Autonomous Mobile Manipulator: What It Is and When to Combine an AMR and a Cobot

The robot that cannot move and the one that cannot pick

A plant has an AMR that carries boxes of components from the warehouse to the lines. It works well, but when it reaches its destination someone has to unload them and put them on the line's shelving. In another area, a cobot loads a machine, but only that machine: when the work moves elsewhere, the robot stands idle.

The autonomous mobile manipulator combines both capabilities: an autonomous mobile robot that moves around the plant and a collaborative arm mounted on top that handles parts when it arrives. It is a solution that makes a lot of sense in certain processes and is worth understanding well before deploying it.

What is an autonomous mobile manipulator?

It is an AMR, that is, a robot that navigates autonomously using LiDAR sensors and maps of the facility, with a cobot, a gripper and usually a vision system installed on top. The AMR provides mobility; the cobot provides the ability to pick, place or operate.

How AMRs work in intralogistics is covered in the article on autonomous mobile robots for internal logistics. Here we focus on what changes when the mobile robot not only transports, but also handles.

Which tasks it handles

• Line feeding. Transporting components and placing them directly on the line's shelving or workstations.

• Tending several machines. Serving several CNC or injection moulding machines with a single robot, moving between them as demand requires.

• Moving samples. In laboratories or quality control, taking parts to measuring stations and collecting them.

• Order picking. Picking items from shelving and placing them in containers.

• Shifts without staff. Keeping processes running that previously required someone to move material between workstations.

Accuracy: the main challenge

An AMR positions itself with a repeatability in the order of millimetres or centimetres, depending on the model and navigation system. For example, the base of the Rebots AMR+COBOT mobile manipulator states an indoor repeatable positioning accuracy of ±10 mm. A cobot, by contrast, works with repeatabilities of hundredths of a millimetre.

This means that when it reaches its destination, the robot does not know exactly where the station is relative to its arm. To pick or place accurately, that difference has to be compensated. The usual solutions are:

• Vision on the cobot's wrist, which locates reference markers or the part itself at the station and corrects the position.

• Physical markers or references at each station, which the robot detects before working.

• Docking stations, where the AMR positions itself against mechanical stops.

Without this compensation, a mobile manipulator is only useful for tasks with very wide tolerances.

Power and autonomy

The AMR powers its own drive as well as the cobot, gripper and vision. The arm's consumption reduces autonomy compared with an AMR that only transports. Charging cycles must be planned: automatic charging at a station, charging during periods of low activity or quick battery swap, depending on the model. Youibot bases, for example, use lithium iron phosphate (LiFePO4) batteries and support automatic, manual or quick-swap charging.

Safety on the move and during handling

A mobile manipulator combines two types of risk: those of a vehicle moving among people and those of an arm handling parts. The AMR includes obstacle and person detection for navigation, and the cobot provides power and force limiting. But the complete application must be assessed: the arm should not move while the AMR is travelling, the transported part must not protrude dangerously and the working areas at each station must be analysed one by one.

Fleet management and integration

When there are several mobile robots, a fleet management system assigns tasks, manages priorities and battery charging, and prevents gridlock in the aisles. In Youibot's case, the YOUI-FLEET system can coordinate up to 100 AMRs. Integration with plant systems such as MES, ERP or WMS allows tasks to be generated automatically according to production.

When it makes sense and when it doesn't

It is a good fit when the same handling task is repeated at several points in the plant, when volumes at each point do not justify a fixed robot, or when the aim is to keep processes running outside staffed shifts. If the task is concentrated at a single high-throughput workstation, a fixed cobot will be faster and simpler. If you only need transport, an AMR without an arm is enough; the different models are compared in the article comparing Youibot's AMRs.

At Rebots we work with Youibot mobile bases and Dobot cobots, although the arm can be adapted to the brand the customer prefers, along with vision systems for position compensation. You can see the solution on the autonomous mobile manipulator page. If you have a task spread across several points in your plant, tell us about your process and we will study whether it fits.