Autonomous mobile robot vehicle (AMR) and method for detecting inconsistency in motion of an amr

Abstract

The invention relates to an autonomous mobile robot vehicle (AMR). The AMR (30) comprises a vehicle body (13), at least one steerable traction wheel (STW) (21, 21a-d) having a steering axis (41), and a controller (31). Each STW comprises at least one traction wheel (44, 44l, 44r), which comprises a traction actuator (43, 43l, 43r) and has a traction axis (42) orthogonal to and intersecting the steering axis (41). The total number of traction wheels (44, 44l, 44r) is at least two. The controller (31) is configured to receive, from at least one of the STWs (21, 21a-d), a signal representing a steering angle (53 or 46) around the steering axis and signals representing a traction speed (45, 45l, 45r) of the at least one traction wheel of the STW, and to detect inconsistency in motion between individual traction wheels of the at least one traction wheel and the vehicle body based on the received signal representing a steering angle and the signals representing a traction speed.

Description

[0001] ABB Schweiz AG

[0002] Bruggerstrasse 66, 5400 Baden, Switzerland

[0003] Autonomous mobile robot vehicle (AMR) and method for detecting inconsistency in motion of an AMR

[0004] Technical Field

[0005] The invention relates to an autonomous mobile robot (AMR) and method for detecting inconsistency in motion of an AMR.

[0006] Background

[0007] Autonomous mobile robots (AMR) including sub-categories such as Automated Guided Vehicles (AGV) can be designed with steerable traction wheels (STW) and supporting casters. Steering angles and traction speeds have to be coordinated in order to ensure rolling motion along a planned path. Otherwise, there is unwanted sliding between tires and floor, resulting in poor path accuracy, increased energy consumption, and wear. Furthermore, at standstill, steering angles can be coordinated such that they interlock and prevent rolling motion of the vehicle. A vehicle controller is used to coordinate and monitor motion. STWs for AMRs include sensors in each STW to signal steering angle and traction speed to the vehicle controller. These signals may need to qualify for functional safety, which adds cost to the individual STW. In addition, traction wheels require a mechanical safety brake which engages at power-loss or when triggered by a safety function. Such a brake adds to the cost and to energy consumption.

[0008] Summary of the invention

[0009] There may be a desire to provide an improved AMR.

[0010] The described embodiments similarly pertain to the autonomous mobile robot (AMR) and the method for detecting inconsistency in motion of an AMR.

[0011] Synergetic effects may arise from different combinations of the embodiments although they might not be described in detail. Further on, it shall be noted that all embodiments of the present invention concerning a method might be carried out with the order of the steps as described, nevertheless this has not to be the only and essential order of the steps of the method. The herein presented methods can be carried out with another order of the disclosed steps without departing from the respective method embodiment, unless explicitly mentioned to the contrary hereinafter.

[0012] Technical terms are used by their common sense. If a specific meaning is conveyed to certain terms, definitions of terms will be given in the following in the context of which the terms are used.

[0013] According to a first aspect, an autonomous mobile robot vehicle (AMR) is provided. The AMR comprises a vehicle body, at least one steerable traction wheel (STW) having a steering axis, and a controller. Each STW comprises at least one traction wheel, which comprises a traction actuator and has a traction axis intersecting and preferably orthogonal to the steering axis. The total number of traction wheels is at least two. The controller is configured to receive from at least one of the STWs a signal representing a steering angle around the steering axis and signals representing a traction speed of the at least one traction wheel of the STW, and to detect inconsistency in motion between individual traction wheels of the at least one traction wheel and the vehicle body based on the received signal representing a steering angle and the signals representing a traction speed.

[0014] The term “AMR” includes Automated Guided Vehicles (AGVs) and driverless transportation vehicles.

[0015] The STWs are mounted on the vehicle body. A local coordinate system can be assigned to the vehicle body such that one axis points, for example, the x-axis, to the nominal forward direction, i.e., the direction in which the vehicle would move when the STWs are not steered (0°). This axis may also be designated as “reference axis”. The steering angle of an STW in this disclosure is measured with respect to a line parallel to this reference axis through the corresponding STW.

[0016] Each STW comprises one or more traction wheels. Although the STW are “steerable traction wheels”, the “traction wheels” in this disclosure differ and are not to be confused with the steerable traction wheels (STW). To avoid confusion, in this disclosure, the term “steerable traction wheel” is never abbreviated as “traction wheels”. The minimum number of STWs is one and the minimum total number of traction wheels with traction actuators is two. For example, in a minimum configuration where the vehicle has one STW, the STW has two traction wheels. In another example, the vehicle has two STWs with one traction wheel each. In further examples the vehicle has two, three or four STWs, each equipped with one or two traction wheels.

[0017] The vehicle may further comprise passive wheels, so-called “caster wheels” that are not driven by a motor, but which stabilize the vehicle.

[0018] The STWs have an axis perpendicular to a ground plane, and around which the STW moves. The movement may be obtained passively or actively, i.e., driven without or with using an actuator such as a motor. The traction wheels are always driven by a motor, where the motor is centered on the turning axis of the traction wheel, and either centered at the midpoint of the wheel or offset to the side.

[0019] The STWs signal their steering angle and their traction speeds to the controller. The controller calculates the motion of the vehicle and detects an inconsistency between individual traction wheels, for example their velocity vector, including measured speed of the traction wheel and direction, which is based on the measured rotation angle defined by the steering angle of the STW, and the motion of the AMR vehicle, if there is an inconsistency. In brief, an inconsistency in motion means that the motion of the vehicle, i.e. speed and direction, does not fit to the steering angle of at least one of the STWs and / or the speed of at least one of the traction wheels or other parameters as presented in this disclosure. The motion of the vehicle may be defined, for example, by the actual radius and / or center-point of the path of the vehicle body, its path speed, and its direction. In other words, the controller determines whether the movement of the vehicle, which is the same movement as the vehicle body, for example, in terms of path speed fits to the steering of the STWs, i.e., around the axis of an STW perpendicular to the ground, and to the angular speed of the traction wheel. The angular velocity and angular speed can be determined from successive angular position measurements. Under “movement of the vehicle”, for example path speed, direction or direction angle, and radius of curvature of path is understood, where “path speed” is the speed along the trajectory or path the AMR is moving, as planned or calculated by the controller Embodiments are described in the following and examples are given and explained in the figures. All signals representing a steering angle and all signals representing a traction speed are obtained by independent measurements. The measurements may be performed by sensors arranged on the STWand traction wheels. The sensors or communication units connected to the sensors then generate the signals and provide them to the controller over a wire or wirelessly.

[0020] By the term “individual traction wheels”, the single traction wheels of the at least one traction wheel of any STW is understood.

[0021] By the term “traction speed”, the angular speed of the traction wheels is understood. A skilled person is aware that the angular velocity corresponds to a Cartesian velocity via the radius of the traction wheel or a linear velocity, dependent on the radius of the traction wheel, which again corresponds to a velocity of the vehicle body at the position along a vertical axis, which coincides with the point, where the traction wheel has contact with the ground.

[0022] The controller may be further configured to calculate the kinematics of the AMR according to a kinematic model, which contains in particular the geometry of the vehicle body, the STWs, the position of the STWs on the vehicle, etc.

[0023] According to an embodiment, the controller is further configured to calculate the actual radius and / or center-point (51) of the path of the vehicle body (13), a path speed (56) of the vehicle body (13), and the direction angle (55) of the vehicle body (13) using the signals received from the at least one STW.

[0024] These parameters are the essential parameters to compute the vehicle motion for a comparison with, for example, expected values or allowed values or ranges and / or for calculating corresponding values for the expected motion of the traction wheels.

[0025] According to an embodiment, signals representing a steering angle and / or signals representing a traction speed are available only from a part of the STWs and traction wheels, and the controller is configured to use only these available signals for detecting the inconsistency.

[0026] Expressed the other way round, only some of the STWs may comprise sensors that deliver the signals for calculating the inconsistency. For calculating the speed, direction and radius of the vehicle, it is not necessary to have sensors on each STW and on each actuation wheel. For example, sensors are arranged only on one STW that deliver the signals for the STW steering angle and speed of the traction wheel. In another example, there is a sensor for the steering angle of one STW and two sensors for sensing the speed of the traction wheels of other STWs. That is, several combinations are possible. As a simple example for inconsistency, when the steering angle of an STW is large, the speed of a traction wheel can only have values in a certain range, dependent on the kinematic model, i.e. the geometry of the vehicle and the arrangement of the STWs on the vehicle. As another example, if the vehicle moves but the STWs are steered nearly 90° to each other, there may be an inconsistency.

[0027] According to an embodiment, the controller is configured to calculate steering angles and traction speeds that are not covered by the available signals, using the available signals, where the available signals represent a steering angle and / or a traction speed.

[0028] The controller is capable to determine the steering angles of the STW and the angles or speed of the traction wheels if no signals for them are available, based on the available signals. For example, the steering angle of four STWs and the speed of one traction wheel is available. Then, the controller calculates the steering angle and / or speed of the further traction wheels based on the available signals and therefore to control motion of the vehicle by controlling the steering of all STWs and traction wheels. Regarding consistency, in this case, the controller can detect an inconsistency of the motion of the vehicle with respect to the received signals, and it can determine whether a plausible solution for the further traction wheels can be found. Of course, the example can also be applied accordingly to receiving signals from four traction wheels and one STW or other constellations.

[0029] According to an embodiment, the controller is further configured to calculate the actual radius and / or center-point of the vehicle body, the path speed, and the direction angle of the vehicle body using the signals from each STW and each traction wheels, wherein a part of the signals are received from sensors with high quality and a part of the signals are received from sensors that are of lower quality than the high quality sensors.

[0030] In other words, instead of no further sensors, sensors of lower quality may be used such that signals for each STW and traction wheel are available. This allows using economic sensor, where, nevertheless, an inconsistency can be detected. With such a configuration, the consistency regarding any traction wheel and STW can be proven. For example, the expected values forthe steering angle and speed of the STWs and traction wheels with low quality sensors can be determined and compared to a threshold that includes the allowed tolerance and that takes into account the tolerance of the low quality sensors. Compared to a configuration with no sensors instead of lower quality sensors, the actual traction speed and steering angles of all traction wheels and STWs can be measured, and therefore the consistency of each individual traction wheel and STW.

[0031] According to an embodiment, the controller is further configured to determine the inconsistency of motion as follows: in case that the number of STWs is more than one, form pairs of STWs and to calculate values of the actual radius, center-point of the vehicle body, the path speed, and / or the direction angle of the vehicle body using the signals from each pair of STWs, and in case that one or more of these values exceed a pre-set limit, determine that there is inconsistency of motion with respect to excess slip or a malfunction and to reduce speed of the vehicle or perform a protective stop.

[0032] For example, if the AMR has three STWs up to three pairs may be formed, and if the AMR has four STWs up to 6 pairs may be formed.

[0033] According to an embodiment, the controller is further configured to weight or filter the signals.

[0034] Dependent on the geometry of the arrangement of STWs and traction wheels and the movement of the vehicle, small changes of the values may have a greater or a lower impact on a stable solution of the calculations. I.e., the sensitivity increases or decreases. This may be handled by weighting the signals in the calculations, e.g., applying a low weight to the pair of STWs, for which the sensitivity is high, or by varying the pre-set limits. E.g., the calculations may prefer those signals that are from sensors of traction wheels in an insensitive geometrical condition, or the limits are set more generously for signals from sensors where the conditions are sensitive, e.g., by applying a weight to the range of the limits. The weighting may include a mapping in steps or continuous mapping between the sensitivity and the weight. The signals may further be filtered over time or number of samples such that peaks are filtered out and the signal is smoother.

[0035] For example, if, for a pair of STWs, the center point of their circular paths, which is in this disclosure also referred to as instantaneous center of rotation (ICR), one of its STWs is close to a steering axis, the significance of the pair of STWs is lowered and the controller reduces the weight of the signals of this STW pair. In other words, the significance of the signals of an STW pair is lowered in dependence on the distance of the STW that is nearest to or coincidences with a steering axis.

[0036] According to an embodiment, the controller is configured to determine inconsistency of motion if one or more differences between the calculated values, which are at least direction of the vehicle body and the path speed, and corresponding set values exceed pre-set limits.

[0037] If difference exceeds the pre-set limits, then there is an excessive path error, which triggers a corrective set-motion, or a speed reduction, or a protective stop.

[0038] According to an embodiment, the controller is configured to determine inconsistency of motion using only one pair of STWs or two traction wheels of one STW, and to correct the set-motion, or reduce the speed, or to perform a protective stop.

[0039] The controller may consist of two components, a functional controller responsible for motion planning and control and a safety controller, responsible for safety reactions, e.g., protective stop. Changing the planned path or speed is the responsibility of the functional controller; supervising all motion, including such adaptations, is the responsibility of the safety controller. In an example for this embodiment, only signals from sensors of only one pair of STWs or two traction wheels of one STW are qualified for functional safety. Then, the safety controller may use only these qualified signals. The “functional controller” can use any available signals.

[0040] The decision whether to correct the set-motion, or the speed reduction, or the protective stop may depend on a policy taking into account which limit is exceeded, which signal led to exceeding a limit, the type of limit, etc. This applies also to other embodiments in this disclosure.

[0041] According to an embodiment, the controller is further configured to calculate the rate of change of the steering angle of each of the STWs, and, if the rate of change of at least one STW exceeds a pre-set limit, to reduce speed, to perform a protective stop, to perform a coordinated motion of each of the STWs to compensate for the path error, or to boost traction power.

[0042] A high rate of change is usually not planned by the navigation routine of the controller but indicates that the AMR or a wheel has hit an obstacle or has no contact to the floor.

[0043] According to an embodiment, the controller is configured to, if the set speed of the AMR is zero but the calculated speed exceeds a pre-set limit, to steer the pairs of STWs to interlocking steering angles.

[0044] If the set speed of the AGV is zero but the calculated speed exceeds a pre-set limit then the AGV is in uncontrolled motion, which causes the controller to trigger the above-mentioned functions.

[0045] According to an embodiment, the controller is further configured to steer one or more STWs quickly and, in case of more than one STW, simultaneously to an angle orthogonal to the current angle of motion, in case a protective stop is triggered while the AMR is driving.

[0046] This is done with a speed and in a steering direction such that it results in a sliding motion of the one or more STWs on the ground. Ideally, the AMR will keep the current direction of motion and will reach a faster stop compared to braking with the traction motors.

[0047] “Quickly” means faster than the vehicle is capable to follow the rotation of the wheels, i.e., it performs no curve motion, but either stops immediately or slides until it stops in the direction of motion before the steering, according to inertia of mass.

[0048] According to an embodiment, the AMR comprises further a safety brake acting on the traction tires, a traction shaft of the traction tires or on the ground or floor.

[0049] The brakes are an additional measure to the above-mentioned measures. The brake may be engaged, for example, if the AMR is intended to park, if power fails, or as part of safety functions, which include removing power from the traction motors. The safety brake acting on the ground or floor may be obtained by disengaging the traction wheels from the floor, e.g. by lifting up the AMR.

[0050] In embodiments, the controller is further configured to execute a navigation function on an independent core or as an independent task.

[0051] Expressed the other way round, the controller is configured to execute the functions specified in the embodiments above in one or several tasks, threads or cores different from the navigation. In particular, one of these independent cores or tasks execute at least one safety function as part of an implementation of functional safety according to IEC 61508 or associated standards. Such safety functions monitor actual values and trigger or resume from safe states if certain conditions are met.

[0052] According to a further aspect, a method for detecting inconsistency in motion of an AMR is provided. The AMR comprises a vehicle body, at least one steerable traction wheel (STW) having a steering axis, and a controller. Each STW comprises at least one traction wheel, which comprises a traction actuator and has a traction axis intersecting and preferably orthogonal to the steering axis. The total number of traction wheels is at least two. The method comprises the steps carried out by the controller: receiving, from at least one of the STWs, a signal representing a steering angle around the steering axis and signals representing a traction speed of the at least one traction wheel of the STW; and detecting inconsistency in motion between individual traction wheels and the vehicle body based on the received signal representing a steering angle and signals representing a traction speed.

[0053] In embodiments, the method further comprises steps corresponding to the configuration of the controller as described herein.

[0054] The controller may comprise circuits without programmable logics or may be or comprise a micro controller, a field programmable gate array (FPGA), an ASIC, a Complex Programmable Logic Devices (CPLD), or any other programmable logic devices known to person skilled in the art.

[0055] An executable program may be provided which, when running on the controller or on a computer, causes the controller or computer to perform the above-mentioned method steps. The executable program may be stored on a computer readable medium.

[0056] The computer readable medium may be seen as a storage medium, such as for example, a USB stick, a CD, a DVD, a data storage device, a hard disk, or any other medium on which a executable program as described above can be stored.

[0057] According to a further aspect, a use of sensors attached to an STW of an AMR as described herein is provided, to determine the steering angle of the STW and traction speed of a traction wheel of the STW, to calculate the steering angle of a further STW if the AMR comprises more than two STWs and the traction speed of a further traction wheel. The sensors may be used by a controller that receives the corresponding signals to detect inconsistency in motion between individual traction wheels and the vehicle body based on the received signal representing a steering angle and signals representing a traction speed. For the calculations, a kinematic model may be used that comprises the geometry of the AMR including, for example form factors of the vehicle body and any devices such as STWs and traction wheels mounted on the AMR as well as positions of the devices with respect to the vehicle body. It may further comprise additional static information required for calculating the status and behavior of the vehicle, the STWs and traction wheels. The kinematic model can be created and applied by a skilled person within the scope of engineering and according to his professional skills.

[0058] The presented AMR proposes an AMR vehicle with one or more steerable traction wheels (STWs) and methods for motion supervision and motion control, which eliminate the need for mechanical brakes for the traction wheels and which reduce the cost of components to measure actual steering angles and traction speeds. These methods are implemented in the motion controller of the vehicle. Preferably, they are implemented to execute on two or more independent cores or tasks where at least one is executing safety functions as part of an implementation of functional safety according to IEC 61508 or associated standards. Such safety functions monitor actual values and trigger or resume from safe states if certain conditions are met. The methods are based on the availability of redundant signals about the actual motion.

[0059] It is assumed that the sensing and control system work properly, and the signals are correct. The sensor signals are used to determine any number of application related quantities, e.g., vehicle speed, vehicle direction, vehicle path radius, etc. To enforce conformity of such quantities with limits imposed according to the requirements of the specific application and operating environment, the values thus obtained from the sensing and control system are compared to the predetermined limits (safety functions make such comparisons). Here is where the functional controller could alter the path or reduce the speed preemptively to avoid tripping a safety function. The safety controller simply observes the situation, continuous comparing to the given limits, and does nothing if the limits are obeyed, and triggers a protective stop when the limits are violated.

[0060] These and other features, aspects and advantages of the present invention will become better understood with reference to the accompanying figures and the following description. Short Description of the Figures

[0061] Fig. 1a shows a sketch with first bottom view of the bottom side of a vehicle.

[0062] Fig. 1 b shows a sketch of a first STW type.

[0063] Fig. 1c shows a sketch of a second STW type.

[0064] Fig. 2 shows a sketch with second bottom view of the vehicle.

[0065] Figs. 3a and 3b show a sketch with two further bottom views where the STWs are steered such that they interlock.

[0066] Figs. 4a, 4b and 4c show a sketch with bottom views that illustrate further exemplary embodiments of robotic vehicles with steerable traction wheels.

[0067] Fig. 5 shows a flow diagram of the method for detecting an inconsistency in motion of an AMR.

[0068] Detailed Description of Embodiments

[0069] Corresponding parts are provided with the same reference symbols in all figures.

[0070] Figs. 1 a, shows a bottom view 10 of the bottom side of a vehicle with vehicle body 13 showing the vehicle body 13 of an AMR and two different types of STWs. Fig. 1 b shows a sketch of a first STW type, and Fig. 1 c shows a sketch of a second STW type.

[0071] Four STWs 21a, b, c, d are mounted near the corners of the vehicle body 13. The STWs are of different types 11 , 12: STWs 21a, b of type 11 as shown in Fig. 1 b have two traction wheels 44I, 44r each, whereas STWs 21c, d of type 12 as shown in Fig. 1c have only one traction wheel 44 each. STWs of type 11 may be steered passively. In this case it comprises no actuator or motor, as shown in Fig. 1 b, or it may be steered actively and therefore comprise an actuator 47, as the one shown in Fig. 1c.

[0072] The STWs comprise further sensors (not shown in Figs. 1a-1c) that measure the steering angles 46 of the STWs around a rotary axis 41 , herein referred to as steering axis 41 and the traction speed deduced, for example by the angle or rotational speed 45I, 45r or 45 around rotation axis 42, herein referred to as axis of traction 42. The traction axis 42 is intersecting orthogonal to the steering axis. As a further variation, the STW may have a single traction wheel 44 and a steering actuator 47 as shown in Fig. 1c, the one traction wheel 44 sits on the ground where also the steering axis 41 hits the ground. However, in variants, the traction wheel has an offset to the side instead of being aligned with the steering axis 41 . However, the most advantageous variant is an STW with a wheel aligned with the steering axis, i.e., without offset. In this context, the angle between steering and traction axes may be any angle that is not zero, but preferably a right angle.

[0073] Further, a controller 31 is mounted at the vehicle body 13.

[0074] Fig. 1a further shows a communication link 32 between the STWs 21 a, b, c, d and the controller 31 as indicated by double arrows. The communication link 32 is used to transmit the signals of the sensors to the controller, and to transmit control signals for controlling the steering angles of the STWs and the traction speed of the traction wheels. That is, the STWs and / or sensors and the controller 31 comprise communication circuits. These circuits may communicate using a wired or a wireless communication protocol according to a corresponding communication standard or a proprietary protocol.

[0075] Fig. 1a further shows the steering angles 53a-d of each STW 21 a-d and the momentary circular paths 52c-d. The momentary circular paths 52c-d have one ICR 51 .

[0076] Although Fig. 1a shows communication links to and from each STW 21 a, b, c, d, the controller 31 does not necessarily communicate with STWs 21a, b, c, d, that are fully equipped with sensors. Moreover, the sensors may be sensors of different quality such that not each of the sensors has to be qualified for functional safety. In other words, the controller is configured to receive from at least one of the STWs 21a, b,c,d, a signal representing a steering angle around the steering axis 41 and signals representing a traction speed 45 of the at least one traction wheel of the STW. It is further configured to detect an inconsistency in motion between individual traction wheels and the vehicle body based on the received signal representing a steering angle and the signals representing a traction speed.

[0077] For detecting an inconsistency, the controller 31 runs a computation in parallel to motion planning and motion control. This computation uses a kinematic model of the AMR vehicle and all available signals from the STWs. The computation comprises at least one of the following functions.

[0078] Fig. 2 shows a bottom view of the vehicle 13. Due to the type of STWs, there are two momentary circular paths 52dr, dl for steering wheel number d, i.e. for the right and left traction wheels, and two momentary circular paths 52cr,cl for steering wheel number c, i.e., for the right and left traction wheels. 53a and 53b indicate again the steering angle of for steering wheel numbers a and b.

[0079] As a first function, herein also referenced as function a), the controller calculates the actual radius of the vehicle body or center-point 51 , the path speed 56, and the direction, i.e., the direction angle 55 of the AMR vehicle with respect to the forward direction, when no curve is performed. This includes a first extreme case where the vehicle turns on the spot. In this case, the radius is zero and a second case when the vehicle travels on straight line. In this case, the radius is infinite. For the calculations, the controller uses signals from each pair of STWs. That is, if the AMR has three STWs, there are three pairs, if the AMR has four STWs

[0080] / NA _ NI there are six pairs, or in general V2 J 2!GV-2)I .. if the results differ above a pre-set limit, i.e., the calculated actual radius, the path speed 56 and / or the direction angle 55, then there is an inconsistency in motion which is expressed in excess slip or which is caused by a malfunction of an actuator or which causes a malfunction of the vehicle, which triggers a speed reduction or a protective stop by the controller. The excess slip or a malfunction lead to inconsistencies.

[0081] Two circumstances or items should be taken into account. i. The larger the distance of the ICR 51 to the vehicle becomes the more sensitive or “jumpy” it becomes to small changes in the signals from the STWs. Therefore, the sensitivity of the trigger for speed reduction or protective stop as described for function a) decreases accordingly. This may be either accepted or, preferably, countered by weighting the impact of the signals, i.e., the pair of STW in this condition, or by filtering the signal. For example, the pre-set limits may be varied according to the weight. The weighting may include mapping by steps or continuous mapping between the sensitivity and the weight. Filtering may include a statistical filtering over time or samples. E.g., a mean value may be calculated over a small number of samples within a sliding window, or a predictive filter may be applied. ii. The Kinematics of the vehicle have the limitation that if the ICR 51 becomes close or coincides with the steering axes 41 of one of the STWs, which is referenced as STWX in the following, then calculating the actual path speed 56 and direction 55 of the vehicle becomes very sensitive to the measured steering angle 46, 53 of STWX and traction speed 43 of STWX, and therefore inaccurate. Function a) therefore includes, that if the actual centerpoint 51 is close or equal to one of the steering axes 41 , then it lowers the significance of those pairs of STWs, which include signals from STWX.

[0082] As a second function, referenced herein as function b), the controller carries out the same calculations as for function a) but uses signals from all STWs directly without pairing them. As a third function, referenced herein as function c), the controller performs the functions a) or b) and compares the results of the corresponding function with set values of direction and path speed. If the difference exceeds pre-set limits, then there is an inconsistency, which is expressed in an excess path error, which triggers a corrective set-motion, or a speed reduction, or a protective stop.

[0083] As a fourth function, referenced herein as function d), the controller calculates the actual radius, path speed, and direction of the AMR vehicle 13 with signals from only one pair of STWs of type 12 or with signals from one STW of type 11 and compares the results with corresponding set values of direction and path speed. If the difference exceeds pre-set limits, the controller determines that there is an inconsistency, which is expressed in an excess path error, which triggers a corrective set-motion, or a speed reduction, or a protective stop. If signals from only one pair of STWs of type 12 or signals from only one STW of type 11 are qualified for functional safety, sensing of all other signals can be implemented using less expensive components.

[0084] As a fifth function, referenced herein as function e), the controller calculates the rate of change of the steering angle (53 or 46) of all STWs continuously. A sudden change of one STW above a pre-set limit may cause an inconsistency of motion and indicates that the STW hit a step, a gap on the floor, or an object, depending on the patterns of all signals. The controller 31 will trigger a safety response such as a speed reduction, or protective stop, or a coordinated motion of all STWs to compensate for the path error, or to boost traction power to push the AMR across the step or gap.

[0085] As a sixth function, referenced herein as function f), the controller monitors the speed if the set speed of the AMR is zero. If the calculated speed exceeds a pre-set limit when the set speed of the AMR is zero, then the AMR is in uncontrolled motion, which triggers the following function: the pairs of STWs are set to steer to interlocking steering angles. This may compromise path accuracy but will cause a reliable stop without the need for a mechanical brake. Interlocking steering angles are, for example, preferably 120° if the AMR has three STWs towards the other STWs. As another example, if the AMR has four STWs then the interlocking steering angle is preferably 90° towards the other STWs.

[0086] Figs. 3a and 3b show two bottom views 14 and 15, respectively, where the vehicle 13 is in exemplary situations of interlocking steering angles with respect to the vehicle body, which may be applied for a forced standstill. In the examples, the AMR has four STWs, which are steered by an interlocking angle of 90° to each other. In Fig. 3a, the angles with respect to the forward direction of the vehicle are 0° and 90°, and in Fig. 3b, the angles with respect to the forward direction of the vehicle are 45° and 135°.

[0087] Figs. 4a, 4b and 4c illustrate further exemplary embodiments of robotic vehicles with steerable traction wheels. Fig. 4a shows a bottom view 16 of a vehicle 13 with 2 STWs 21 a

[0088] 21 b and four supporting caster wheels 22. Fig. 4b shows a bottom view 17 of a vehicle 13 with one STWs 21 of type 11 with two traction actuators 43r 43I and one steering actuator 47 and four supporting caster wheels 22. Fig. 4c shows a bottom view 18 of a vehicle 13 with three STWs 21 .

[0089] Fig. 5 shows a flow diagram of the method 60 for detecting an inconsistency in motion of an AMR as described herein. The method 60 comprises a first step 61 carried out by the controller, which is receiving, from at least one of the STWs, a signal representing a steering angle around the steering axis and signals representing a traction speed of the at least one traction wheel of the STW. The method 60 comprises further a second step 62, which is detecting inconsistency in motion between individual traction wheels and the vehicle body based on the received signal representing a steering angle and signals representing a traction speed. The method 60 may further comprise additional steps according to the configuration of the controller as described herein.

[0090] Reference Numerals

[0091] 11 STW type 11

[0092] 12 STW type 12

[0093] 13 AMR vehicle body

[0094] 14, 15 bottom views of an AMR vehicle with four STWs in steered interlocking

[0095] 16 bottom view of an AMR vehicle with two STWs of different types

[0096] 17 bottom view of an AMR vehicle with one STW having two traction wheels

[0097] 18 bottom view of an AMR vehicle with three STW, each having one traction wheel

[0098] 21 , 21 a-d steerable traction wheels (STWs), steering wheel number a, b, c, d

[0099] 22 caster wheels

[0100] 30 AMR 31 motion controller

[0101] 32 bi-directional signal connection

[0102] 41 steering axis

[0103] 42 axis of traction

[0104] 43, 43I, 43r traction motor, left motor, right motor

[0105] 44, 44I, 44r traction wheel, left wheel, right wheel

[0106] 45, 45I, 45r rotational or traction speed / signal representing rotation of the traction wheel(s), left wheel, right wheel

[0107] 46 actual rotational position of the steering joint I signal representing rotation about the steering axis 41

[0108] 47 steering motor

[0109] 51 center point of the circular paths 52a-d, instantaneous center of rotation (ICR)

[0110] 52c, d momentary circular paths for steering wheel number c, d

[0111] 52dr,d I momentary circular paths for steering wheel number d, right and left traction wheels

[0112] 52cr,cl momentary circular paths for steering wheel number c, right and left traction wheels

[0113] 53a-d steering angle of for steering wheel number a, b, c, d,

[0114] 54 momentary circular path

[0115] 55 vehicle body angle or directional angle

[0116] 56 direction / speed of vehicle body at vehicle body angle 55

[0117] 60 Method

[0118] 61 , 62 Method steps

Claims

Claims1 . AMR (30) comprising: a vehicle body (13) at least one steerable traction wheel (21 , 21-d), STW, having a steering axis (41), each STW (21 , 21 a-d) comprising at least one traction wheel (44, 44I, 44r) comprising a traction actuator (43, 43I, 43r) and having a traction axis (42) and intersecting the steering axis (41); wherein the total number of traction wheels (44, 44I, 44r) is at least two; and a controller (31), which is configured to receive, from at least one of the STWs (21 , 21-d), a signal representing a steering angle (53 or 46) around the steering axis (41) and signals representing a traction speed (45, 45I, 45r) of the at least one traction wheel (44, 44I, 44r) of the at least one STW (21 , 21 a-d); and to detect inconsistency in motion between individual traction wheels (44, 44I, 44r) of the at least one traction wheel (44, 44I, 44r) and the vehicle body (13) based on the received signal representing a steering angle (53 or 46) and signals representing a traction speed (45, 45I, 45r).

2. AMR (30) according to any one of the previous claims, wherein the controller (31) is further configured to calculate the actual radius and / or center-point (51) of the path of the vehicle body (13), a path speed (56) of the vehicle body (13), and the direction angle (55) of the vehicle body (13) using the signals received from the at least one STW.

3. AMR (30) according to claim 1 or 2, signals representing a steering angle (53 or 46) and / or signals representing a traction speed (45, 45I, 45r) are available only from a part of the STWs (21 , 21 a-d) and traction wheels (44, 44I, 44r), and the controller (31) is configured to use only these available signals (53 or 46; 45, 45I, 45r) for detecting the inconsistency.

4. AMR (30) according to claim 3, wherein the controller (31) is configured to calculate steering angles and traction speeds that are not covered by the available signals, using the available signals.

5. AMR (30) according to any one of the previous claims, wherein the controller is further configured tocalculate the actual radius and / or center-point (51) of the vehicle body, the path speed (56), and the direction angle (55) of the vehicle body using the signals from each STW and each traction wheels, wherein a part of the signals are received from sensors with high quality and a part of the signals are received from sensors that are of lower quality than the high quality sensors.

6. AMR (30) according to any of the previous claims, wherein the controller is further configured to determine the inconsistency of motion as follows: in case that the number of STWs (21 , 21 a-d) is more than one, form pairs of STWs (21 , 21a-d) and to calculate values of the actual radius, center-point of the vehicle body, the path speed, and / or the direction angle of the vehicle body using the signals from each pair of STWs (21 , 21 a-d), and in case, one or more of these values exceed a pre-set limit, determine that there is inconsistency of motion with respect to excess slip or a malfunction and to reduce speed of the vehicle or to perform a protective stop.

7. AMR (30) according to any of the previous claims, wherein the controller is further configured to weight or filter the signals.

8. AMR (30) according to any of the previous claims, wherein the controller is further configured to determine inconsistency of motion if one or more differences between the calculated values, which are at least direction of the vehicle body and the path speed, and corresponding set values exceed pre-set limits.

9. AMR (30) according to any of the previous claims, wherein the controller is further configured to determine inconsistency of motion using only one pair of STWs or two traction wheels of one STW, and to correct the set-motion, or reduce the speed, or to perform a protective stop.

10. AMR (30) according to any of the previous claims, wherein the controller is further configured to calculate the rate of change of the steering angle of each of the STWs, and, if the rate of change of at least one STW exceeds a pre-set limit, to reduce speed, to perform a protective stop, to perform a coordinated motion of each of the STWs to compensate for the path error, or to boost traction power.11 . AMR (30) according to any of the previous claims, the controller is configured to, if the set speed of the AMR is zero but the calculated speed exceeds a pre-set limit, to steer the pairs of STWs to interlocking steering angles.

12. AMR (30) according to any of the previous claims, wherein the controller is further configured to steer one or more STWs quickly and, in case of more than one STW, simultaneously to an angle orthogonal to the current angle of motion, in case a protective stop is triggered while the AMR is driving.

13. AMR (30) according to any of the previous claims comprising further a safety brake acting on the traction tires, a traction shaft of the traction tires or on the ground or floor.

14. Method (60) for detecting inconsistency in motion of an AMR, wherein the AMR comprises a vehicle body, at least one steerable traction wheel, STW, having a steering axis, each STW comprising at least one traction wheel comprising a traction actuator and having a traction axis (42) intersecting the steering axis (41); wherein the total number of traction wheels is at least two; and a controller (31), wherein the method comprises the steps carried out by the controller: receiving (61), from at least one of the STWs, a signal representing a steering angle (53 or 46) around the steering axis and signals representing a traction speed of the at least one traction wheel of the STW; and detecting (62) inconsistency in motion between individual traction wheels and the vehicle body based on the received signal representing a steering angle (53 or 46) and signals representing a traction speed.

15. Use of sensors attached to an STW of an AMR according to claim 1 to determine the steering angle of the STW and traction speed of a traction wheel of the STW, to calculate the steering angle of a further STW if the AMR comprises more than two STWs and the traction speed of a further traction wheel.

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