Abstract:
An industrial vehicle including an axle to which wheels are mounted and supported to pivot vertically relative to a body of the vehicle. The vehicle includes a pivot control apparatus for controlling pivoting of the axle. The industrial vehicle further includes a controller for determining whether an abnormality has occurred in the pivot control apparatus. A warning lamp having two or more different modes is located in the cabin. The modes include an abnormality mode for indicating the occurrence of an abnormality in the pivot control apparatus and a normality mode for indicating normal functioning of the pivot control apparatus. The controller also notifies the operator of two or more additional different pieces of information using the modes of the warning lamp. That is, the single warning lamp also indicates the reason for restricting the axle by its color.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to industrial vehicles such as forklifts, and more particularly, to an apparatus for restricting pivoting of a pivotal axle. 
     Known industrial vehicles, such as forklifts, employ pivotal axles to improve vehicle stability. For example, an axle, to which rear wheels are mounted, is supported pivotally with respect to the body of a forklift. However, if the axle pivots when the forklift turns and tilts the body, this may destabilize the vehicle. 
     Accordingly, Japanese Unexamined Patent Publication No. 58-211903 proposes a forklift employing an axle pivot control apparatus that restricts pivoting of its axle in accordance with the centrifugal force produced when the forklift turns. The forklift includes a centrifugal force sensor and an apparatus for locking the axle. The sensor detects the centrifugal force applied to the forklift. If the centrifugal force detected by the sensor exceeds a predetermined value, the control apparatus locks the axle to restrict futher pivoting. The axle is thus locked when the forklift turns. This suppresses tilting of the forklift in its lateral direction when the forklift turns and thus maintains vehicle stability. 
     Japanese Unexamined Patent Publication No. 58-167215 teaches a forklift employing an axle pivot control apparatus that locks the axle in accordance with the weight of the load carried on the forks and the position of the forks. 
     The axle is locked when the vehicle&#39;s center of gravity is raised causing instability of the vehicle. That is, the axle is locked if the forks are lifted to a high position when carrying a heavy load. This keeps the forklift stable when handling loads. 
     When employing an axle pivot control apparatus such as those described above, the forklift operator should be notified of abnormalites in and actuation of the apparatus. For example, if the operator completely relies on the control apparatus, the operator may continue operation of the forklift even under undesirable circumstances. Alternatively, the operator may become overcautious when unaware of whether the control apparatus has been actuated. In such case, the functions of the axle pivot control apparatus are less effective. 
     Additionally, in a forklift employing an axle pivot control apparatus that is actuated by more than one factor, the forklift operator should be notified of the factor actuating the control apparatus. For example, if the operator knows whether the axle is locked due to a raised center of gravity or a large centrifugal force, the operator can respond appropriately. This would allow the functions of the control apparatus to be fully effective. 
     Thus, there are various kinds of information related to the axle pivot control apparatus of which the operator should be notified such as the occurrence of an abnormality, the factor actuating the control apparatus, and the current status of the control apparatus. However, many meters and indicators must be arranged on the instrument panel in addition to the indicators related to the axle pivot control apparatus. This restricts available space and makes it difficult to display all information related to the axle pivot control apparatus. To provide sufficient space, the size of the meters must be minimized. However, this would lower the visibility of the meters and indicators on the instrument panel. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide an industrial vehicle axle pivot control apparatus that notifies the operator of more than one piece of information related to pivoting of the axle. 
     To achieve the above objective, the present invention provides an industrial vehicle, including an axle to which wheels are mounted and supported to pivot vertically relative to a body of the vehicle, and a pivot control apparatus for controlling pivoting of the axle. The industrial vehicle further includes a determiner for determining whether an abnormality has occurred in the pivot control apparatus. A warning lamp has two or more different modes. The modes include an abnormality mode for indicating the occurrence of an abnormality in the pivot control apparatus and a normality mode for indicating normal functioning of the pivot control apparatus. A notifier for notifying an operator of two or more different pieces of information using the warning lamp. The warning lamp enters the abnormality mode to indicate the occurrence of an abnormality when the determiner determines that an abnormality has occurred in the pivot control apparatus, and the warning lamp enters the normality mode to indicate normal functioning of the pivot control apparatus when the determiner determines that the pivot control apparatus is functioning normally. 
     In another aspect of the present invention, a method for controlling axle pivot in an industrial vehicle including an axle to which wheels are mounted and supported to pivot vertically relative to a body of the vehicle is provided. The method includes determining whether an abnormality has occurred in a pivot control apparatus that regulates pivotal motion of the axle and controlling a warning lamp to indicate the occurrence of an abnormality, if an abnormality has occurred, and to indicate normal functioning of the pivot control apparatus when no abnormality has occurred. 
    
    
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a plan view showing an instrument panel employed in a first embodiment according to the present invention; 
     FIG. 2 is a circuit diagram showing the electric structure of the instrument panel of FIG. 1; 
     FIG. 3 is a diagrammatic view showing the axle pivot control apparatus according to the present invention; 
     FIG. 4 is a diagrammatic view showing the axle pivot control apparatus according to the present invention; 
     FIG. 5 is a side view showing a forklift to which the present invention is applied; 
     FIG. 6 is a block diagram showing the flow of electric signals in the axle pivot control apparatus; 
     FIG. 7 is a map used when restricting pivoting of the axle; 
     FIG. 8 is a diagram showing the ranges in which pivoting of the axle is permitted and prohibited; and 
     FIG. 9 is a flowchart showing a warning light control routine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment according to the present invention will now be described with reference to the drawings. As shown in FIG. 5, an industrial vehicle, or forklift  1  is a typical front-drive, rear-steer, four-wheel vehicle (two front wheels  7  and two rear wheels  11 ). Two outer masts  2  are arranged in front of the front wheels  7 . The forklift  1  has a body  1   a  to which each outer mast  1  is inclinably connected by a tilt cylinder  5 . Two inner masts  3  are arranged between the outer masts  2 . The inner masts  3  are supported by the outer masts  1  such that the inner masts  3  can be lifted and lowered. A fork  4  is supported by each inner mast  3 . A sprocket is installed at the upper portion of the inner mast  3  to move the fork  4  along the inner mast 
     As shown in FIG. 3, the front wheels  7  are operably connected to an engine  9  by a differential ring gear  8  and a transmission (not shown). As shown in FIGS. 3 and 4, a rear axle  10 , on which the rear wheels  11  are mounted, extends laterally at the rear portion of the body  1   a  and is pivotal about a center pin  10   a.  Thus, the rear axle  10  pivots with respect to the body  1   a.  A pair of stoppers  1   b  are secured to the lower portion of the body  1   a  to restrict pivoting of the rear axle  10  within an angular range of ±4°. 
     As shown in FIG. 4, a hydraulic damper  13  connects the body  1   a  to the rear axle  10 . The damper  13  includes a housing  13   a,  a piston  13   b,  and a piston rod  13   c.  The housing  13   a  is coupled to the body  1   a.  The piston rod  13   c  extends from the piston  13   b  and is coupled to the rear axle  10 . The piston  13   b  defines a first chamber R 1  and a second chamber R 2  in the housing  13   a.  The damper  13  is connected to an electromagnetic switch valve  14  by way of a first passage P 1  and a second passage P 2 . The first passage P 1  is connected with the first chamber R 1 , while the second passage P 2  is connected with the second chamber R 2 . A third passage P 3  extends from the second passage P 2  and leads to an accumulator  17 , which reserves hydraulic oil, by way of a check valve  18 . The accumulator  17  compensates for loss of hydraulic oil, due to leakage or other reasons. A throttle valve  19  is arranged in the second passage P 2 . 
     The electromagnetic switch valve  14  has a solenoid  14   a  and a spool, which is shifted between a first position  15  and a second position  16  by the solenoid  14   a.  A controller is installed in front of an operator cabin (FIG. 5) to control the electromagnetic switch valve  14 . The spool is shown at the first position in FIG.  4 . In this state, the movement of hydraulic oil between the first and second chambers R 1 , R 2  is prohibited to lock the piston  13   b  in the housing  13   a.  This, in turn, locks, or restricts, pivoting of the rear axle  10 . If the spool is moved to the second position  16 , the movement of the hydraulic oil between the chambers R 1 , R 2  is permitted. This unlocks the piston  13   b  and allows the rear axle  10  to pivot freely. The damper  13 , the electromagnetic switch valve  14 , and the hydraulic circuit therebetween forms a locking device. 
     As shown in FIG. 3, a yaw rate sensor  21 , a vehicle velocity sensor  22 , fork position sensors  23 ,  24 , a pressure sensor  25 , and a pivot angle sensor  26  are installed in the forklift  1 . The sensors  2 - 26  detect the current operation status and load status of the forklift  1 . Each sensor  21 - 26  is connected to the controller  20 . 
     The yaw rate sensor  21  detects the yaw rate (angular velocity) Y (rad/sec) of the forklift  1 . A gyroscope (e.g., piezoelectric type, or an optical type gyroscope) may be employed as the yaw rate sensor  21 . The vehicle velocity sensor  22  detects the rotating speed of the differential gear  8  and thus indirectly detects the velocity V of the forklift  1 . 
     The first and second fork position sensors  23 ,  24  are each attached to the outer masts  2  at different heights. Limit switches may be employed as the fork position sensors  23 ,  24 . The forks  4  may be lifted to a maximum height H max  of about six meters. The first fork position sensor  23  is actuated when the forks  4  are lifted to a height of two meters or higher and de-actuated when the forks  4  are located below the height of two meters. The second fork position sensor  24  is actuated when the forks  4  are lifted to a height of four meters or higher and de-actuated when the forks  4  are located below the height of four meters. Thus, the states of the two position sensors  23 ,  24  indicate the zone in which the forks  4  are located. The forks  4  are located in a low zone when positioned between a height of zero meters to two meters, an intermediate zone when positioned between a height of two meters to four meters, and a high zone when positioned at a height of four meters or higher. The controller  20  determines which zone the forks  4  are located in. 
     The pressure sensor  25  is arranged at the bottom portion of the lift cylinder  6  to detect the hydraulic pressure in the cylinder  6 . The weight w of the load carried on the forks  4  is obtained from the detected hydraulic pressure. 
     As shown in FIGS. 3 and 4, the pivot angle sensor  26  is supported at one side of the body  1   a.  A potentiometer may be employed as the pivot angle sensor  26 . Pivoting of the rear axle  10  is converted to rotary movement by a link mechanism  27 . The pivot angle sensor  26  detects the rotary movement to obtain the pivot angle θ. A signal representing the detected motion is transmitted to the controller  20 . The pivot angle θ is included in the range of −4° to 4°. 
     An instrument panel  28 , which is shown in FIG. 1, is installed in the cabin. Indicators  29 , which include various type of warning lights (warning lamps), and a liquid crystal display  30 , which displays predetermined information with symbols and characters, are arranged on the instrument panel  28 . An axle warning lamp  31  for warning the operator of an abnormality in the axle pivot control apparatus is also arranged on the instrument panel  28 . 
     As shown in FIG. 2, the warning lamp  31  includes a first light-emitting diode (LED)  32  and a second light-emitting diode (LED)  33 . The first LED  32  emits a red light while the second LED  33  emits a green light. The color of the warning lamp  31  is determined by the combination of the lights emitted by the first and second LEDs  32 ,  33 . If the first LED  31  is lit solely, the warning lamp  31  emits a red light. If the second LED  33  is lit solely, the warning lamp  31  emits a green light. If the first and second LEDs are lit together, the warning lamp  31  emits a yellow light, which is the combination color of red and green. A green LED and a red LED are selected since these colors and its combination color each have a different hue. Thus, each color is easily distinguished from the others. 
     The controller  20  incorporates a lighting circuit  34 . The LEDs  32 ,  33  each have an anode, which is connected to the lighting circuit  34 , and a cathode, which is grounded by way of a resistor R. A battery (not shown) applies an electric potential of +B to the lighting circuit  34 . The lighting circuit  34  incorporates two switch elements (not shown). Each switch element is associated with one of the LEDs  32 ,  33  to actuate or de-actuate the LED  32 ,  33 . There are three possible combinations for the actuation of the LEDs  32 ,  33 . The first combination is only the first LED  32  being lit. The second combination is only the second LED  33  being lit. The third combination is both first and second LEDs  32 ,  33  being lit. Accordingly, the color of the light emitted by the warning lamp  31  depends on the status of the two switch elements. 
     As shown in FIG. 3, an actuating switch  35  is connected to the controller  20 . The actuating switch  35  is manually shifted between either an OFF position or an ON position. When the actuating switch  35  is shifted to the OFF position, the axle pivot control apparatus is de-actuated thus enabling the rear axle  10  to pivot freely. When the actuating switch  35  is shifted to the ON position, the axle pivot control apparatus is actuated to control pivoting of the rear axle  10 . 
     The electric structure of the axle pivot control apparatus will now be described with reference to FIG.  6 . The controller  20  incorporates a microcomputer  36 , analog-to-digital (AD) converter circuits  37 ,  38 ,  39 ,  40 , the lighting circuit  34 , and an actuator  41 . The microcomputer  36  includes a central processing unit (CPU)  42 , a read only memory (ROM)  43 , a random access memory (RAM)  44 , a clock circuit  45 , an input interface  46 , and an output interface  47 . Signals representing the values detected by the sensors  21 ,  22 ,  25 ,  26  are sent to the AD converter circuits  37 ,  38 ,  39 ,  40 , respectively. 
     The signals from the fork position sensors  23 ,  24  and the actuating switch  35  are sent to the CPU  42 . If the actuator  41  receives a de-excitation command from the CPU  42 , the actuator  41  impedes the flow of current to the solenoid  14   a  and moves the spool of the electromagnetic switch valve  14  to the first position  15 . On the other hand, if the actuator  41  receives an excitation command from the CPU  42 , the actuator  41  commences the flow of current to the solenoid  14   a  and moves the spool of the electromagnetic switch valve  14  to the second position  16 . Accordingly, the electromagnetic switch valve  14  is controlled by signals sent to the actuator  41  from the CPU  42 . 
     The ROM  43  stores a program used to control pivoting of the axle and another program used to control the warning light (refer to FIG.  9 ). When the actuating switch  35  is located at the ON position, the CPU  42  executes these programs in a cyclic manner at predetermined time intervals (e.g., 10 to 90 milliseconds). The warning light control routine is executed to control the warning lamp  31  and will described later. 
     The axle pivot control routine will first be described. The CPU  42  first reads the yaw rate Y, the vehicle velocity V, the load weight w, and the pivoting angle θ. The CPU  42  also determines the vertical position H of the forks  4  based on the signals sent from the fork position sensors  24 ,  25 . The CPU  42  then judges whether the load carried on the forks  4  is light or heavy. If the value of the load weight w is lower than a reference value w o  (w&lt;w o ), the load is light. On the other hand, if the value of the load weight w is equal to or higher than a reference value w o  (w≧w o ), the load is heavy. 
     The CPU  42  determines the operation status of the forklift by computing a lateral acceleration G s  and a yaw acceleration ΔY/ΔT, which indicates the rate of change of the yaw rate Y, based on the signals sent from the sensors  21 - 29 . The lateral acceleration G s  is computed from the yaw rate Y and the vehicle velocity V using the equation G s =V×Y. The yaw acceleration ΔY/ΔT is computed from the difference between the current yaw rate Y and the previous yaw rate Y, which was obtained in the preceding cycle, relative to the elapsed time therebetween. The rear axle  10  is locked, or restricted from pivoting, either when the lateral acceleration G s  exceeds its threshold value g o  or when the yaw acceleration ΔY/ΔT exceeds its threshold value y o . The threshold value g o  of the lateral acceleration G s  is selected by referring to a map M, which is illustrated in FIG.  7 . 
     The threshold value g o  is set at a first value (e.g., 0.18N) when the vertical position H of the forks  4  is lower than a height of two meters and set at a second value (e.g., 0.08N) when the vertical position H of the forks  4  is located at a height of two meters or higher. The threshold values g o , y o  are determined through experiments and theoretical calculations and differ in accordance with the type of vehicle or other factors. Reference to the yaw acceleration ΔY/ΔT when determining the operation status of the forklift  1  enables pivoting of the rear axle  10  to be restricted before the lateral acceleration G s  becomes too high and also continuously restricts pivoting of the rear axle  10  when the forklift changes directions. 
     The CPU  42  also judges whether or not the vehicle&#39;s center of gravity is raised (a state in which a heavy load is lifted to a high position). If it is determined that the center of gravity is raised, that is, if the state of the load is in a lock range (FIG.  7 ), the CPU  42  locks the rear axle  10 . However, if the absolute value of the rear axle pivot angle θ exceeds 2°, the rear axle  10  remains unlocked even when the load status is in the lock range. This is to prevent locking of the rear axle  10  when one of the rear wheels  11  rides over a bump. If the rear axle  10  is locked while one is raised by a bump, the elevated wheel  11  will remain elevated even if the wheel  11  is moved to a level surface. If the absolute value of the pivot angle θ is 2° or lower, the elevated rear wheel  11  will descend onto the road surface regardless of the rear axle  10  being locked. 
     The CPU  42  stores three flags F g , F y , and F n . Flag F g  is set when the lateral acceleration G s  exceeds its threshold value g o . Flag F y  is set when the yaw acceleration ΔY/ΔT exceeds its threshold value y o . Flag F n  is set when the absolute value of the pivot angle θ is 2° or lower while the load status is included in the lock range. In other words, the flags F g , F y , and F n  are set when the rear axle  10  should be locked. 
     The warning light control routine will now be described. Testing of the axle pivot control apparatus is carried out during the warning light control routine to test for abnormalities. Thus, if the rear axle  10  remains unlocked while it should be locked, the axle pivot control apparatus is diagnosed as having an abnormality. 
     The warning lamp  31  emits a red light when there is an abnormality. If the operation status (lateral acceleration G s  and yaw acceleration ΔY/ΔT) meets the axle locking conditions, the warning lamp  31  emits a green light. If the load status (weight w and position H) meets the axle locking condition, the warning lamp  31  emits a yellow light. The operator can thus distinguish three types of information from these three colors. The warning light control routine is executed in accordance with the flowchart illustrated in FIG.  9 . 
     As shown in FIG. 9, the microcomputer  36  first performs step S 10  to carry out testing of the axle pivot control apparatus. The testing includes two processes. In the first process, sensors are tested for abnormalities. For example, if the value of the vehicle velocity is zero but the value of the yaw rate Y is not, the microcomputer  36  determines that there is an abnormality in the related sensors. In the second process, the locking device is tested for abnormalities. More specifically, the microcomputer  36  determines whether the rear axle  10  is locked when it should be. For example, the microcomputer  36  monitors changes in the pivot angle θ when the rear axle  10  should be locked. If changes in the pivot angle θ are detected, the microcomputer  36  determines that there is an abnormality in the axle pivot control apparatus. If an abnormality is found during any one of the two testing processes, this indicates that the axle pivot control apparatus, which includes the sensors tested in the first process and the locking device tested in the second process, is not functioning properly. 
     At step S 20 , the microcomputer  36  determines whether or not the axle pivot control apparatus has an abnormality. If it is determined that there is an abnormality, the microcomputer  36  proceeds to step S 30  and lights the first LED  32  so that the warning lamp  31  emits a red color. If it is determined in step S 20  that there is no abnormalities, the microcomputer  36  proceeds to step S 40 . 
     At step S 40 , the microcomputer  36  determines whether the vehicles&#39;s center of gravity is raised, that is, whether the load status is included in the lock range. The microcomputer  36  determines that the load status is in the lock range if the flag F n  is set and that the load status is outside the lock range if the flag F n  is not set. If it is determined that the load status is in the lock range, the microcomputer  36  proceeds to step S 50  and lights both the first LED  32  and the second LED  33  so that the warning lamp  31  emits a yellow color. If it is determined that the load status is not in the lock range in step S 40 , the microcomputer  36  proceeds to step S 60 . 
     At step S 60 , the microcomputer  36  determines whether or not the rear axle  10  is being locked due to the operation status of the forklift  1 . In other words, the microcomputer  36  determines that the rear axle  10  is in a locked state if either flag F g  or flag F y  is set. If neither flag F g  or flag F y  is set, the microcomputer  36  determines that pivoting of the rear axle  10  is permitted. If it is determined that the rear axle  10  is locked, the microcomputer  36  proceeds to step S 70  and lights the second LED  33  so that the warning lamp  31  emits a green color. The microcomputer  36  then completes the present routine cycle. If it is determined that the rear axle  10  is not locked in step S 60 , the microcomputer  36  completes the present routine cycle. 
     As described above, the color of the warning lamp  31  when the rear axle  10  is locked due to the load status differs from that when the rear axle  10  is locked due to the operation status of the forklift  1 . When the warning lamp  31  emits a yellow light or a green light, the rear axle  10  is locked. In this state, the operator may rely on the axle pivot control apparatus when maneuvering the forklift  1  and handling loads. However, if the warning lamp  31  emits a yellow light, this indicates that the operator should take precautions when steering the forklift  1  or when handling loads. If the warning lamp  31  emits a green color, this indicates that the operator should take precautions when steering the forklift  1 . Accordingly, the color of the light emitted by the warning lamp  31  advises what precautions the operator should take if necessary. Furthermore, the warning lamp  31  emits a red light when there is an abnormality in the axle pivot control apparatus. Thus, the operator is immediately notified of an abnormality if one occurs. 
     The warning lamp  31  turns into various different colors. Thus, the operator is warned of an occurrence of an abnormality and notified whether the rear axle  10  is in a locked state by the same warning lamp  31 . Accordingly, the space occupied by the warning lamp  31  need not be increased. This is significant since the space provided for the instrument panel  28  is limited. 
     The warning lamp  31  emits light of three different colors. That is, the warning lamp  31  emits the color of the first LED  32 , the color of the second LED  33 , and the combination color of these colors. Thus, in addition to being warned of an abnormality in the axle pivot control apparatus, the operator can learn why the rear axle  10  is being locked. 
     The LEDs  32 ,  33  are selected such that they emit colors that are clearly distinguished from one another when lit. If the LEDs  32 ,  33  are lit independently, the first LED  32  emits a red light and the second LED  33  emits a green light. The LEDs  32 ,  33  emit a combination color of yellow when lit together. Therefore, different information is accurately communicated to the operator. 
     Signals sent from the pivot angle sensor  26  are used during testing of the axle pivot control apparatus. Thus, separate sensors for testing purposes need not be employed. 
     The yaw rate sensor  21  and the vehicle velocity sensor  22  are tested for abnormalities by comparing the signals sent from these sensors. Thus, separate sensors for testing purposes need not be employed. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. More particularly, the present invention may be embodied as described below. 
     In the preferred and illustrated embodiment, the warning lamp  31  emits light of three different colors. However, if the axle pivot control apparatus is functioning properly, the warning lamp  31  may be lit by another color. For example, a further color may be used to indicate that pivoting of the rear axle  10  is permitted due to the absolute value of the pivot angle θ exceeding 2° despite the forks  4  being lifted to a high position while carrying a heavy load. Accordingly, if the warning lamp  31  is lit by the further color, this would notify the operator that the rear axle  10  is free to pivot despite the heavy load being lifted to a high position. Thus, the operator can take the necessary precautions when handling the load. 
     In the preferred and illustrated embodiment, the warning lamp  31  may be lit by a further color to indicate that the actuating switch  35  has been turned off and pivoting of the rear axle  10  will thus not be controlled. This lets the operator know that pivoting of the rear axle  10  will not be restricted. 
     The warning lamp  31  may be lit such that it emits different colors when pivoting of the rear axle  10  is prohibited and when pivoting of the rear axle  10  is permitted. This notifies the operator of the state of the rear axle  10 . 
     Two LEDs are employed in the preferred and illustrated embodiment. However, more than two LEDs may be employed. 
     For example, the warning lamp  31  may be lit by three LEDs. In such case, the warning lamp  31  emits light of four or more different colors. Thus, the operator can be informed of four or more types of different information. The combination of a red LED, a green LED, and a blue LED will produce a maximum of seven different colors. 
     In the preferred and illustrated embodiment, the LEDs  32 ,  33  emit a red light or a green light. However, the colors of the LEDs  32 ,  33  are not limited to these two colors. For example, a red LED and a blue LED may be used together. Alternatively, a green LED and a blue LED may be used together. 
     The light emitting body accommodated in the warning light is not limited to an LED. For example, a bulb or a miniature fluorescent tube may be used instead. 
     A compact color liquid crystal display device may be employed as the warning light. In such case, the color shown on the liquid crystal display device changes. 
     The warning lamp  31  may be constructed by a single light emitting body that emits a white light through a plurality of different color filters. In such case, the color of the warning lamp  31  is changed by selecting the appropriate color filter or color filters. 
     The warning lamp  31  incorporates two light emitting bodies and is thus capable of emitting three different colors. Thus, if there are two different types of information including information regarding to abnormalities, two colors may be selected to indicate the two different type of information simultaneously. 
     Testing of the axle pivot control apparatus may be performed by testing the fork position sensors or the pressure sensor, which are employed to detect the status of the carried load. 
     In the preferred and illustrated embodiment, the detected pivot angle θ is used to detect abnormalities related to pivoting of the rear axle  10 . However, abnormalities may be detected by using additional sensors. For example, the movement of the damper piston rod  13   c  may be detected by a sensor to test for abnormalities. 
     Abnormalities in the locking and unlocking of the rear axle may also be tested. For example, the position of the spool in the electromagnetic switch valve  14  may be detected to judge whether the rear axle  10  should be locked or unlocked. 
     In the preferred and illustrated embodiment, different colors are used for each type of information. The color used to indicate that the rear axle is locked due to the operation status differs from that used to indicate that the rear axle is locked due to the load status. However, the color is not limited to one for each type of information. For example, different colors may be used to indicate different types of abnormalities. Alternatively, locking of the rear axle  10  may be indicated by a single color regardless of whether the rear axle  10  is locked by the operation status of the load status. 
     Any type of sensor can be used to estimate the lateral acceleration and the yaw acceleration. For example, instead of using a yaw rate sensor, a tire angle detector may be employed to detect the steering angle (tire angle) of the rear wheels  11 . In this case, the tire angle and the vehicle velocity V are used to compute the lateral acceleration G s (=V 2 /r) and the yaw acceleration ΔY/ΔT(=V·Δ(1/r)/ΔT). When calculating the yaw acceleration ΔY/ΔT, r represents the turning radius of the vehicle. Alternatively, the values detected by an acceleration sensor and a yaw rate sensor may be used to calculate the lateral acceleration G s  and the yaw acceleration ΔY/ΔT. 
     Lateral acceleration may be relied on as the sole physical quantity indicative of the operation status. The yaw acceleration does not necessarily have to be used. Furthermore, the rate in which the lateral acceleration fluctuates (ΔG/ΔT) may be used in lieu of the yaw acceleration ΔY/ΔT. 
     In the preferred and illustrated embodiment, pivoting of the rear axle  10  is restricted by the operation status and by the load status. However, the axle pivot control apparatus may be constructed such that the rear axle  10  is locked solely by either one of these conditions. 
     In the preferred and illustrated embodiment, the pivot angle θ of the rear axle  10  is referred when judging whether to restrict pivoting. However, the pivot angle θ does not necessarily have to be referred to when locking the rear axle  10 . 
     In the preferred and illustrated embodiment, the warning lamp  31  can be continually turned on and off to indicate information. For example, the warning lamp  31  can be lit constantly or turned on and off depending on the type of abnormality. Furthermore, the lighting pattern of the warning lamp  31  may be used to convey three types of information even when using only one color. For example, a first piece of information may be conveyed by turning the warning lamp  31  on and off continually, a second piece of information by continuously turning on the warning lamp  31 , and a third piece of information by turning off the warning lamp  31 . 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.