , the two rows are in quadrature with each other. In other words, the two rows are approximately ninety degrees out of phase with each other, meaning that a centerline of a slot in the first row  310  is circumferentially offset from a centerline of a slot in the second row  320  by one-quarter of the distance between slots. However, the slot offset need not be ninety degrees, and may be as little as ten degrees. In any case, the offset is skewed so that each slot in one row is closer to a slot in the other row than to other slots. Thus, the signals from the two rows  310  and  320  appear as slightly offset pairs of signal peaks. In a working example, the encoder disk has two rows, the diameter of the encoder disk  220  is 1.1 inches and each row contains  250  slots. 
     FIG. 4 illustrates the configuration of the encoder disk  220 , a radiation source and a radiation detector of the encoder assembly  127  shown in FIG.  2 . The encoder disk  220  is positioned on the shaft  210  using the adapter  215 . A radiation source  410  is located on one side of the encoder disk  220  and situated so that radiation from the radiation source can pass through some of the slots in each row. In general the radiation source  410  may be located on either side of the encoder disk  220 . Preferably, as shown in FIG. 4, there is an outer radiation source  412  and an inner radiation source  414  for each row of slots although a single radiation source or more than one radiation source per row may be used. In addition, the radiation source  410  is preferably an infrared source although other types of radiation such as, for example, visible light, may be used. 
     In an alternative embodiment, non-optical radiation sources and sensing devices, or their equivalent may be utilized. These include, by way of example and not limitation, magnets, capacitive plates, inductive coils, electrical current measurements, voltage measurements, and resistance measurements. In addition, MEMs (microelectictro-mechanical) switches and systems may be utilized to carry out the present invention, as well as nano scale sensors and devices. Further, the radiation source and/or sensor need not be separate from the encoder disk. By way of example, a radiation source could be mounted upon the encoder disk, while the sensor is a unit separate form the encoder disk. Conversely, a sensor could be mounted upon the encoder disk, while the radiation source is a unit separate from the encoder disk. In these alternate embodiments, the apertures in the encoder disk may not be necessary, as either the radiation source or detector is mounted directly upon the encoder disk and may rotationally pass either the detector or radiation source respectively. 
     On the other side of the encoder disk  220  and directly opposite the radiation source  410  is a radiation detector  420 . The radiation detector  420  is positioned such that the encoder disk  220  lies between the detector  420  and the radiation source  410  and is capable of detecting radiation from the radiation source  410 . In a preferred embodiment, there are two radiation detectors, an outer radiation detector  422  and an inner radiation detector  424 . Referring to FIGS. 4 and 5, in a preferred embodiment a stationary aperture  500  is fixed between the encoder disk  220  and the radiation detector  420  and is aligned with the rows of slots in the encoder disk  220 . In particular, the stationary aperture  500  includes the same number of rows as the encoder disk  220  with each row having a plurality of slots. The slots of the stationary aperture  500  approximately line up with the slots of the encoder disk  220 , and serve to collimate the radiation from the emitter. As shown by the dotted lines in FIG. 3, the stationary aperture  500  is essentially a small portion of encoder disk  220 . In a preferred embodiment, the slots in the stationary aperture  500  are smaller than the slots in the encoder disk  220 . In a working example the number of slots in each row of the stationary aperture  500  is five slots. 
     A block diagram of a signal processing module is depicted in FIG.  6 . As discussed above, as the open slots in the encoder disk  220  rotate over the radiation source  410 , radiation is detected by the sensors  420 . Hereinafter, a set consisting of a radiation source and sensor, with its corresponding row of slots, will be referred to as a “channel”. Thus, there are two channels, one for each row of slots in the encoder disk  220 . When radiation from the source is detected, the sensors  420  emit a pseudo-sinusoidal signal corresponding to the strength of the detected source radiation. The signal from the sensor  420  is transmitted to a signal conditioner  600 . Each channel has its own conditioner  600 . 
     The signal conditioner  600  is of the conventional type which receives a signal from a sensor  420 , and removes background noise and other anomalies using techniques well known in the art. The conditioned signal is then received by the decode logic module  602 , the operation of which will be discussed in detail below. 
     In general, the decode logic module  602  utilizes the conditioned analog signals from signal conditioners  600  associated with each channel to determine when to send a digital pulse to a counter  604  or  606 , and to determine the rotational direction of the encoder disk  220 . Rotational direction of the encoder disk  220  determines whether the pulse sent to a counter  604  or  606  increments or decrements the measuring count. Further, as discussed in detail below, the decode logic module  602  also performs error detection and jitter compensation. 
     Once a counter  604  or  606  has been incremented/decremented, the resulting counter value is sent to a switch module  608 . The switch module  608  determines the value from counter  604 , representing English units, or  608 , representing SI units, to be displayed to a user. 
     By depressing the mm/inch button  610 , the user determines what types of units will be displayed. The switch module  608  then transfers the value stored in counter  604  or  608  respectively, to the LCD Driver module  612 . The LCD Driver module  612  converts the received value into signals which will allow the LCD Display  614  to display the corresponding value to a user. Implementation of the counter  604  or  606 , switch module  608 , mm/inch button  610 , LCD Driver  612  and LCD Display  614  are well known in the art. 
     FIG. 7 is a block diagram of the decode logic module  602 . The operational details are discussed below, in the Functional Overview section. As mentioned earlier, the there are two channels from which signals are received. One channel corresponds with signals generated by the outer slot ring (hereinafter “outer slot channel”) while the other channel corresponds with the inner slot ring (hereinafter “inner slot channel”). The signal generated by the outer slot channel is received by an outer comparator  700 , while the signal generated by the inner slot channel is received by an inner comparator  702 . The comparator  700  or  702 , samples the received signal level and compares a sampled signal to signal threshold levels. The comparator function may be realized in dedicated logic circuitry or in a programmed microprocessor. 
     Referring to FIG. 8, when a signal  800  or  802  generated by a sensor  420 , exceeds a high threshold  804 , the signal is considered by the comparator  700  or  702 , respectively, to be high. When a signal exceeds a low threshold  806 , the signal is considered by the comparator  700  or  702 , respectively, to be low. However, the comparator indicates a change from high to low only when the signal crosses the low threshold  806 , and indicates a change from low to high only when a high threshold  804  is crossed. The comparator thus generates “high” and “low” pulses. This manner of changing states allows the system to compensate for signal jitter, as will be discussed in detail below. 
     When a signal  800  or  802  goes high or low, a pulse is sent to the counter  604  or  606 , respectively, of FIG.  6 . Additionally, when a signal  800  or  802  goes low, a pulse is sent to a counter  604  or  606 . 
     A pulse sent to a counter  604  or  606  can either increment or decrement the value of the counter  604  or  606 . Whether the count is incremented or decremented is determined by the direction of rotation of the encoder disk  220 . Referring again to FIG. 7, the direction detection module  706  receives signals directly from the signal conditioners  600 , one from the outer channel and one from the inner channel. The sequence of relatively high and low signals from each channel is used to determine rotational direction of the encoder disk  220 , utilizing the quadrature offset of the slots to offset signals generated by rotation of the disk in time. By way of example, if the slot pattern in FIG. 3 shown by an outer slot  300  and an inner slot  302  were utilized, the direction detection module receives a high signal from the inner encoder row  320  followed by a high signal from the outer encoder row  310  of FIG. 3, the encoder disk  220  is rotating in a clockwise direction. Determination of direction of rotation of the encoder disk  220  is discussed in detail below. Once the direction of rotation of the encoder disk  220  is determined, an up/down signal is sent to a counter  604  or  606 . This up/down signal will set the counter  604  or  606  to either increment or decrement when a counting pulse is received by a counter  604  or  606  from a comparator  700  or  702 . 
     As the openings of the encoder wheel  220  and aperture  500  come into registration, radiation passes to the sensor  420 . When the openings in the encoder wheel  220  and aperture  500  are not in registration, radiation is prevented from passing to the sensor  420 . As the encoder wheel  220  rotates, the positions of the radiation source  410  and sensor  420  pairs enter different states. These states are defined by positions constituting a complete exposure, or complete lack of exposure, of the radiation source  410  by the encoder disk  220 . As can be seen by one skilled in the art, there are a minimum of four distinct states, given that there are two channels, the slots of which are in quadrature with each other. The present invention utilizes these states to detect errors and compensate for signal jitter, and, preferably, to determine rotational direction of the encoder disk  220 . In order to monitor and store these states, the present invention utilizes a state machine  708 , shown in FIG.  7 . 
     The decode logic  602  of FIG. 6 contains the state machine  708  of FIG.  7 . The state machine  708  receives the high and low pulses from each comparator  700 , and  702 , and uses these pulses to set the state of the system. Preferably, this state is represented by a two-bit state value. When a comparator  700  or  702  receives a high signal as defined above, the state machine  708  receives a high or “1” pulse from that comparator. Conversely, when a comparator  700  or  702  has received a low signal as defined above, the state machine  708  receives a low or “0” pulse from that comparator. The state machine  708  concatenates the signals, or bits, received from each comparator  700  and  702  to construct the two-bit state value. In a preferred embodiment, the signal received from the outer comparator  700  occupies the first position in the two bit state value, while the signal received from the inner comparator  702  occupies the second position in the two bit state value. The state machine  708  is discussed directly below and in detail in the Functional Overview section. 
     As the encoder disk  220  rotates and passes by an emitter  410 , a sensor  420  receives radiation from the emitter  410  and generates a signal. The signal generated by the sensor  420  is received by the signal conditioner  600  and then by a comparator  700  or  702 . The signal received by each comparator will appear generally sinusoidal as shown in FIG.  8 . The relative peaks and valleys in the sinusoidal waveform correspond with the positions of the emitter  410  as detected by a sensor  420 , relative to a slot in the encoder disk  220 . 
     FIG. 9 shows the digital time domain waveforms generated by the outer ring  310  and inner ring  320  of slots of the encoder wheel  220  of FIG. 3, as they rotatably pass between a stationary emitter  410  (outer emitter  412 , an inner emitter  414 ) and a stationary sensor pair  420  (outer sensor  422 , inner sensor  424 ), seen in FIG.  4 . The dashed vertical lines of FIG. 9 delineate different segments of the signals generated. The segment labeled “A” corresponds to a state in which a solid portion of the outer slot ring  310  and inner slot ring  320  block the transmission of radiation from the outer emitter  412  and inner emitter  414  to the sensors  422  and  424 . The segment labeled “B” corresponds to a state in which a solid portion of the outer slot ring  310  blocks the transmission of radiation from the outer emitter  412  to the outer sensor  422 , while an aperture of the inner slot ring  320  allows transmission of radiation from the inner emitter  414  to the inner sensor  424 . The segment labeled “C” corresponds to a state in which an aperture of the outer slot ring  310  and inner slot ring  320  allow transmission of radiation. The segment labeled “D” corresponds to a state in which an aperture of the outer slot ring  310  allows transmission of radiation, while a solid portion of the inner slot ring  320  inhibits transmission of radiation. 
     Referring to FIGS. 9 and 7, when both emitters  412  and  414  are underneath a solid (non-slotted) portion of the encoder disk  220 , each comparator  700  and  702  receives a low signal from each sensor  422  and  424 , shown in FIG. 4, and each comparator  700  and  702  sends a low, or 0, signal to the state machine  708 . This corresponds with signal segment “A” of FIG.  9 . At this point the state machine  708  would contain a state represented by the two bit value  00 . 
     With the encoder disk  220  moving in a clockwise direction while the emitters  412  and  414  remain stationary, the next position corresponds to signal segment “B” of FIG. 9 when the outer emitter  412  is beneath a solid portion of the encoder disk  220  while the inner emitter  414  is beneath a slot. The outer comparator  700  receives a low signal from the outer sensor  422 , and sends a low, or 0, signal to the state machine  708 . The inner comparator  702  receives a high signal from the inner sensor  424 , and sends a high, or 1, signal to the state machine. At this point the state machine  708  would contain a state represented by the two bit value  01 . 
     As the encoder disk moves further in a clockwise direction, the next position corresponds to signal element “C” of FIG. 9 when the outer emitter  412  is beneath a slot of the encoder disk  220 , with the inner emitter  414  also beneath a slot. The outer comparator  700  receives a high signal from the outer sensor  422 , and sends a high, or 1, signal to the state machine  708 . The inner comparator  702  also receives a high signal from the inner sensor  424 , and sends a high, or 1, signal to the state machine  708 . At this point the state machine  708  would contain a state represented by the two-bit value  11 . 
     As the encoder disk moves yet further in a clockwise direction, the next position corresponds to signal element “D” of FIG. 9 when the outer emitter  412  is beneath a slot of the encoder disk  220 , with the inner emitter  414  beneath an un-slotted solid portion. The outer comparator  700  receives a high signal from the outer sensor  422 , and sends a high, or 1, signal to the state machine  708 . The inner comparator  702  receives a low signal from the inner sensor  424 , and sends a low, or 0, signal to the state machine  708 . At this point the state machine  708  would contain a state represented by the two-bit value  10 . 
     III. Functional Overview 
     Encoder Assembly 
     Referring to FIGS. 3-5, a rotational displacement of the encoder disk  220  is determined using the radiation source  410  and the radiation detector  420 . In particular, the radiation source  410  emits radiation toward the slots of the encoder disk  220  and this radiation is detected by the radiation detector  420  on the other side of the disk and opposite the radiation source  410 . As the encoder disk  220  rotates, the radiation sensed by the radiation detector  420  varies in a sinusoidal manner. For example, as a slot becomes aligned with the detector  420  the signal intensity is approximately at its maximum and as the solid portion between the slots becomes aligned with the detector  420  the signal intensity is approximately at its minimum. 
     In a preferred embodiment, the stationary aperture  500  is positioned between the encoder disk  220  and the radiation detector  420 . Preferably, the stationary aperture  500  has two rows with five slots per row, and has a slot size slightly smaller than that of the slots in the encoder disk  220 . The stationary aperture  500  serves to further restrict the flow of radiation from the source  410  to the sensor  420 . The causes a sensor  420  to receive a strong signal from the radiation source  410  only when the slot in the stationary aperture  500  and encoder disk  220  are very closely aligned. This benefits the system as a whole in that the sensor  420  will emit more definite high and low signals to the signal processor. An “open” condition is defined as when the slots of the encoder disk  220  are lined up with the slots of the stationary aperture  500  and radiation from the radiation source  410  passes through the aligned slots. Conversely, a “closed” condition is defined as when the slots of the encoder disk  220  are lined up with the solid portion between the slots of the stationary aperture  500 . Ideally, during this closed condition very little radiation can pass through the encoder disk/stationary aperture combination. 
     Signal Processing Module 
     The structural overview of the Signal Processing module has been discussed above with regards to FIG.  6 . The present discussion will primarily concern the Decode Logic module  602  within the Signal Processing Module. The physical structure of the Decode Logic module has been discussed above and is graphically set forth in FIG.  7 . This discussion will concern the functioning of the various components of the Decode Logic module  602  as they interact with the measuring system of the present invention. 
     As discussed above, the Decode Logic module  602  preferably contains two comparators, an outer comparator  700 , which compares a signal from the outer channel, and an inner comparator  702 , which compares a signal from the inner channel. 
     The following discussion of FIG. 10 will concern only a single comparator. It should be noted that this discussion will be equally applicable to both the outer comparator  700  and inner comparator  702  of the preferred embodiment. In Block  1000  a signal threshold T 1  is set to a preset level. (For purposes of this discussion, the preset level T 1  is a level above which the signal to the comparator is considered high as discussed above, corresponding to the a high threshold  804 , of FIG. 8. T 2  is the level below which the signal to the comparator is considered to be low as discussed above, corresponding to the low threshold  806 , of FIG. 8.) In Block  1002 , the comparator determines if the signal received from appropriate sensor is above T 1 . If the received signal is not above T 1  (NO branch of Block  1002 ), the comparator continues to monitor the signal. If the received signal is above T 1  (YES branch of Block  1002 ), the comparator outputs a binary  1 , or high signal (Block  1004 ). The signal threshold of interest is then changed from T 1  to T 2  in Block  1006 . In Block  1008 , the comparator determines if the signal level received from the sensors is below T 2 . If the signal is not below T 2  (NO branch of Block  1008 ), the comparator continues to monitor the signal. If the received signal is below T 2  (YES branch of Block  1008 ), the comparator outputs a binary  0 , or low signal (Block  1010 ). 
     In accordance with the above discussion, it should be noted that the output of a comparator remains high until a low threshold is crossed, and once the low threshold has been crossed, the comparator output remains low until a high threshold is crossed. 
     In a preferred embodiment, there is only a single signal threshold to be compared with the signal. This threshold changes from a high to a low threshold, or low to high, once a particular threshold has been crossed. That is, once a high threshold has been crossed, the threshold to be compared with the signal changes to a low threshold. Once a low threshold has been crossed, the threshold to be compared with the signal changes to a high threshold. 
     The signal states (1 and 0, high and low) output by a comparator as mentioned above are utilized (as will be described) in incrementing/decrementing a counter  604  or  606 , updating the emitter state in the state machine  708 , and error detection. 
     With regard to incrementing/decrementing the counter  604  or  606 , when the output of a comparator is either high or low, the comparator outputs a single pulse to a counter. The pulse will either increment or decrement the counter, depending upon the current direction of rotation of the encoder disk  220 . Determination of direction of rotation is discussed below. 
     Direction is determined by observing the signals from the signal conditioner of FIG.  7 . The preferred method of direction determination depends upon the offset of the slot rows in the encoder wheel. Using the slot offset pattern of FIG. 3 shown by an outer slot  300  and an inner slot  302  as an example, if the wheel is rotating in a clockwise direction, the Direction Determination module  706  of FIG. 7 will detect a high signal from the channel associated with the inner slot ring  320 , followed by a high signal from the channel associated with the outer slot ring  310 . If the encoder disk  220  is rotating in a counter clockwise direction, a high signal will be detected on the channel associated with the outer slot ring, followed by a high signal on the channel associated with the inner slot ring. (An alternative method of determining rotational direction is to deduce the direction from the sequence of states within the state machine  708 .) Once direction of rotation of the encoder disk  220  is determined, a signal is sent by the Direction Detection Module  706  to the counter  604  or  606  to use incoming pulses from the comparator  700  or  702  to increment or decrement the measuring count. 
     Referring to FIG. 11, the state machine  708  (FIG. 7) determines if the outputs of the comparators  700  and/or  702  have changed (Block  1100  of FIG.  11 ). If the outputs of the comparators  700  and/or  702  have not changed (NO branch of Block  1100 ) the state machine  708  will continue to monitor the outputs of the comparators  700  and  702 . If the output of one of the comparators has changed (YES branch of Block  1100 ) the state machine  708  will store the current state as the previous state (Block  1102 ). Next, the state machine will get the state bit from the outer comparator  700  (Block  1104 ). The state machine will then get the state bit from the inner comparator  702  (Block  1106 ). The state machine then concatenates the state bits generated by the outer  700  and inner  702  comparators to form a two-bit state and set this value to be the current state (Block  1108 ). The state machine  708  then outputs the previous state and current state (Block  1110 ). 
     The output from the state machine  708  is used by the error detection module  710  of FIG. 7 to determine if an error in counting has occurred. As discussed previously, an error can occur if the jaws of the measuring device are opened too quickly. Referring to FIG. 12, the error detection module  710  gets the value of the previous state from the state machine  708  (Block  1200 ). Next, the error detection module  710  retrieves the current state from the state machine  708  (Block  1202 ). The error detection module next retrieves the direction of rotation information from the Direction Detection module  706  (Block  1204 ). Based upon the direction of travel of the encoder disk  220 , the error correction module  710  logic accesses a table which compares the previous state to the current state, and determines if the current state is expected after the previous state (block  1206 ). When determining if an error has occurred while traveling in the clockwise direction, the table in FIG. 13 is used, while the table in FIG. 14 is used when the encoder disk  220  is traveling in a counter-clockwise direction. The states depicted in each of these tables corresponds with the depiction of signal segments set forth in FIG.  9 . Based upon the table entries, the previous state and the current state are compared in Block  1208  of FIG.  12 . In Block  1210 , based upon the table entries, it is determined whether or not the current state is expected after the previous state. If the current state is expected after the previous state (YES branch of Block  1210 ), the error detection module continues to analyze the next state. If the current state is not expected after the previous state (NO branch of Block  1210 ), an error has occurred. In a preferred embodiment, upon occurrence of an error, a message is displayed to the user, requiring the user to close the jaws of the calipers and re-zero the system. 
     If the alternative method of direction determination is used, i.e. by deducing direction from the sequence of states, then the tables of FIGS. 13 and 14 determine the state sequence for the clockwise or counterclockwise directions, respectively. 
     The addition of the error detection system as set forth above removes the need for a mechanical motion dampening system. If the user opens, or closes, the jaws too quickly, causing an error to occur, an error message is displayed to the user requiring him to close the jaws of the device and re-zero the system. As a motion dampener is not required, a measuring device utilizing the present invention will cost substantially less and have less mechanical complexity than a unit requiring a motion dampener. 
     The present invention further obviates the need for motion dampening systems and other hardware by compensating for signal jitter though the inherent operation of the comparators  700  and  702  in the preferred embodiment. As discussed above, in order for a signal to go from high to low, the signal must first cross the low threshold. By way of example, if a signal were to jitter about the high threshold, the comparator would continue to construe the signal as high until the low threshold was actually crossed. Preferably, the high and low thresholds are far enough apart such that both cannot be crossed when signal jitter occurs Returning to the example of a signal jittering about the high threshold, the only way for a signal to go from a high state to a low state would be for a valid signal to be generated due to rotation of the encoder disk. Signal jitter alone cannot trigger a change in states or thresholds. By setting the high and low thresholds significantly far apart (e.g. 25% or more of the peak signal excursion), and by not changing signal states until the next threshold is crossed, the effects of jitter upon the system are minimized or eliminated. 
     The decode logic  602 , or its individual components, may be implemented as dedicated logic circuitry or a programmed microprocessor, for example. 
     The foregoing description of the preferred embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description of the invention, but rather by the claims appended hereto.