Patent Application
20120211301
1. Technical Field
The multiple embodiments disclosed herein relate to a leveling system for a vehicle with a structure to be leveled.
2. Background Art
A vehicle has a structure to be leveled, such as an aerial work platform or a telehandler fork. If the vehicle is an aerial work platform, it typically has a self propelled drive or unpowered ground chassis, a swing chassis, a boom lift structure and an operator platform. As the boom structure is deployed to lift the platform up and/or out, a reference angle of the joint supporting the platform changes. To maintain a level platform for the operator the joint angle has to be adjusted in sync with relevant boom lift functions.
The majority of aerial work platform models utilize master/slave leveling for the platform. The master/slave system typically has two hydraulic cylinders connected to the boom lift structure. A master cylinder is connected to a level reference on the primary pivot. A platform level cylinder is hydraulically connected in parallel to the master cylinder as a slave. As the master cylinder is extended the slave cylinder is retracted. The master/slave system does not work for lift structures without a level reference on the primary pivot, for example non-parallelogram 4-bar risers. Additionally the master/slave system adds weight and inefficiencies of the master cylinder, load holding valves and hydraulic hoses. The system inherently levels to the reference, which is generally the ground slope as it affects the drive chassis. Leveling to gravity is generally not possible. The master/slave system is subject to drift, the operator may manually adjust the platform to compensate.
Electronic platform leveling is a method that employs an angle sensor at the platform and generally a second sensor at the chassis. Hydraulic flow to the platform leveling cylinder is controlled to maintain the relative angle between chassis and platform. This is done to give the operator awareness of the ground level that the machine is operating on. Optionally the platform may be leveled to gravity. In order to level to gravity only the angle sensor at the platform is needed. The sensor employed to determine angle of the platform is conventionally accelerometer based. The sensor may measure acceleration in one or more directions. Gravity acceleration is sensed as a static component of total acceleration. A number of channels may be combined into a single angle measurement by vector addition depending on alignment of the axes of the accelerometer. The accelerometer cannot distinguish between short term accelerations of the platform due to propel or boom functions and changes in platform angle. A low pass filter is applied to the angle measurement to minimize the effects of this measurement error. The filter is designed to trade off smooth operation and stability with responsiveness and accuracy of the platform level control. The resulting performance limits of this system are generally detectable by the operator as lag in the level adjustment, undesirable adjustments when braking or accelerating and oscillations. The setup and tuning of the measurement algorithm is critical to achieve reasonable performance. This method is implemented on a number of aerial work platforms and vehicles. A vehicle with a telehandler fork operates similarly to an aerial work platform for leveling the telehandler fork.
The platform level cylinder is generally controlled by a proportional flow control and directional valves. To ensure safety a set of counterbalance valves is located at the cylinder.
In at least one embodiment, a leveling system for a vehicle with a structure to be leveled is disclosed. The leveling system has at least one linear accelerometer mounted to measure platform acceleration in at least one principal plane of motion of a lift structure, an angular rate sensor mounted to measure angular velocity about an axis perpendicular to the principal plane of motion of the lift structure, and an electronic control module configured to use measurements from angular rate and accelerometer sensors to produce a level angle output. The level angle output is used to adjust and control the angle of a structure to be leveled.
In another embodiment, a leveling system for a vehicle with a lift structure is disclosed. A linear accelerometer is mounted to measure acceleration of a lift structure to be leveled in a principal plane of motion of the lift structure. An angular rate sensor is mounted to measure angular velocity of the structure to be leveled about an axis perpendicular to the principal plane of motion of the lift structure. An electronic control module is configured to use measurements from angular rate and accelerometer sensors to produce a level angle output. The level angle output is used to adjust and control the angle of the structure to be leveled. The electronic control module updates a first compensated angle to a subsequent compensated angle by adding a product of loop time and angular rate to a first compensated angle. The compensated angle is compared to a low pass filtered accelerometer based angle to produce a resulting error. The resulting error adjusts the compensated angle to the level angle output which approaches the accelerometer based angle using a compensation coefficient.
In yet another embodiment, a method of leveling a lift structure is disclosed. The method includes providing a linear accelerometer mounted to measure acceleration in a principal plane of motion of a lift structure, providing an angular rate sensor mounted to measure angular velocity about an axis perpendicular to the principal plane of motion of the lift structure, and providing an electronic control module configured to use measurements from angular rate and accelerometer sensors to produce a level angle output. The level angle output is used to adjust and control the angle of the lift structure.
As required, detailed embodiments are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary and may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims and/or as a representative basis for teaching one skilled in the art to variously employ the disclosed embodiments.
With reference now to
The multiple embodiments disclosed herein include a first angle sensor module 28 and a second angle sensor module 30 that are utilized with an algorithm to combine several sensor readings into a stable and accurate measurement of the motion of the platform 12. Redundant sensor information can be used to diagnose sensor faults and/or enable safety modes that allow the operator to lower the platform 12 even when some of the sensing elements have been determined to be faulty. The first sensor module 28 and/or the second sensor module 30 may include rate sensors and/or accelerometers to obtain measurements for all six (6) degrees of freedom to enable determination of the full state of motion of the platform 12.
In at least one embodiment, the first angle sensor module 28 is mounted to a point referenced to inclination of the platform 12. The first angle sensor module 28 may be referred to as the platform referenced sensor 28. The second angle sensor module 30 may be mounted to a point referenced to the ground level, such as the chassis of the vehicle V. The second angle sensor module 30 may be referred to as the ground referenced sensor 30. Mounting positions of the first angle sensor module 28 and the second angle sensor module 30 may be chosen such that axes of each sensor 28, 30 remain aligned in any position of the lift structure 16 or alignment can be inferred from additional measurements. In an embodiment of a platform leveling system 10 having a swing chassis, the ground referenced sensor 30 will generally be on the swing chassis. The platform referenced sensor 28 can be located on a fixed side of a platform rotator 22 or jib rotator, if present.
An embodiment of the angle sensor module 28 is illustrated in
In another embodiment the linear acceleration may be measured along one or more non-orthogonal directions in the measurement plane and the results combined by vector addition to provide an in-plane acceleration measurement and direction. In other embodiments, sensor 28 may be replaced with multiple or separate sensor modules capable of providing similar data.
In one embodiment, an output of the angle sensor module 28 of the acceleration of the two orthogonal axes O1, O2 may be an accelerometer output. The accelerometer output can be used to determine an angle referenced to gravity. The measurement of the angle sensor module 28 is a static measurement so that the accelerometer output is stable over a long term without intermittent service, but may be subject to error from transient linear accelerations of the angle sensor module 28. Another output of the angle sensor module 28 that is the angular rate around an axis O3 orthogonal to the first two axes may be the angular rate output. The angular rate output measures angular velocity of the angle sensor module directly, which can be numerically integrated to determine angle. The angular rate output of the angle sensor module 28 may be insensitive to linear accelerations and the integration process inherently reduces measurement noise and/or error. However, over the long-term small errors in sensor offset calibration can accumulate to a large error in the result. Effective and accurate offset calibration is therefore critical to measurement accuracy (see
The illustrated platform leveling system 10 of
With reference to
Linear acceleration is measured by the accelerometer within the angle sensor module 28 as acceleration 40 in a first direction X and acceleration 42 in a second direction Y. The acceleration 40 in the first direction X and acceleration 42 in the second direction Y is then low pass filtered 44. A gravity angle 46 is calculated to produce an angle reference output 48.
In at least one embodiment, the accelerometer based gravity angle 46 is obtained from two orthogonally arranged accelerometers in the angle sensor module 28, 30. The angle may be calculated as alpha=arctan(ax/ay). This arrangement has the benefit that the result is relatively insensitive to calibration of the accelerometers. In the case of the failure of one of the accelerometer elements, the controller 24 may switch to a mode where the gravity angle 46 is calculated from a single accelerometer (alpha=arcsin(ax/g) or arccos(ay/g). This mode will allow safe descent for the operator with additional limitations (reduced velocity, descent only etc.). Similarly in case of failure of the angular rate sensor, a safe descent mode can be activated that will only use accelerometer based angles.
The angle estimate output 38 and the angle reference output 48 are both combined, along with a predetermined compensation coefficient 50 as a dynamic compensation 52. The output of the dynamic compensation 52 is the compensated angle 34.
Aerial work platform systems, such as the platform leveling system 10 illustrated in
In one embodiment, the angle reference output 48 is more accurate when the machine is not moving and inaccurate when the machine is moving due to non-vertical accelerations. The angle estimate 38 is precise during short term changes regardless of acceleration, but drifts over time. The two angles 38, 48 may be weighted to consider the vehicle control state from the vehicle control module 24. Since aerial work platforms, material lifts, and the like are inactive for much of the time, this approach is appropriate. The accelerometer based angle 48 is weighted when the underlying support structure is stationary and the angular rate 38 is weighted when the underlying support structure is in motion or shaking.
Referring now to
The platform leveling system 10 and method thereof provides for a generally lag-free and accurate angle measurement for the level of the platform 12. Generally lag-free angle measurement shall mean that there is generally not a delay between the occurrence of the angle and the measurement thereof. Of course, a small amount of time delay between the occurrence of the angle and the measurement thereof is to be expected but is minimized. The accuracy of the compensated angle 34 is principally limited by the calibration accuracy of angular rate offset 64. The angular rate offset 64 may, depending on the angle sensor module 28, 30, be sensitive to temperature and drift over time. To maintain accuracy of the calibration setpoint, the angular rate sensor offset 64 can be occasionally estimated and updated. The estimation algorithm may be based on but not limited to linear or advanced filtering methods, such as adaptive or Kalman filtering. In the current embodiment, heuristics are applied that are based on a rest state of the platform leveling system 10 and dynamics captured by the angle sensor module 28, 30 itself. When the platform leveling system 10 is static, as determined by monitoring function switches and inertial sensor excitation, the angular rate sensor output 56 is low pass filtered and the result used to update the sensor offset 64. The time constant of the low pass filter is optimized to match the noise characteristics of the sensor element. This approach ensures that a slow drift in offset is captured while external excitations are not allowed to distort the result.
The compensation coefficient 50 may be determined using information indicating whether the vehicle V is at rest 60, and an accelerometer excitation measurement 62. The machine motion input 60 may be provided by a motion sensor, a vehicle control module, or be based on user inputs, such as engaging a drive transmission, engaging a park function, or actuating the vehicle accelerator or parking brake or the like. The accelerometer excitation measurement 62 may be provided by accelerometers measuring vibratory or other shaking motion of the vehicle V or from an existing input such as acceleration 40, 42.
The same method of measurement may be applied to the platform referenced sensor 28 and the ground level referenced sensor 30. A conventional accelerometer only sensor may be used for the ground level referenced sensor because dynamics of the swing chassis are relatively low. Depending on user selectable options, the platform 12 may be controlled to be parallel to the ground under the vehicle V or level to gravity. Optionally, the operator may also manually trim the angle of the platform 12 to suit their preferences. Based on these inputs, a setpoint is calculated and used by the controller 24 to drive the output to the hydraulic valves and actuator 18. The controller 24 may use feedback from the angle sensor modules 28, 30 and feedforward information derived from other sensors, as well as monitoring the vehicle V state (joystick input, toggle switches) to determine an appropriate output command. The level of the platform 12 can be controlled to optimize a variety of objectives such as, e.g. minimize error or minimize energy consumption and meet operator comfort and safety requirements.
Accurate knowledge of motion of the platform 12 in the boom plane also enables the enforcement of operational limits of the platform 12. Compliance with regulations can be ensured by limiting function speed for the worst case scenario. By using the speed of the platform 12, as derived from sensor measurements, the function speed can be adapted such that velocity is optimized through the motion range. This allows for faster time to height, while ensuring operator safety and comfort.
When compared to prior art, such as filtered accelerometer systems, the disclosed embodiments of the platform leveling system 10 for the vehicle V, can eliminate issues of delayed response, excessive error, hunting, sensitivity to linear accelerations, stability. Platform leveling can be enabled while driving, as well as extending the main boom and other functions that previously had to limit platform leveling because of their sensitivity to linear accelerations.
In addition, linear platform velocity can be determined directly and used for vehicle V control. The main application is to limit maximum linear speed of the platform 12 for operator safety without sacrificing speed in mechanically disadvantaged portions of the workspace.
While embodiments disclosed herein have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, features of various implementing embodiments may be combined to form further embodiments of the invention.
