Abstract:
An angular position indicator for cranes which uses a combination of electronic, optical and mechanical components. It is intended for use on fixed or mobile Cranes and designed to operate in harsh industrial environments. Wireless communication replaces fixed electrical hardwiring that would otherwise be required between system components. The angular position indicator uses an angular displacement transducer of unique design. The angular position indicator performs pattern recognition of optical apertures using analog measurements of the illumination of optical detectors. The design allows for a substantial increase in measurement resolution as compared to digital optical encoders with the same number of optical channels.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an angular position indicator and, in particular, an angular position indicator suitable for use on a crane boom.  
         BACKGROUND OF THE INVENTION  
         [0002]    In order to calculate the lifting capacity of a crane several parameters must be measured, one of which is the angular position of the crane&#39;s boom.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is a angular position indicator for a crane.  
           [0004]    According to the present invention there is provided an angular position indicator for a crane boom which includes a base adapted for mounting to a crane boom that is to have its angular position measured. A pendulum is pivotally mounted to the base and hanging freely in a vertical orientation by force of gravity. An array of sensors for determining the angular positioning of the pendulum. A first set of processing electronics including a transmitter. A second set of processing electronics including a human readable display and a receiver. One of the first set of processing electronics and the second set of processing electronics calculates angular positioning of the crane boom from data received from the array of sensors. The second set of processing electronics is remote from the first set of processing electronics. The first set of processing electronics receives data from the array of sensors and transmits a signal which is received by the second set of processing electronics. The second set of processing electronics displays angular positioning of the crane boom on the human readable display.  
           [0005]    The angular position indicator, as described above, is a wireless angular position indicator. This system provides numerous advantages over hardwired systems. Hardwired electrical cabling is difficult to install and is subject to physical damage and weathering which requires maintenance. Hardwired systems are difficult and, sometimes impossible, to install on cranes that have operator controls that do not move with the boom turret.  
           [0006]    Although beneficial results may be obtained through the use of the angular position indicator, as described above, provision must be made to permit a number of cranes to operate in the same vicinity all of which are using a wireless system. Even more beneficial results may, therefore, be obtained when the signal passing from the first set of processing electronics to the second set of processing electronics has a unique identification code that preserves data integrity.  
           [0007]    Although beneficial results may be obtained through the use of the angular position indicator, as described above, generally the higher the resolution obtained the more costly the angular position indicator. Even more beneficial results may be obtained when the array of sensors includes optical emitters and optical detectors fixed to the base and an optical encoder mounted to the pendulum. The optical emitters and optical detectors are angularly displaced in relation to the optical encoder mounted on the pendulum should any movement of the structure occur. The optical encoder has a series of optical apertures that generate unique identifiable light patterns detectable by the optical detectors. The unique identifiable light patterns including some optical apertures fully illuminated by the optical emitters, some optical apertures not illuminated by the optical emitters and at least one optical aperture partially illuminated by the optical emitters. The processing electronics assigns a fractional value to the degree of illumination of the optical aperture that is partially illuminated to enhance the resolution of the angular measurement. This approach enables higher resolution to be obtained using lower cost equipment with fewer optical channels.  
           [0008]    Although beneficial results may be obtained through the use of the angular position indicator, as described above, oscillatory motion caused by vibration, shock and angular acceleration can adversely affect the accuracy and repeatability of the angular position measurement. Even more beneficial results may, therefore, be obtained when the mean angular displacement is displayed in order to compensate for oscillatory motion caused by vibration, shock and angular acceleration. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:  
         [0010]    [0010]FIG. 1 is a block diagram of an angular position indicator constructed in accordance with the teachings of the present invention.  
         [0011]    [0011]FIG. 2 is a block diagram of a display for the angular position indicator illustrated in FIG. 1.  
         [0012]    [0012]FIG. 3 is a detailed side elevation view of the optical encoder disk and pendulum illustrated in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    The preferred embodiment, an angular position indicator for a crane boom generally identified by reference numeral  10 , will now be described with reference to FIGS. 1 through 3.  
         [0014]    Structure and Relationship of Parts:  
         [0015]    Referring to FIG. 1, there is provided an angular position indicator  10 , that includes a base  12  mountable to a structure  14  that is to have its angular position measured. For example, angular position indicator  10  is often used to measure the angular position of a boom of a crane boom  14 . A pendulum  16  is pivotally mounted to base  12  and hangs freely in a vertical orientation by force of gravity. Pendulum  16  is pivotally mounted by means of a shaft  18  journaled by bearings  20 , thereby reducing dampening of the movement of pendulum  16  due to friction.  
         [0016]    An array of sensors  22  is provided for determining the angular positioning of pendulum  16 . Array of sensors  22  includes optical emitters  24  and optical detectors  26  fixed to base  12 . An optical encoder  28  is mounted to pendulum  16 , such that optical emitters  24  and optical detectors  26  are angularly displaced in relation to optical encoder  28  mounted on pendulum  16  should any movement of crane boom  14  occur. Angular position indicator  10  is supplied with power by a battery  30 .  
         [0017]    Referring to FIG. 3, optical encoder  28  has a series of optical apertures  32  that, when exposed to optical emitters  24 , generate unique identifiable light patterns detectable by optical detectors  26 . There are unique identifiable light patterns for every angular position, and specific illumination of the unique identifiable light patterns causes optical detectors  26  to generate a specific electrical current which is a function of angular displacement. Optical encoder  28  also includes a calibration aperture  34  that allows for full simultaneous illumination of optical detectors  26 .  
         [0018]    Referring to FIG. 1, a first microprocessor  36  is provided that receives data from array of sensors  22  and calculates an angular position. Optical detectors  26  and optical emitters  24  have several optical channels  38 .  
         [0019]    An analog signal amplifier  40  and an analog to digital converter  42  are provided. The electric current is passed from optical channels  38  through optical detector channel selectors  44  to analog signal amplifier  40  and through analog to digital converter  42  to convert the electric current to data in the form of binary code. First microprocessor  36  receives data from only one of optical channels  38  at a time. First microprocessor  36  includes an identification encoder  46  which supplies an identification code to be associated with data.  
         [0020]    In the illustrated embodiment, an analog multiplexer  48  is interposed between analog signal amplifier  40  and analog to digital converter  42 . Analog multiplexer  48  serves to route information concerning the condition of battery  30  to first microprocessor  36 . First microprocessor  36  calculates mean angular displacement in order to compensate for oscillatory motion caused by vibration, shock and angular acceleration. First microprocessor  36  has a radio transmitter  50  with an antenna  52  for transmitting data along with the associated identification code, and information concerning the condition of battery  30 .  
         [0021]    Referring to FIG. 2, there is provided a display unit generally referenced by numeral  54 , that is separate from and operates at a distance from angular position indicator  10 . Display unit  54  includes a second microprocessor  56  and a LCD display  58 . Second microprocessor  56  has a radio receiver  60  with an antenna  62  for receiving data along with associated identification code, and information concerning condition of battery  30  from first microprocessor  36 . Second microprocessor  56  has an identification encoder  64  which references the identification code associated with data received from first microprocessor  36 . If the identification code is valid, LCD display  58  on display unit  54  displays the angular position in a readable format for viewing by an operator. It will be appreciated that displays other than LCD can be used, so long as display is readable by operator. A display driver  66  controls LCD display  58 .  
         [0022]    An auditory alarm  68  and a control voltage alarm  70  are connected to display unit  54 . Auditory alarm  68  and control voltage alarm  70  are activated when angular position measured is outside of operator selected limits or when condition of battery  30  deteriorates. Operator selected limits are entered via input keys  72  on display unit  54  and are stored in non-volatile memory  74  such that second microprocessor  56  retains operator selected limits in the event supply of power to display unit  54  is interrupted. Power to display unit  54  is supplied externally through power connector  76  and regulated through power supply regulator  78 .  
         [0023]    Operation:  
         [0024]    Operation: Mechanical.  
         [0025]    The angle transducer consists of the following component groups:  
         [0026]    shaft  18 ;  
         [0027]    bearings  20   
         [0028]    pendulum  16   
         [0029]    optical encoder disk  28   
         [0030]    digital to analog converter  41   
         [0031]    optical emitter channel selector  43   
         [0032]    8 element optical emitter  24   
         [0033]    8 element optical detector  26   
         [0034]    optical detector channel selector  44   
         [0035]    analog signal amplifier  40   
         [0036]    analog channel multiplexer  48   
         [0037]    analog to digital converter  42   
         [0038]    microprocessor  36  with ID encoder and battery pack  
         [0039]    radio transmitter  50   
         [0040]    i) The base  12  of ANGULAR POSITION INDICATOR  10  is mounted to a crane boom  14  that is to have its angular position measured. (eg. Crane Boom)  
         [0041]    ii) The SHAFT  18  and BEARINGS  20  are mounted to the base  12 .  
         [0042]    iii) The OPTICAL ENCODER  28  is mounted to the PENDULUM  16 .  
         [0043]    iv) The PENDULUM  16  is coupled to the SHAFT  18  by the BEARINGS  20 .  
         [0044]    v) The OPTICAL ENCODER  28  and PENDULUM  16  are free to rotate about the SHAFT  18 .  
         [0045]    vi) Gravity causes the PENDULUM  16  to hang perpendicular to the ground. The orientation of the OPTICAL ENCODER  28  and PENDULUM  16  assembly is thus fixed with respect to the ground.  
         [0046]    Operation: Sensing of Angular Displacement.  
         [0047]    i) The OPTICAL EMITTERS  24  and OPTICAL DETECTORS  26  are mounted to the base  12 . The orientations and positions of the OPTICAL EMITTERS  24 , OPTICAL DETECTORS  26 , and the base  12  are fixed with respect to each other and do not change.  
         [0048]    ii) Angular movement of the crane boom  14  introduces an angular displacement of the OPTICAL EMITTERS  24  and OPTICAL DETECTORS  26  with respect to the fixed orientation of the OPTICAL ENCODER  28  and PENDULUM  16  assembly.  
         [0049]    iii) The OPTICAL ENCODER  28  has a series of optical apertures  32  that form specific patterns at different angular positions on the OPTICAL ENCODER  28 . For any specific angular position on the OPTICAL ENCODER  28  there is a corresponding specific pattern of optical apertures  32 .  
         [0050]    iv) The OPTICAL EMITTERS  24  illuminate the OPTICAL ENCODER  28  at an angular position that depends on the angular displacement between the OPTICAL EMITTERS  24  and the OPTICAL ENCODER  28 .  
         [0051]    v) The specific pattern of optical apertures  32  at any specific angular position on the OPTICAL ENCODER  28  causes a specific illumination of the OPTICAL DETECTORS  26 . This specific illumination causes the OPTICAL DETECTORS  26  to generate specific electric currents.  
         [0052]    vi) The value of these specific electric currents is a function of the angular displacement between the optical emitters  24  and optical detectors  26  and the OPTICAL ENCODER  28 . Thus, an ABSOLUTE ANGULAR POSITION to ELECTRICAL CURRENT conversion has been performed.  
         [0053]    vii) The electric currents from the OPTICAL DETECTORS  26  are passed through the OPTICAL DETECTOR CHANNEL SELECTOR  44  to the ANALOG SIGNAL AMPLIFIER  40 .  
         [0054]    viii) The ANALOG SIGNAL AMPLIFIER  40  transforms the electric currents into electric voltages and amplifies these voltages to levels suitable for input to the ANALOG TO DIGITAL CONVERTER  42 .  
         [0055]    ix) The ANALOG TO DIGITAL CONVERTER  42  transforms the amplified electric voltages into the binary equivalents of their numeric values. Thus, the ABSOLUTE ANGULAR POSITION is represented by a group of specific binary numbers.  
         [0056]    x) There are eight OPTICAL CHANNELS  38 . Seven of these OPTICAL CHANNELS  38  are used to sense the pattern of optical apertures  32  on the OPTICAL ENCODER  28 . Thus, there are seven binary numbers that represent the illuminance of the OPTICAL DETECTORS  26 . One number for each optical channel  38 . Each of the seven numbers has a value that ranges from a minimum of zero to a maximum of 255. The value of the number is proportional to the illuminance of the corresponding OPTICAL DETECTOR  26 .  
         [0057]    xi) The eighth OPTICAL CHANNEL  38  is used for calibration.  
         [0058]    xii) The binary numbers from the seven OPTICAL CHANNELS  38  are presented to the FIRST MICROPROCESSOR  36 . The FIRST MICROPROCESSOR  36  executes a software algorithm that uses the binary numbers to determine the ABSOLUTE ANGULAR POSITION of THE ANGULAR POSITION INDICATOR  10 .  
         [0059]    xiii) The output of the software algorithm is a single number that is equal to the angular displacement between the base  12  and the OPTICAL ENCODER  28 . This number is temporarily stored in processor memory.  
         [0060]    Operation: Data Transmission.  
         [0061]    i) The FIRST MICROPROCESSOR  36  reads an IDENTIFICATION NUMBER from the ID ENCODER  46  and stores this number in processor memory.  
         [0062]    ii) The FIRST MICROPROCESSOR  36  determines the condition of the battery  30  by instructing the ANALOG CHANNEL MULTIPLEXER  48  to route the battery output voltage to the ANALOG To DIGITAL CONVERTER  42 . The ANALOG TO DIGITAL CONVERTER  42  digitizes the battery voltage and presents the data to the FIRST MICROPROCESSOR  36 . This data is used to determine the condition of the battery  30 .  
         [0063]    iii) The FIRST MICROPROCESSOR  36  forms a data packet that consists of the following information:  
         [0064]    a) ID Code.  
         [0065]    b) Battery Condition  
         [0066]    c) Angular Position.  
         [0067]    iv) The FIRST MICROPROCESSOR  36  sends the data packet to the RADIO TRANSMITTER  50 .  
         [0068]    v) The RADIO TRANSMITTER  50  sends the data to the DISPLAY UNIT  54 .  
         [0069]    Operation: Detailed Example of Angle Measurement.  
         [0070]    The pattern of optical apertures  32  at a specific location on the OPTICAL ENCODER  28  is sensed by recording the illumination of the OPTICAL DETECTORS  26  at that location. The illuminance depends on the amount of light that passes through an optical aperture  32  to an OPTICAL DETECTOR  26 . If the optical aperture  32  is completely closed then the optical channel  38  is blocked and the illuminance is zero. If the optical aperture  32  is completely open then the optical channel  38  is clear and the illuminance is maximized.  
         [0071]    The present design uses seven optical channels  38  to sense the pattern of optical apertures  32 . The illuminance through each optical channel  38  is resolved to 1 part in 256, (8 bit resolution, 0.4%).  
         [0072]    The FIRST MICROPROCESSOR  36  records the illuminance by gathering data from the OPTICAL DETECTORS  26 . A software algorithm determines the ABSOLUTE ANGULAR POSITION of the ANGULAR POSITION INDICATOR  10  using the illuminance data.  
         [0073]    In order to prevent undesired cross modulation between optical channels  38 , the FIRST MICROPROCESSOR  36  enables and records data from only one optical channel  38  at a time.  
         [0074]    Operation: Detailed Example of Angle Measurement.  
         [0075]    The illuminance data is collected as follows:  
         [0076]    i) The FIRST MICROPROCESSOR  36  instructs the ANALOG CHANNEL MULTIPLEXER  48  to pass output from the ANALOG SIGNAL AMPLIFIER  40  to the ANALOG TO DIGITAL CONVERTER  42 .  
         [0077]    ii) The FIRST MICROPROCESSOR  36  instructs the emitter channel selector  43  to enable a current path through the first OPTICAL EMITTER  24 .  
         [0078]    iii) The FIRST MICROPROCESSOR  36  instructs the DIGITAL TO ANALOG CONVERTER  41  to pass current through the first OPTICAL EMITTER  24 .  
         [0079]    iv) The first OPTICAL EMITTER  24  converts the current passed through it to light. The light illuminates the optical aperture  32  immediately in front of the OPTICAL EMITTER  24 .  
         [0080]    v) The FIRST MICROPROCESSOR  36  instructs the DETECTOR CHANNEL SELECTOR  44  to enable a current path from the first OPTICAL DETECTOR  26  to the ANALOG SIGNAL AMPLIFIER  40 .  
         [0081]    vi) The ANALOG SIGNAL AMPLIFIER  40  converts the current from the first OPTICAL DETECTOR  26  to a voltage and amplifies the voltage to a level suitable for input to the ANALOG TO DIGITAL CONVERTER  42 . This voltage is routed to the ANALOG TO DIGITAL CONVERTER  42  through the ANALOG CHANNEL MULTIPLEXER  48 .  
         [0082]    vii) The ANALOG TO DIGITAL CONVERTER  42  digitizes the voltage at its input. The output of the ANALOG TO DIGITAL CONVERTER  42  is a binary number that ranges from a value of 0 to 255 depending on the magnitude of the voltage at its input. The magnitude of the voltage depends on the illumination of the OPTICAL DETECTOR  26 , thus the binary output of the ANALOG TO DIGITAL CONVERTER  42  is a number that corresponds to the amount of light that reached the OPTICAL DETECTOR  26  through the first channel of the optical aperture  32 . The position of the first channel of the optical aperture  32  with respect to the OPTICAL DETECTOR  26  is represented by the value of the number.  
         [0083]    viii) The FIRST MICROPROCESSOR  36  records the binary output of the ANALOG TO DIGITAL CONVERTER  42  in processor memory.  
         [0084]    ix) The FIRST MICROPROCESSOR  36  repeats steps to for each of the six remaining optical channels  38 . The illumination data recorded by the FIRST MICROPROCESSOR  36  contains information regarding the specific pattern and location of the optical apertures  32  on the OPTICAL ENCODER  28 .  
         [0085]    x) The FIRST MICROPROCESSOR  36  executes a software algorithm that uses the illumination data to determine the angular position of the ANGULAR POSITION INDICATOR  10 . The algorithm proceeds as follows:  
         [0086]    a) Each of the seven illumination numbers is compared with a threshold number. A number equal to, or greater than, the threshold corresponds to an optical path where the position of the optical aperture  32  is such that more than 66% of the light emitted by the OPTICAL EMITTER  24  has illuminated the OPTICAL DETECTOR  26 . A number less than the threshold corresponds to an optical path where the position of the optical aperture  32  is such that less than 66% of the light has illuminated the OPTICAL DETECTOR  26 .  
         [0087]    b) The results of the seven comparisons are used to form a 7 bit binary number. The value of this number is equal to the integer value of the angular displacement. The range is from 0 degrees to 127 degrees with a resolution of 1 degree. The pattern of optical apertures  32  on the OPTICAL ENCODER  28  is duplicated every 128 degrees so that a total of 256 degrees can be decoded.  
         [0088]    c) Each of the seven illumination numbers is compared with two more threshold numbers. Illumination numbers that are between the threshold numbers correspond to optical paths where the position of the optical aperture  32  is such that 33% to 66% of the light has illuminated the OPTICAL DETECTOR  26 . The FIRST MICROPROCESSOR  36  makes a record of the optical paths that have illumination numbers between the two threshold numbers.  
         [0089]    d) For each integer value of the angular displacement obtained in (b) there is a corresponding set of numbers obtained in (c). The algorithm uses the information obtained in (c) to determine if the angular displacement obtained in (b) lies between two integer values. Thus, the algorithm is able to resolve the angular displacement to a resolution of ½ degree.  
         [0090]    xi) The FIRST MICROPROCESSOR  36  stores the angular displacement in memory.  
         [0091]    Operation: Damping of the PENDULUM  16 .  
         [0092]    i) The OPTICAL ENCODER  28  and PENDULUM  16  are free to rotate about the SHAFT  18 . Vibration, shock, angular acceleration, or other such mechanical movements can cause the OPTICAL ENCODER  28  and PENDULUM  16  to swing in an oscillatory manner. Such oscillatory motion will cause errors to be introduced into the angular position measurement since the position of the PENDULUM  16  is assumed to be parallel to the local gravitational field and perpendicular to the ground. Oscillatory motion of the PENDULUM  16  is recorded by the FIRST MICROPROCESSOR  36  since the rate at which the software algorithm determines the angular displacement is much quicker than the natural period of oscillation.  
         [0093]    ii) The FIRST MICROPROCESSOR  36  retains a record of angular displacement measurements and executes a software algorithm that calculates the mean angular displacement. The resolution of the calculation is ½ degree.  
         [0094]    Operation: System Resolution.  
         [0095]    i) The present design uses a software algorithm that resolves the angular displacement to ½ degree. The resolution can be increased by increasing the number of window comparisons made in and making the appropriate calculation.  
         [0096]    ii) The ultimate system resolution is determined by the resolution of the ANALOG TO DIGITAL CONVERTER  42  and the size of the optical apertures  32  on the OPTICAL ENCODER  28 . The present design has an ultimate system resolution of {fraction (1/256)} of a degree. (14 arc seconds)  
         [0097]    iii) Note that a conventional optical encoder using seven optical channels has a resolution of only 1 part in 128. This resolution corresponds to 2.8 degrees. (approximately 10,000 arc seconds)  
         [0098]    Operation: Automated Correction for Variations in Optical Coupling.  
         [0099]    i) The OPTICAL ENCODER  28  has an optical channel  38  that is dedicated to monitoring the degree of coupling from the OPTICAL EMITTERS  24  to the OPTICAL DETECTORS  26 . Illumination numbers from this optical channel  38  are used to calibrate the other optical channels  38  so that the response of all optical channels  38  is the same The calibration is done by controlling the current that the DIGITAL TO ANALOG CONVERTER  41  passes through the OPTICAL EMITTERS  24 .  
         [0100]    ii) Variations in optical coupling can occur due to several factors. Output power of optical emitters tends to vary with time, temperature, and individual component tolerances. Induced photo current in optical receptors varies with temperature and individual component tolerances. Optical and mechanical alignment vary during production, as does the mechanical tolerances of encoder discs. A software algorithm is executed periodically to determine if the optical coupling has changed. The algorithm adjusts the OPTICAL EMITTER  24  current as necessary in order to maintain equal response from all optical channels  38 .  
         [0101]    Operation: Individual Optical Channel Signatures:  
         [0102]    i) The OPTICAL ENCODER  28  has a calibration aperture  34  that allows full illumination of all OPTICAL DETECTORS  26  simultaneously. Illumination numbers are taken from the calibration aperture  34  during production. These numbers represent the individual responses of each optical channel  38 . The numbers correspond to the efficiency of the optical emitters  24  and optical detectors  26  and are used for calibration.  
         [0103]    7) Display Unit  54 . Detailed Description and Operation.  
         [0104]    A) The Display Unit  54  consists of the following component groups:  
         [0105]    i) Radio Receiver  60 .  
         [0106]    ii) Second Microprocessor  56 .  
         [0107]    iii) Display Driver  66 .  
         [0108]    iv) LCD Display  58 .  
         [0109]    v) Backlight for LCD  59 .  
         [0110]    vi) ID Encoder  64 .  
         [0111]    vii) Non-Volatile-Memory  74 .  
         [0112]    viii) Audible Alarm  68 .  
         [0113]    ix) Control Voltage Alarm  70   
         [0114]    X) Input keys  72   
         [0115]    xi) Power Supply Regulators  78 .  
         [0116]    Operation: Data Flow.  
         [0117]    i) The RADIO RECEIVER  60  receives data from the ANGULAR POSITION INDICATOR  10 . This data is presented to the SECOND MICROPROCESSOR  56  where it is temporarily stored in internal processor memory.  
         [0118]    ii) The SECOND MICROPROCESSOR  56  searches the data for a specific IDENTIFICATION CODE. If the ID CODE in the data matches the ID code that is set by the ID ENCODER  64  then the SECOND MICROPROCESSOR  56  accepts the data as valid. If the ID CODE does not match then the data is rejected. This scheme enables the SECOND MICROPROCESSOR  56  to discriminate between valid data and noise, interference, or either such irrelevant data that may came from the RADIO RECEIVER  60 .  
         [0119]    iii) The SECOND MICROPROCESSOR  56  sends valid data to the DISPLAY DRIVER  66 .  
         [0120]    iv) The DISPLAY DRIVER  66  controls the LCD DISPLAY  58 . Data from the DISPLAY DRIVER  66  is shown on the LCD DISPLAY  58 . This data is the angle that was measured and transmitted by the ANGULAR POSITION INDICATOR  10 .  
         [0121]    Operation: Left/Right Configuration.  
         [0122]    i) The user can select Left or Right mounting configuration of the ANGULAR POSITION INDICATOR  10  using a specific sequence of the INPUT KEYS  72 . The selection is stored in the NON-VOLATILE MEMORY  74  so that the system remembers its configuration when the power is removed.  
         [0123]    ii) Date received from the ANGULAR POSITION INDICATOR  10  is modified according to the mounting selection. The purpose of the modification is to give the correct sense for which direction of rotation represents positive angular displacement.  
         [0124]    Operation: Zero Adjustment.  
         [0125]    i) The user can select an offset that is to be added or subtracted from the angle measurement received from the ANGULAR POSITION INDICATOR  10 . The offset is entered using the INPUT KEYS  72 .  
         [0126]    ii) This offset is stored in the NON-VOLATILE MEMORY  74  so that the system remembers its configuration when the power is removed.  
         [0127]    Operation: Alarm Indication.  
         [0128]    i) The user can select MAXIMUM and MINIMUM limits for comparison with the angle measurement received from the ANGULAR POSITION INDICATOR  10 . The limits are entered using the INPUT KEYS  72 .  
         [0129]    ii) The limits are stored in the NON-VOLATILE MEMORY  74  so that the system remembers its configuration when the power is removed.  
         [0130]    iii) If the measured angle is beyond either of these limits then the AUDIBLE ALARM  68  and the CONTROL VOLTAGE ALARM  70  are both activated. The alarm condition results in removal of the control voltage.  
         [0131]    Operation: Low Battery Indication.  
         [0132]    i) Part of the data sent by the ANGULAR POSITION INDICATOR  10  contains information regarding the condition of its BATTERY  30 . The SECOND MICROPROCESSOR  56  examines this data.  
         [0133]    ii) If the SECOND MICROPROCESSOR  56  determines that the BATTERY  30  in the ANGULAR POSITION INDICATOR  10  is near the end of its service life then an error code is shown on the LCD DISPLAY  58  and the AUDIBLE ALARM  68  is momentarily activated thus indicating to the user that the battery  30  for the ANGULAR POSITION INDICATOR  10  requires replacement.  
         [0134]    Operation: Loss of Radio Communication.  
         [0135]    i) If the DISPLAY UNIT  54  does not receive any data from the ANGULAR POSITION INDICATOR  10  for any period of 30 seconds or more then the SECOND MICROPROCESSOR  56  determines that there has been a loss of radio communication with the ANGULAR POSITION INDICATOR  10 . The loss of radio communication may be the result of one or more of the following situations:  
         [0136]    a) Radio channel corrupted by noise or interference.  
         [0137]    b) Component failure in the ANGULAR POSITION INDICATOR  10 .  
         [0138]    c) Component failure in the DISPLAY UNIT  54 .  
         [0139]    ii) When a loss of communication condition has been detected by the SECOND MICROPROCESSOR  56  an error code is shown on the LCD DISPLAY  58 , the AUDIBLE ALARM  68  is momentarily activated, and the control voltage is removed thus indicating to the user that communication with the ANGULAR POSITION INDICATOR has failed.  
         [0140]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.  
         [0141]    It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.