Increas­ingly flex­ible produc­tion processes mean that the flow of goods is also being re-imag­ined – and real­ized by AGVs. Auto­mated guided vehi­cles enable complex trans­port tasks to be completed effi­ciently in factory build­ings and logis­tics areas without rigid mate­rial flow solu­tions. Ideally, the AGVs navi­gate completely freely to their desti­na­tions, inde­pen­dently avoiding obsta­cles and stop­ping safely in front of any people that suddenly appear.

Safety must there­fore be top of the agenda for AGVs. Nothing would be more fatal for AGV manu­fac­turers and oper­a­tors than if people were injured through contact with the vehicle. Colli­sions with machines or hazardous goods could also result in signif­i­cant damage. AGVs must there­fore satisfy specific safety require­ments in terms of control, navi­ga­tion, braking, and speed moni­toring. The corre­sponding require­ments in the indus­trial sector are defined in DIN EN ISO 3691-4 “Driver­less indus­trial trucks and their systems”. The stan­dard also spec­i­fies the required perfor­mance level for moni­toring func­tions, oper­ating modes, and brake control.

AGVs must there­fore satisfy specific safety require­ments in terms of control, navi­ga­tion, braking, and speed moni­toring.

To meet the safety require­ments, the compo­nents of an AGV should be able to be inte­grated into different solu­tions as simply as possible. This requires the support of industry stan­dards and, above all, exten­sive safety features that are already embedded in the compo­nents them­selves. The drive concept of an AGV plays an essen­tial role here, enabling the vehi­cles to be config­ured cost-effec­tively and as simply as possible.

Safety role of the drive solu­tion

As far as the safety of an auto­mated guided vehicle is concerned, the safety controller and laser scan­ners for moni­toring its surround­ings play a central role. However, any actions must then be imple­mented ‘safely’ via the drives. Safety-crit­ical factors such as braking capacity, speed, steering angle and loading weight are inter­de­pen­dent and must be taken into account when executing AGV maneu­vers.

In order to be able to trigger these safe states via the safety controller when required, the drive tech­nology must be able to handle safety commands. In prac­tice, however, drives with no bus inter­faces or the asso­ci­ated safety protocol are typi­cally used, which means that addi­tional safety encoders have to be installed. For reasons of redun­dancy, a second signal is always neces­sary for moving axles. These safety encoders for moni­toring the direc­tion of travel and the safe speed of the drive cost money, take time and effort to inte­grate, and use up valu­able instal­la­tion space.

The Argo­Drive driving/steering system from ebm-papst for AGVs is a unit consisting of motors, special trans­mis­sions, sensors and all the neces­sary connec­tions, and it allows for free-range navi­ga­tion. (Graphic: ebm-papst)

For this reason, ebm-papst has already inte­grated the neces­sary compo­nents for imple­menting a redun­dancy-based safety concept in its compact drive solu­tion Argo­Drive. This driving/steering system combines propul­sion and steering func­tions in a single assembly and has the neces­sary sensors and all neces­sary connec­tions. Every Argo­Drive – designed to make it very easy to imple­ment AGVs with omni­di­rec­tional maneu­ver­ability – already has redun­dant encoder systems.

In addi­tion to the motors’ hall sensors, an addi­tional second encoder provides the required redun­dancy to reli­ably detect the exact speed. An encoder system designed for redun­dancy is also avail­able for deter­mining the steering angle. This means that the Argo­Drive already provides the most impor­tant funda­men­tals for gener­ating safety signals for the higher-level safety control of AGVs. As a result, AGV manu­fac­turers can save them­selves increased inte­gra­tion effort with sepa­rate encoders, for example, in order to record the steering angle of a slewing ring and the safe speed. Thanks to the avail­able MTTFd values from the sensors, the achiev­able perfor­mance level can also be calcu­lated using common tools.

The Argo­Drive already provides the most impor­tant funda­men­tals for gener­ating safety signals for the higher-level safety control of AGVs.

In order to offer AGV manu­fac­turers maximum freedom in the choice of safety controller, the most impor­tant commer­cially avail­able commu­ni­ca­tion stan­dards must be supported. This is why with the Argo­Drive, ebm-papst offers support for CANopen as well as EtherCAT or inte­gra­tion into the Siemens control envi­ron­ment with Profinet.

Safe stop

In an emer­gency, an AGV must come to a halt within a short distance that is appro­priate for the rele­vant situ­a­tion. Of course, if the braking char­ac­ter­is­tics of the AGV are poorer, the protec­tive field detected by the safety scan­ners can be increased, but the oper­a­tional effi­ciency and compet­i­tive­ness of the vehicle is then signif­i­cantly reduced.

AGVs can be equipped with two, three or four Argo­Drives, depending on the required perfor­mance and weight class. (Graphic: ebm-papst)

Regard­less of the size of the protec­tive field, the drive system must reli­ably execute the safe stop command in the event of danger. In this situ­a­tion, high braking power is key. The Argo­Drive can initiate imme­diate emer­gency braking as well as controlled motor decel­er­a­tion at up to 2.5 m/s2. The mechan­ical emer­gency braking torque is 60 Nm, while the motor can decel­erate with up to 30 Nm. The mechan­ical brake is also used as a holding brake, for example, to secure damaged AGVs against unin­ten­tional rolling away, even on ramps.

In the event of emer­gency braking, an AGV with four Argo­Drives installed will conse­quently come to a stand­still even faster than with two units plus support wheels – because the braking torque of all four drives is avail­able. This is where a signif­i­cant advan­tage of this solu­tion becomes apparent, as an AGV can be real­ized with just one Argo­Drive thanks to its combined driving and steering func­tion­ality.

With just two Argo­Drives, the AGV offers full omni­di­rec­tional maneu­ver­ability. And with four drive units, not only can the possible trans­port weight be scaled up, but also, as mentioned, the maximum braking decel­er­a­tion. Even if the power supply to the entire vehicle fails, the Argo­Drive goes into a safe stop to prevent uncon­trolled move­ments. The desired safe state is thus reached.

In addi­tion to a high braking decel­er­a­tion, the drive must also easily tolerate a large number of emer­gency stops without any restric­tions on func­tion­ality. In the event of an emer­gency stop, the Argo­Drive mechan­i­cally decel­er­ates the wheel to a maximum via a brake pad. For the vast majority of braking oper­a­tions, however, the elec­tro­mo­tive decel­er­a­tion of 2.5 m/s2 is always suffi­cient, because the decel­er­a­tion perfor­mance of an AGV is matched to the size of the protec­tive fields set for normal driving speed. The safety controller only initi­ates emer­gency braking if objects or people suddenly breach the narrow protec­tive field in front of the AGV, where highly dynamic real-time moni­toring takes place.

ebm-papst offers the Argo­Drive in Light, Stan­dard and Heavy versions. All of them have the same omni­di­rec­tion­ality. “Heavy” allows a maximum weight of 500 kg per drive unit. (Graphic: ebm-papst)

Inte­grated fail­safe capa­bility

In addi­tion to the redun­dancy of the encoder systems for the steering angle and speed of the inte­grated motors, the complete drive system must be highly fail­safe. Key values such as the MTTFd (mean time to dangerous failure) of elec­tronic compo­nents are a must for AGV designers.

This means that trans­porting heavy body shells or Euro pallets, designed for a weight of up to 2 metric tons, is ideally suited to AGVs with the Heavy version. (Graphic: ebm-papst)

This is because these values inform their safety calcu­la­tions just as much as the B10 values for the purely electro­mechan­ical compo­nents of the drive system, such as the brake. The B10 value indi­cates the nominal service life of the compo­nent at a survival prob­a­bility of 90 percent. SISTEMA safety soft­ware tools use these values to deter­mine whether the required fail­safe capa­bility of the AGV is possible with the drive system – or indeed any other installed compo­nent. ebm-papst has these impor­tant values for the Argo­Drive, such as MTTFd for the sensors and B10 for the brake, ready for AGV devel­opers.

The high quality stan­dards of ebm-papst ensure that the drive solu­tion is fail­safe and has a long service life. There is no need for main­te­nance during the ArgoDrive’s entire service life. This sets it apart from more mechan­i­cally complex drive concepts such as the Mecanum wheel, where regular cleaning and relu­bri­ca­tion with oil are neces­sary, for example. In addi­tion, the ArgoDrive’s unique design means that, unlike other omni­di­rec­tional drive units, it has no moving cables or contact points in the driving/steering system. The drive assem­blies and sensors are also enclosed in the housing and are there­fore not exposed to ambient condi­tions. This elim­i­nates another poten­tial cause of failure.

More than just a compo­nent supplier

Imple­menting the required safety measures is undoubt­edly a chal­lenge for AGV devel­opers, so it is impor­tant that the drive solu­tion is inte­grated into the overall concept as simply as possible. With the Argo­Drive, ebm-papst already offers a variety of inte­grated safety features that make life easier for AGV manu­fac­turers.

The Argo­Drive compact drive solu­tion from ebm-papst for AGVs has an inte­grated safety concept. (Graphic: ebm-papst | Gernot Walter)

These include simpli­fied instal­la­tion by oper­ating the drive with protec­tive extra-low voltage of 48 V DC. Design constraints for drives oper­ating at 230 V AC or higher are less of a concern, and the low voltage rating also places fewer demands on main­te­nance personnel. This not only reduces time and effort – thereby saving costs – but also increases fail­safe capa­bility and the safety level (protec­tive extra-low voltage only). Another advan­tage of the Argo­Drive is its multiple mounting points, both at the side and on the top and bottom.

AGV manu­fac­turers can also draw on ebm-papst’s inte­gra­tion expe­ri­ence. Indeed, the wealth of project-based know-how at the latter’s disposal means that it is well placed to provide recom­men­da­tions for imple­menting AGV devel­opers’ safety concepts. This applies, for example, to the readout of safety signals: what needs to be taken into account here and what pitfalls can arise with various bus systems. With the Argo­Drive, ebm-papst sees itself not only as a compo­nent supplier for the drive system, but increas­ingly also as a devel­op­ment partner for AGV manu­fac­turers.