AGVs use various move­ment concepts to navi­gate flex­ibly and effi­ciently in various appli­ca­tions. In intral­o­gis­tics, two methods are gener­ally used: differ­en­tial or omni­di­rec­tional move­ment. These methods differ signif­i­cantly in terms of maneu­ver­ability, space require­ments and control complexity. In a differ­en­tial move­ment, the AGV is controlled by two inde­pen­dently driven wheels and their speed differ­ences. This means that the vehicle has a high degree of maneu­ver­ability and can rotate on the spot, which is partic­u­larly advan­ta­geous in tight spaces.

However, this move­ment requires complex control algo­rithms and can cause stability prob­lems on uneven surfaces. Omni­di­rec­tional AGVs, on the other hand, can move in any direc­tion, including side­ways and diag­o­nally, thanks to special driving steering systems. This type of move­ment provides maximum flex­i­bility and enables precise navi­ga­tion in confined envi­ron­ments. The disad­van­tage is higher costs and increased control effort, as several drives have to be coor­di­nated simul­ta­ne­ously.

„Thanks to the ECI motors, it is currently the most compact system on the market, it is easy to inte­grate and is modu­larly designed.“

Dr. Ing. Robert Michel, Product Manager at b-drives

Differ­en­tial move­ment

Wheel motor systems for auto­mated guided vehi­cles (AGVs) must fulfil key require­ments: This calls for compact solu­tions that can be flex­ibly adapted to the specific appli­ca­tion require­ments thanks to their different motor­iza­tion. High load capac­i­ties are often required, as well as over­load capacity when braking and accel­er­ating. The wheel motor systems must also always guar­antee reli­able and safe oper­a­tion and also impress with their price/performance ratio.

A successful example is the very compact eWheel devel­oped by b-drives GmbH for bidi­rec­tional navi­gating vehi­cles with BLDC motors from ebm-papst installed. “With the eWheel, trans­port systems can reach their desti­na­tion safely even in the tightest of spaces. Thanks to the ECI motors, it is currently the most compact system on the market, it is easy to inte­grate and is modu­larly designed,” as stated by Dr. Robert Michel, Product Manager at b-drives.

With stator lengths of 20, 40 or 60 mm, the ECI 63 motors cover the power range from 150 to 370 W. (Image | ebm-papst)

The rated output of the ECI 80 motors is up to 750 W. (Image | ebm-papst)

To ensure that the wheel motors can with­stand the tough oper­ating condi­tions in logis­tics, the actu­a­tion elec­tronics for the two motors in the wheel motor are housed in a solid housing. Plan­e­tary gear­boxes ensure the neces­sary reduc­tion ratio. (Image | b-drives)

Flex­ible design options

The modules with two driven wheels are designed as a modular system and are avail­able in six motor sizes, three gearbox reduc­tions and five wheel diam­e­ters. This enables the user to choose between different lengths of 109 to 238 mm at maximum speeds of 0.7 to 3.7 m/s/ Since the torques required in the appli­ca­tions are also different, the modular system also offers a selec­tion here. For example, for the smallest length, the maximum torques are between 29 and 100 Nm and the largest between 53 and 100 Nm. All versions have a high load capacity of up to 500 kg per wheel. In line with the motors, b-drives also offers a double-axle controller with up to 80 Aeff peak current, CANopen and Safe Torque Off (STO). If other bus systems such as ProfiNET or EtherCAT are desired, the corre­sponding indi­vidual elec­tronics are also avail­able for this purpose.

Modular drives

Two types of motor are used in the wheel motors: The brush­less, elec­tron­i­cally commu­tated ECI 63 internal rotor motor with 63 mm diam­eter (Fig. 3) and the larger ECI 80 with 80 mm diam­eter (Fig. 4). With stator lengths of 20, 40 or 60 mm, the larger ECI 63 motors cover the power range from 150 to 370 W at up to 880 mNm nominal torque and 4,000 rpm nominal speed. Their effi­ciency is over 90 %. The same three stator lengths are avail­able for the ECI 80 motors; the rated output ranges from up to 750 W.

“The motors are avail­able in safety extra-low voltage of 24 and 48 V, meaning that b-drives with motors from ebm-papst can also easily adapt the wheel motors to the rele­vant appli­ca­tion at this point”, adds Steffen Schmidt, Market Manager Intral­o­gis­tics at ebm-papst. Thanks to the safety extra-low voltage, the require­ments for the docu­men­ta­tion are manage­able and commis­sioning can be simpli­fied. The over­load capacity of the motors up to three times the rated torque is also impor­tant for the appli­ca­tion area, espe­cially when starting up with large loads or if the AGV has to over­come bevels or uneven­ness.

The STO func­tion (Safe Torque Off) inte­grated into the drive controller also ensures a safe stop (PLe). Holding brake and STO can also be trig­gered together. In addi­tion, the speed of the drive can be reli­ably moni­tored by a suit­able control system (Safe Limited Speed) and ensure that the vehicle only moves in the approved direc­tion (SDI, Safe Direc­tion). The motors have two inde­pen­dent encoder systems as stan­dard: the inte­grated hall sensors for commu­ta­tion and a high-reso­lu­tion incre­mental coder for control. By comparing both signals, the elec­tronics can reli­ably record the speed in a func­tional manner.

An alter­na­tive encoder can be used if required, e.g. a Sin/Cos encoder or a TTL or HTL incre­mental encoder. It is also possible to use safety func­tions via PROFIsafe. “The ECI motors are compat­ible with Simatic Micro-Drive; we can then install the corre­sponding encoder in the wheel module,” explains Dr Michel. The inno­v­a­tive, compact and versa­tile wheel module will open up a wide range of appli­ca­tions thanks to this flex­i­bility, espe­cially as it also impresses with its good price/performance ratio.

Omni­di­rec­tional move­ment

In appli­ca­tions where maneu­ver­ability, load capacity, dynamics or good fine posi­tioning are required, omni­di­rec­tional vehicle concepts are usually used. Ebm-papst offers the ArgoDrive the right driving steering system including wheel. Each drive unit consists of two brush­less DC motors, trans­mis­sion, sensors and connec­tion plugs. The two ECI 63 motors with a stator length of 20 mm contribute to steering, accel­er­a­tion, move­ment or braking with a super­po­si­tion gear, depending on require­ments. If the wheel is aligned or no steering move­ment takes place, the entire power of both motors can be used for driving; this makes the ArgoDrive unique and partic­u­larly effi­cient. With a rated output of just 300 W for both motors, it can move loads of up to 100 kg, 300 kg or 500 kg per drive unit in the Light, Stan­dard and Heavy versions. The infi­nite steering angle enables omni­di­rec­tional vehicle move­ment, even from a stationary posi­tion.

Omni­di­rec­tional move­ment requires at least two Argo­Drives per vehicle. This enables partic­u­larly narrow and maneu­vering AGVs to be achieved with two driving steering systems arranged oppo­site each other and two cam rollers. The number of systems used can also be increased as required to increase the load capacity of the vehicle. With four driving steering systems and four fixed rollers, for example, loaded vehi­cles with a total weight of 4 t can be moved extremely dynam­i­cally, omni­di­rec­tion­ally and with high posi­tioning accu­racy.