GB2276854A

GB2276854A - Omnidirectional drive and steering unit.

Info

Publication number: GB2276854A
Application number: GB9307413A
Authority: GB
Inventors: George Robert Kiss
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-04-08
Filing date: 1993-04-08
Publication date: 1994-10-12
Also published as: GB9307413D0

Abstract

A vehicle or conveyor is driven by one or more drive units each having a pair of wheels 3 which are driven independently eg by respective electric motors 2 and gearboxes 4. The motors 2 may be mounted on plates 5 via telescopic suspension elements 6. The or each drive unit is thus rotated about a steering axis 7 by the torque resulting from the differential drive forces. A control system monitors the rotation of the drive unit about the steering axis, and other relevant variables, in order to control the motion of the drive unit relative to the driven surface. The analogue or digital motion control signals can specify instantaneous motion parameters or higher-level, macro-motion commands for specific path types. <IMAGE>

Description

Self-Steering Drive Unit This invention relates to the field of steered drive units for vehicles and material transportation (conveyor) systems.

Two main forms of steering are commonly used on vehicles. In the first form some drive element, for example a wheel, is rotated about a steering axis using an externally applied torque. The second form is often called "tank steering". Tank steering develops and applies a steering torque to the whole vehicle by using a pair of drive elements, for example wheels, belts or tracks, and by controlling the motion of the pair relative to each other.

It is already known that the use of individually steered multiple drive elements on vehicles has advantages in terms of manoeuvrability, as is the case, for example, with four-wheel steering on cars. These advantages are enhanced by omnidirectional drive elements where the steering angle can vary over 360 degrees. Omnidirectional drive elements are used in mobile robots and in material transportation (conveyor) systems. The main characteristic of these drive elements is that they can be used to drive a vehicle, or an object to be transported on a conveyor, in any direction. Such drives are also called holonomically constrained drives in contrast to non-holonomic ones, where the direction of the instantaneous velocity vector is constrained. Although a vehicle with non-holonomic drives still has two degrees of freedom (i.e. rotation and displacement), it cannot move off from its current position in an arbitrary direction and thus often has to go through more or less complex manoeuvres, like three-point turns, in order to reach specified positions in a workspace.

Omnidirectional drive units are often implemented in the form of a driven wheel and a steering mechanism, such that the steering angle can vary over 360 degrees. Such drive units are usually externally steered using a steering motor that rotates the drive wheel around a steering axis using some form of steering mechanism. The steering motor is separate from the drive motor that develops the driving torque for the wheel.

The present invention applies the principle of tank steering to drive units that can freely rotate about a steering axis relative to the chassis of a vehicle or of a conveyor system. The steering torque is developed by applying differential driving forces to the driving components by means of a control system. Such a drive unit will be called self-steering (or internally steered), because no external steering torque and steering mechanism are needed. One or more such drive units are then used to implement a complete vehicle or conveyor system.

The advantages of the drive unit described in this invention compared to externally steered drive units are: 1. Reduced energy consumption resulting from the fact that the driving torque is also used as steering torque during curvilinear motion.

2. Reduced component and production costs because of the elimination of a separate steering motor and associated steering mechanism.

3. Reduced motor cost because of the use of two drive motors of halved power output instead of one larger one, resulting in larger volume discounts. A cost saving also arises from the fact that motor cost is often a convex (from below) nonlinear function of power output rating.

A single drive unit is sufficient if the rotational degree of freedom is not a requirement. Two drive units used together on a vehicle provide both rotation and displacement. Three or more drive units can be used to provide stability and/or greater load carrying capacity.

The essential features of this invention are one or more drive units forming parts of a vehicle or conveyor system, each drive unit consisting of a pair of driving components capable of transmitting independently controllable driving forces to a driven surface and a control system that determines the velocity vector of the drive unit as a whole by controlling the driving forces. The drive unit is rotated about a steering axis by the torque resulting from the differential drive forces. A control system monitors the rotation of the drive unit about the steering axis, and other relevant variables, in order to control the motion of the drive unit relative to the driven surface. The analogue or digital motion control signals can specify instantaneous motion parameters or higher-level, macro-motion commands for specific path types.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows a close-up view of a drive unit constructed using wheels and electric motors; Figure 2 shows three drive units mounted on a vehicle chassis; Figure 3 shows the use of sensors and an analogue control system to determine the relative velocity vector between a drive unit and a driven surface; Figure 4 shows a microprocessor based digital control system for the drive unit, permitting macro motion commands to be used.

Referring to the drawing, in Figure 1 a steering axis 7 can rotate relative to a fixed element 1 that might be the chassis of a vehicle or of a conveyor system. The lower end of the steering axis is rigidly fixed to the rest of the drive unit. Each of the two wheels 3 are independently driven by electric motors 2 through gearboxes 4. The motors are mounted on mounting plates 5, using telescopic suspension elements 6 that also act as shock absorbers. The two motor mounting plates have independent suspension, each being supported by four suspension elements.

To use the drive units in a conveyor system, they can also be turned upside down, the transported object resting on the wheels.

Figure 2 shows an example of a simple vehicle with three drive units, but more or fewer can be used, depending on the application. The number of drive units can be reduced to two if castor wheels are also used to provide stability, or even to one if rotation of the vehicle is not a requirement. Although it cannot provide rotation, a single drive can still execute curvilinear motion.

Figure 3 shows an analogue control system in which a comparator receives two inputs, one from the steering command source of the vehicle (usually a high-level path planner) and one from a shaft encoder sensor driven by the steering axis 7 of Figure 1. The output of the comparator is a steering error signal that forms one of the inputs to a motion control system. The other two command inputs of this motion control system are the forward/reverse command and the road speed command, respectively. From these three inputs the control system determines the output signals applied to two motor speed controller units that might be, for example, of the pulse-width modulation type for DC motors. The sign of this motor speed control signal is determined by the forward/reverse command input. The magnitude is determined by the road speed command, suitably modified by the steering error signal. The resulting motor speed is also under closed-loop control by a feedback loop from drive shaft sensors to the speed controllers.

Figure 4 shows a microprocessor based digital control system for the drive unit, that offers the additional possibility of using higher-level macro commands to control the motion of the drive unit. These macro commands might take the form of: o an instantaneous velocity vector specification for the drive as a whole by means of an angle and magnitude pair, specifications of whole time series of velocity vectors, issued all at once, commanding the drive unit to traverse an arbitrary path at variable velocities, parametric commands for particular types of commonly used path segments like straight lines, circular or elliptic arcs, staircase patterns, etc.

In the case of a vehicle with several drive units, a higher level path planner subsystem coordinates the commands sent to each of the control systems associated with the several drive units in order to ensure coherent control of the vehicle as a whole. Together, the path planner subsystem and the drive control systems form a distributed control system for the vehicle.

Claims

1. One or more drive units (Figure 1) forming parts of a vehicle or conveyor system, each drive unit consisting of: a pair of driving components capable of transmitting independently controllable driving forces to a driven surface and a control system that determines the velocity vector of the drive unit as a whole by controlling the driving forces. The drive unit is rotated about a steering axis 7 in Figure 1 by the torque resulting from the differential drive forces. A control system monitors the rotation of the drive unit about the steering axis, and other relevant variables, in order to control the motion of the drive unit relative to the driven surface. The analogue or digital motion control signals can specify instantaneous motion parameters or higher-level, macro-motion commands for specific path types.

2. A drive unit as claimed in Claim 1 wherein the drive components are wheels, legs or tracks and the drive forces are provided by a pair of motors of any description.

3. A drive unit as claimed in Claim 2 wherein the motors are electric motors of any description.

4. A drive unit substantially as described herein with reference to the accompanying drawing.