Patent Description:
Many types of materials handling vehicles, such as forklift trucks, have been developed wherein material handling devices, typically forks, are elevated to extreme heights to store and retrieve materials at upper levels within a warehouse. Such high lift vehicles commonly use a variety of mast arrangements wherein the forks, and oftentimes also the operator of the vehicle, are elevated high above the floor of the warehouse to perform picking and/or storage operations. In a typical multi-stage mast construction, a movable carriage comprising forks is supported for vertical movement relative to at least one mast section by a chain, where a first end of the chain is attached to the movable carriage and a second end of the chain is anchored to a relatively stationary location. An actuating member includes a vertically movable element, such as the ram of a cylinder assembly, acting on a midsection of the chain, between the first and second ends, to tension the chain and cause the movable carriage to move upward, wherein a controlled tension is maintained on the chain during a downward actuation of the actuating member to lower the movable carriage.

During a typical materials handling vehicle operation, the vehicle is operated to position the movable carriage via horizontal as well as vertical movement. As a result of horizontal movement of the movable carriage toward shelving or a rack for storing products, the forks may be positioned in an overlapping relationship over a shelf or rack. If the movable carriage is then actuated vertically in downward movement, the forks may engage and become caught on the shelf or rack, causing the chain to become slack between the first and second chain ends as the actuating member continues the downward movement. Subsequently, horizontal movement of the movable carriage, moving the forks out of engagement with the shelf or rack could result in the movable carriage dropping or free-falling until chain tension is re-established.

In a known system for detecting chain slack, a compression spring is located at a chain anchor/tensioner for biasing the end of a lift chain relative to a switch. When a chain slack event occurs during a lowering operation, the compression spring pushes the chain anchor/tensioner, and the switch can detect this movement and send a signal to stop the lowering operation. This type of chain slack detection system and similar systems typically require additional hardware with associated expense for implementation.

<CIT> discloses a materials handling machine according to the preamble of claim <NUM>.

In accordance with an aspect of the invention, a materials handling vehicle is provided having chain slack detection. The materials handling vehicle comprises a mast assembly, a load handling structure supported on the mast assembly, one or more operator controls, and a lifting structure having a chain structure for performing a lifting and lowering of the load handling structure relative to the mast assembly. The materials handling vehicle further comprises a height sensor for generating a height signal corresponding to vertical movement of the load handling structure relative to the mast assembly, and a vehicle control module for processing the height signal received from the height sensor and an operator control signal received from the one or more operator controls. The vehicle control module evaluates the height signal and the operator control signal and disables one or more vehicle functions if the height signal does not correspond to the operator control signal.

The one or more vehicle functions may include at least one of lowering movement of the load handling structure or vehicle travel movement.

The vehicle control module may disable the one or more vehicle functions if the operator control signal comprises a load handling structure lower signal, and the height signal comprises one of a lifting state signal, corresponding to a height of the load handling structure increasing relative to an adjacent mast section, or a static state signal, corresponding to the height of the load handling structure not changing relative to the adjacent mast section.

The operator control signal may comprise one of a load handling structure lower signal, or a load handling structure lift signal.

The height signal may comprise one of a lowering state signal, corresponding to a height of the load handling structure decreasing relative to an adjacent mast section, a lifting state signal, corresponding to the height of the load handling structure increasing relative to the adjacent mast section, or a static state signal, corresponding to the height of the load handling structure not changing relative to the adjacent mast section. The lifting state and static state signals may comprise signals that do not correspond to the load handling structure lower signal for disabling the one or more vehicle functions.

The materials handling vehicle may further comprise a hydraulic system for actuating the chain structure, and a pressure sensor in the hydraulic system. The vehicle control module may process and evaluate a pressure signal from the pressure sensor indicative of a pressure present in the hydraulic system, and may disable the one or more vehicle functions if the pressure signal indicates the pressure in the hydraulic system is less than a predetermined pressure value.

The height sensor may comprise a height encoder to sense a position of the load handling structure relative to an adjacent mast section.

The materials handling vehicle may further comprise an operator's compartment.

In accordance with a further aspect of the invention, a method of detecting a chain slack condition in a materials handling vehicle is provided, the materials handling vehicle having a mast assembly, a load handling structure supported on the mast assembly, one or more operator controls, a lifting structure for performing lifting and lowering of the load handling structure relative to the mast assembly, and a vehicle control module. The method comprises detecting an operator control signal from the one or more operator controls, detecting a height signal corresponding to vertical movement of the load handling structure relative to the mast assembly, receiving and evaluating the operator control signal and the height signal in the vehicle control module, and disabling one or more vehicle functions if the height signal does not correspond to the operator control signal, indicating a chain slack condition.

The one or more vehicle functions may include at least one of lowering movement of a load handling structure or vehicle travel movement.

The operator control signal may comprise one of a load handling structure lower signal, or a load handling structure lift signal. The height signal may comprise one of a lowering state signal, corresponding to a height of the load handling structure decreasing relative to an adjacent mast section, a lifting state signal, corresponding to the height of the load handling structure increasing relative to the adjacent mast section, or a static state signal, corresponding to the height of the load handling structure not changing relative to the adjacent mast section.

The lifting state and static state signals may comprise signals that do not correspond to the load handling structure lower signal for indicating the chain slack condition.

The method of detecting a chain slack condition may further comprise detecting a pressure signal from a pressure sensor in a hydraulic system, wherein the vehicle control module may process and evaluate the pressure signal and may disable the one or more vehicle functions if the pressure signal indicates a pressure in the hydraulic system is less than a predetermined pressure value.

The method of detecting a chain slack condition may further comprise detecting a mast switch signal from a mast switch, wherein the vehicle control module may process and evaluate the mast switch signal and may deactivate detection of a chain slack condition if the mast switch signal indicates that the load handling structure is in a predetermined lowered position.

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.

Referring to <FIG>, a materials handling vehicle <NUM> is shown for illustrating aspects of a chain slack detection system described herein. The vehicle <NUM> may include a power unit <NUM>, a mast assembly <NUM>, and a movable carriage comprising a load handling structure <NUM> located on an opposite side of the mast assembly <NUM> from the power unit <NUM>. In one embodiment, the load handling structure <NUM> includes an operator compartment <NUM> having an operator platform <NUM>, although it is understood that the chain slack detection system described herein may be used with other types of load handling structures. A pair of forks <NUM> extends outward from a rear edge of the operator platform <NUM>, wherein the forks <NUM> may be used to support a pallet P, an operator platform (not shown), or other structure for facilitating order picking, load transporting, or other materials handling operations.

Referring to <FIG> and <FIG>, the mast assembly <NUM> may be supported on the power unit <NUM>, and includes plural telescoping sections forming, for the purposes of the present description, a two stage mast. Specifically, the illustrated mast assembly <NUM> comprises a stationary first mast section <NUM> fixed to the power unit <NUM> and a movable second mast section <NUM> supported for vertical movement along the first mast section <NUM>, wherein the load handling structure <NUM> is supported on the second mast section <NUM>. The mast assembly <NUM> further includes a lifting structure <NUM> provided for actuating the load handling structure <NUM> in vertical movement along the adjacent second mast section <NUM>.

As is illustrated in <FIG>, the illustrated lifting structure <NUM> comprises a chain structure <NUM>, i.e., one or more chains, having a first end 30a attached to the first mast section <NUM> and an opposing second end 30b attached to the operator compartment <NUM>. An intermediate section 30c of the chain structure <NUM> extends over a lift pulley <NUM> supported on the second mast section <NUM>. The mast assembly <NUM> additionally includes one or more lift ram/cylinder assemblies <NUM> for effecting movement of the second mast section <NUM> and the load handling structure <NUM> (via the chain structure <NUM>) relative to the first mast section <NUM>. A bottom portion of a cylinder 34a of the lift ram/cylinder assembly <NUM> in the illustrated embodiment is coupled to a frame <NUM> of the vehicle <NUM>. A ram 34b is housed within the cylinder 34a and extends from the cylinder 34a under the control of pressurized hydraulic fluid, and is fixed to the second mast section <NUM>. Hence, actuation of the ram 34b causes the second mast section <NUM> to move vertically relative to the first mast section <NUM>, and the chain structure <NUM> actuates the load handling structure <NUM> to move vertically relative to the second mast section <NUM> as the chain structure <NUM> is drawn over the lift pulley <NUM>. It may be understood that for every unit of vertical movement of the second mast section <NUM>, the load handling structure <NUM> moves twice as far relative to the first mast section <NUM> via the chain structure <NUM> and lift pulley <NUM>.

The operator compartment <NUM> can include operator controls C, see <FIG>, that may be operated by an operator standing on the operator platform <NUM> to control, e.g., the speed, steering direction, and braking of the vehicle <NUM>, as well as mast lift and lower operations for selectively effecting vertical movement of the load handling structure <NUM>. For example, the operator controls C may generate an operator control signal OS (see <FIG>) for controlling hydraulic pressure to the lift ram/cylinder assembly <NUM>, wherein the operator control signal OS may comprise one of a load handling structure lower signal and a load handling structure lift signal. The operator controls C for generating the operator control signal OS may comprise, for example, individual lift/lower buttons, a rocker switch, or any suitable control (not shown in detail).

Referring to <FIG>, an exemplary hydraulic system <NUM> is illustrated for providing pressurized hydraulic fluid to the lift ram/cylinder assembly <NUM>. The illustrated hydraulic system <NUM> includes a hydraulic lift pump <NUM> driven by a pump motor M. The hydraulic lift pump <NUM> draws hydraulic fluid from a hydraulic fluid reservoir <NUM> and circulates the hydraulic fluid to the lift ram/cylinder assembly <NUM> through a solenoid-operated valve <NUM> and a fluid supply line <NUM>. A check valve <NUM> may be located in the fluid supply line <NUM>, and a pressure relief valve <NUM> may be located between the fluid supply line <NUM> and a fluid return line <NUM>. The solenoid-operated valve <NUM> can be actuated to selectively control the hydraulic fluid supplied to the lift ram/cylinder assembly <NUM> for controlling the height of the load handling structure <NUM>. For example, operation of the solenoid-operated valve <NUM> can be controlled by actuator control signals received from a vehicle control module <NUM>. The actuator control signals can be transmitted to the solenoid-operated valve <NUM> in response to an operator control signal OS received at the vehicle control module <NUM> during operation of the operator controls C. A lowering valve <NUM> located in the fluid return line <NUM> opens proportional to the operation of the operator controls C, and a pressure compensator <NUM> can be provided to regulate the flow rate of the lowering valve <NUM>. The solenoid-operated valve <NUM> may be encompassed in a hydraulic manifold <NUM>, and a pressure sensor <NUM> is hydraulically connected to the fluid supply line <NUM> at the hydraulic manifold <NUM> for sensing a hydraulic pressure of the hydraulic fluid supplied to the lift ram/cylinder assembly <NUM> and for providing a pressure signal PS (see <FIG>) to the vehicle control module <NUM>. The pressure sensor <NUM> may comprise redundant pressure sensors 54a, 54b, or the pressure sensor <NUM> may comprise a single pressure sensor, e.g., one of the pressure sensors 54a, 54b depicted in <FIG>.

Referring to <FIG>, the load handling structure <NUM> includes a height sensor <NUM> for generating a height signal HS (see <FIG>) corresponding to vertical movement of the load handling structure <NUM> relative to a portion of the mast assembly <NUM>, e.g., relative to the second mast section <NUM>, wherein the height signal HS may be provided as an input to the vehicle control module <NUM>. The height signal HS may comprise one of: a lowering state signal, corresponding to the load handling structure height decreasing relative to an adjacent mast section, a lifting state signal, corresponding to the load handling structure height increasing relative to the adjacent mast section, or a static state signal, corresponding to the load handling structure height not changing relative to the adjacent mast section, wherein the adjacent mast section comprises the second mast section <NUM> in the illustrated embodiment. It may be understood that the static state signal may comprise the absence of a load handling structure lower signal and a load handling lift signal from the height sensor <NUM>, as monitored at the vehicle control module <NUM>.

The height sensor <NUM> may comprise a height encoder mounted to the load handling structure <NUM>, and, as depicted in <FIG>, may comprise a rotatory shaft encoder including a wheel that rolls along a portion of the adjacent second mast section <NUM> to provide a pulse signal to the vehicle control module <NUM> corresponding to a distance and direction of movement of the load handling structure <NUM> relative to the second mast section <NUM>. However, it is understood that the system described herein is not limited to a particular form of height sensor for determining movement of the load handling structure <NUM> relative to the second mast section <NUM>.

In accordance with an aspect of the chain slack detection system, the vehicle control module <NUM> receives a plurality of sensor signals and command signals from various vehicle components, e.g., operator control signals OS, height signals HS, and pressure signals PS, and can disable one or more vehicle functions based upon an evaluation of one or more of the plurality of signals. For example, the vehicle control module <NUM> can monitor a mast switch <NUM>, see <FIG> and <FIG>, wherein, if the mast switch <NUM> is ON, corresponding to the load handling structure <NUM> being in a lowered position, the vehicle control module <NUM> will deactivate the chain slack detection associated with the operator control signals OS and height signals HS, see blocks <NUM> and <NUM> in <FIG>. If the mast switch <NUM> is OFF, indicating that the load handling structure <NUM> is at or above a predetermined height, e.g., the operator platform <NUM> is above <NUM>, the operator control signals OS and height signals HS can be used to detect a chain slack condition, see blocks <NUM>, <NUM>, and <NUM> in <FIG>. As is described further below, if the pressure signal PS received from the pressure sensor <NUM> does not correspond to one or more predetermined hydraulic fluid pressures for the hydraulic system <NUM> or if the height signal HS received from the height sensor <NUM> does not correspond to the operator control signal OS received from the operator controls C, the vehicle control module <NUM> can disable one or more vehicle functions. The one or more vehicle functions can include at least one of a lowering movement of the load handling structure <NUM> and a vehicle travel function comprising forward and/or reverse vehicle travel movement.

In a first, pressure monitoring system, illustrated in <FIG>, the pressure signal PS from the pressure sensor <NUM> can be monitored, wherein the pressure monitoring system is operable as described herein to disable one or more vehicle functions when the mast switch <NUM> is in the OFF position, indicating that the load handling structure <NUM> is in a raised position. The vehicle control module <NUM> can monitor the pressure signal PS to determine whether the pressure signal PS is below a predetermined value, indicating a condition that could correspond to a chain slack condition, such as may occur if a portion of the load handling structure <NUM> has become caught in or on a portion of a rack R, see <FIG>. For example, a pressure signal PS from the pressure sensor <NUM> may be received and monitored by the vehicle control module <NUM>, see block <NUM> in <FIG>. During a static state condition when the operator control C is not actuated to lower or lift the load handling structure <NUM>, if the pressure signal PS from the pressure sensor <NUM> is less than a predetermined static pressure PStatic, as defined for the static state condition, see block <NUM>, the vehicle control module <NUM> may disable either or both of the load handling structure lower function and the vehicle travel function, see block <NUM>. Further, the pressure signal PS from the pressure sensor <NUM> may be monitored by the vehicle control module <NUM> during a dynamic state condition when the operator control C is actuated to lift or lower the load handling structure <NUM>. If the pressure signal PS from the pressure sensor <NUM> is less than a predetermined dynamic pressure PDynamic, as defined for the dynamic state condition, see block <NUM>, the vehicle control module <NUM> may disable either or both of the load handling structure lower function and the vehicle travel function, see block <NUM>. The predetermined static and dynamic pressures PStatic, PDynamic may be minimum pressures determined with reference to an unloaded or empty, e.g., without an operator, load handling structure <NUM>.

It should be noted that when the load handling structure lower function is disabled, operation of the lift ram/cylinder assembly <NUM> in the lifting direction, i.e., a load handling structure lift function, can be maintained such that the lift ram/cylinder assembly <NUM> can be operated to remove any slack and re-establish tension in the chain structure <NUM>. Once the slack in the chain structure <NUM> is removed, the vehicle control module <NUM> may reactivate the disabled vehicle functions including, for example, the load handling lower function and the drive function. If the pressure signal PS from the pressure sensor <NUM> indicates that the hydraulic fluid pressure in the hydraulic system <NUM> meets the above-described conditions for the static and dynamic pressures PStatic, PDynamic, then the vehicle control module <NUM> continues to enable the load handling lower function and the vehicle travel function, and further monitors the height signals HS and the operator control signals OS for first and second chain slack conditions, as is further described below, see block <NUM>.

In a second, height monitoring system, see <FIG>, the height signals HS and operator control signals OS can be monitored by the vehicle control module <NUM> to detect a condition corresponding to chain slack. During a normal lift/lower operation, using one of the operator controls C, the operator can initiate a lowering of the load handling structure <NUM>, providing a load handling structure lower signal to the vehicle control module <NUM> such that the vehicle control module <NUM> recognizes that the load handling structure <NUM> should be lowering relative to the adjacent mast section. When the load handling structure lower signal is initiated, the vehicle control module <NUM> provides an actuation signal to the solenoid-operated valve <NUM>, see <FIG>, actuating the lift ram/cylinder assembly <NUM> such that the second mast section <NUM> of the mast assembly <NUM> moves down at a selected first rate, and the load handling structure <NUM> moves down at a predetermined second rate that is faster than the first rate, e.g., at twice the first rate relative to the first mast section <NUM> via the chain structure <NUM> and lift pulley <NUM>. As long as the load handling structure <NUM> moves down at the predetermined rate relative to the lowering rate of the adjacent second mast section <NUM>, the chain structure <NUM> is maintained in a taut, non-slack state during lowering of the load handling structure <NUM>, see <FIG>. The height signal HS output from the height sensor <NUM> comprises a lowering state signal corresponding to the operator control signal OS, i.e., corresponding to a load handling structure lower signal from the operator controls C, and the vehicle control module <NUM> allows the vehicle function, i.e., lowering of the load handling structure <NUM>, to continue without interruption, see blocks <NUM> and <NUM> in <FIG>.

Referring to <FIG> and <FIG>, in a first chain slack condition, when the operator has initiated the load handling structure lower signal via an input at the operator controls C, a portion of the load handling structure <NUM> may become caught in or on a portion of a rack R, such as during an order picking operation adjacent to the rack R, preventing the load handling structure <NUM> from lowering. When the load handling structure <NUM> is prevented from lowering, this can cause slack in the chain structure <NUM> as a result of the lift ram/cylinder assembly <NUM> and the second mast section <NUM> lowering at the selected first rate in response to the load handling structure lower signal being initiated via an input at the operator controls C, while the load handling structure <NUM> remains generally stationary, i.e., the load handling structure <NUM> fails to lower. The failure of the load handling structure <NUM> to lower at an appropriate second rate, corresponding to the second mast section <NUM> lowering at the selected first rate, causes a chain slack condition to occur, as illustrated in <FIG>. Left unchecked, the accumulated slack in the chain structure <NUM> can present a substantial danger to the operator, wherein movement of the vehicle <NUM> away from the rack R while the chain structure <NUM> is in a slack condition can cause the load handling structure <NUM> supporting the operator to free-fall when the load handling structure <NUM> separates from engagement with the rack R. Hence, it is beneficial for such a chain slack condition to be identified and to implement a vehicle control to prevent such free-fall from occurring.

In the above-described first chain slack condition, the relative movement between the load handling structure <NUM> and the second mast section <NUM> causes the height sensor <NUM> to produce a height signal HS corresponding to upward movement of the load handling structure <NUM>. In particular, in response to the load handling structure lower signal, the second mast section <NUM> moves downward relative to the load handling structure <NUM>, causing the height sensor <NUM> to provide an apparent indication of upward movement of the load handling structure <NUM>, see block <NUM> in <FIG>. The height signal HS provided to the vehicle control module <NUM> is evaluated by the vehicle control module <NUM> to determine whether the height signal HS corresponds to the operator control signal OS. The vehicle control module <NUM> evaluates the received height signal HS, which in the present case comprises a lifting state signal, with reference to the received operator control signal OS comprising a load handling structure lower signal and determines that the operator control signal OS does not correspond to the height signal HS, and disables the one or more vehicle functions, see block <NUM> in <FIG>. In particular, disabling of the vehicle functions can comprise disabling actuation of the lift ram/cylinder assembly <NUM> in the lowering direction to prevent further formation of chain slack and/or disabling the driving function of the vehicle <NUM> to prevent the vehicle <NUM> from moving the load handling structure <NUM> out of supporting engagement with the rack R. It should be noted that operation of the lift ram/cylinder assembly <NUM> in the lifting direction, i.e., the load handling structure lift function, can be maintained such that the lift ram/cylinder assembly <NUM> can be operated to remove slack and re-establish tension in the chain structure <NUM>. Once the slack in the chain structure <NUM> is removed, the vehicle control module <NUM> may reactivate the disabled vehicle functions including, for example, the load handling lower function and the drive function. Thus, the chain slack detection disclosed herein may be used to prevent a free-fall of the load handling structure <NUM> caused by slack accumulating in the chain structure <NUM> due to the load handling structure <NUM> being prevented from vertical movement.

Referring to <FIG> and <FIG>, in a second chain slack condition, when the operator has initiated the load handling structure lower signal via an input at the operator controls C, a portion of the load handling structure <NUM> may become stuck or caught on a portion of the adjacent second mast section <NUM>. Similar to the first chain slack condition, when the load handling structure <NUM> is prevented from lowering at the second rate corresponding to the lowering of the second mast section <NUM> at the first rate, this can cause slack in the chain structure <NUM>. In particular, a slack condition in the chain structure <NUM> can occur as a result of both the second mast section <NUM> and the load handling structure <NUM> moving downward at the same rate, i.e., the first rate, as illustrated in <FIG>. As with the first chain slack condition, left unchecked, the accumulated slack in the chain structure <NUM> can present a substantial danger to the operator. Specifically, release of the load handling structure <NUM> from its stuck position in relation to the second mast section <NUM>, while the chain structure <NUM> is in a slack condition, can cause the load handling structure <NUM> supporting the operator to free-fall. Hence, it is beneficial for such a chain slack condition to be identified and to implement a vehicle control to prevent such free-fall from occurring.

In the above-described second chain slack condition, in response to the load handling structure lower signal, the second mast section <NUM> and the load handling structure <NUM> are maintained at the same relative position as they move downward together. The absence of movement between the load handling structure <NUM> and the second mast section <NUM> causes the height sensor <NUM> to produce a height signal HS corresponding to non-movement of the load handling structure <NUM>, see block <NUM> in <FIG>. The vehicle control module <NUM> receives and evaluates the height signal HS, which in the present case comprises a static state signal, with reference to the received operator control signal OS comprising a load handling structure lower signal, and determines that the operator control signal OS does not correspond to the height signal HS, and disables the one or more vehicle function, see block <NUM> in <FIG>. In particular, disabling of the vehicle functions can comprise disabling actuation of the lift ram/cylinder assembly <NUM> in the lowering direction to prevent further formation of chain slack. Additionally, disabling the vehicle functions can comprise disabling the driving function of the vehicle <NUM>. It should be noted that operation of the lift ram/cylinder assembly <NUM> in the lifting direction, i.e., the load handling structure lift function, can be maintained such that the lift ram/cylinder assembly <NUM> can be operated to remove the slack and re-establish tension in the chain structure <NUM>. Once the slack in the chain structure <NUM> is removed, the vehicle control module <NUM> may reactivate the disabled vehicle functions including, for example, the load handling lower function and the drive function. Thus, the chain slack detection disclosed herein may be used to prevent a free-fall of the load handling structure <NUM> caused by slack accumulating in the chain structure <NUM> due to the load handling structure <NUM> being stuck relative to the adjacent second mast section <NUM>.

It should be understood that if the pressure signal PS from the pressure sensor <NUM> indicates that the hydraulic fluid pressure in the hydraulic system <NUM> meets the above-described conditions for the static and dynamic pressures and, in the absence of a detected chain slack condition, the vehicle control module <NUM> continues to enable the load handling lower function and the vehicle travel function, and further monitors the height signals HS and the operator control signals OS for the above-described first and second chain slack conditions. It also may be understood that the vehicle control module <NUM> continues to monitor the pressure signals PS at the same time as monitoring of the height signals HS and the operator control signals OS to control the vehicle functions with reference to determined chain slack conditions during operation of the vehicle <NUM>.

As an alternative to the embodiments discussed above, an embodiment is contemplated wherein the height monitoring system may be operable as soon as the load handling structure <NUM> raises above a height of <NUM>, and may be operable from the time when the load handling structure <NUM> is in the lowered position until the load handling structure <NUM> reaches <NUM>, i.e., the mast switch <NUM> is OFF, at which point, as discussed above, the pressure monitoring system becomes operable. In this alternative embodiment, the height monitoring system may continue to operate in conjunction with the pressure monitoring system after the mast switch <NUM> is OFF.

Claim 1:
A materials handling vehicle (<NUM>) comprising:
a mast assembly (<NUM>);
a load handling structure (<NUM>) supported on the mast assembly (<NUM>);
one or more operator controls (C);
a lifting structure (<NUM>) having a chain structure (<NUM>) for performing a lifting and lowering of the load handling structure (<NUM>) relative to the mast assembly (<NUM>);
a height sensor (<NUM>) for generating a height signal corresponding to vertical movement of the load handling structure (<NUM>) relative to the mast assembly (<NUM>); and
a vehicle control module (<NUM>) for processing the height signal received from the height sensor (<NUM>) and an operator control signal received from the one or more operator controls (C), characterized in that
the vehicle control module (<NUM>) evaluates the height signal and the operator control signal and disables one or more vehicle functions if the height signal does not correspond to the operator control signal.