Robots are unlike any other mechatronic system. Robots are typically 5 to 6 axis platforms, which, while they may be made up of the same components as any other mechatronic systems, they are subject to very complex control requirements. The control of the robot has to take into account the weights and sizes of each of the robot actuators as part of the load.
The impact of this is tremendous. Each joint in the robot has incorporated into it the weight and offset distance of the next dependent joint in the mechanical system. These features are the kinematic definition of the robot, where the joints are, how they are connected and what the range of motion is for that axis.
As it is typically the case for the motion control industry, there are some unique exceptions. The hexapod robot uses 6 parallel linear actuators organized in pairs to manipulate a triangular tooling bracket. The other member of the parallel linear robot family is the Stewart platform, or delta robot, which is composed of 3 parallel actuators that are based on rotary motors. And, of course, additional axes for manipulators, end effectors and rotary actuators can all be added to any of these platforms.
In most motion applications the actual path in a Cartesian work area is not an issue. When an automated storage and retrieval system goes to pick up a stored pallet, the exact path that is taken is not a concern. The big distinction in robotic applications is that the actual path of the motion is, in fact, critical.
If, for example, a previously stored pallet were to have been left hanging halfway out of its shelf, it would present an obstacle that the motion system would have to account for to avoid a dangerous collision. And that is the first major difference between general motion control applications and Robot or CNC applications. In Robots and CNCs, there is a potential for collision. And the obstructions can change. So there is often a requirement for vision to be integrated with robots, and special sensors to protect expensive cutting tools in CNCs. The exact trajectory of the motion is not just the shortest distance between two points.
The second major distinction of Robot applications is that unlike most other types of machinery, the relationships between the axes are constantly changing. In the serial type of robot, each axis becomes part of the load for the axis before it. The one issue being that the base axis, for example has to manage the weight of everything else in the robot. Even worse, the responsiveness of the base axis is significantly impacted by where the other axes are geometrically. As the robot reaches further away from the base, the load is increasing. So controlling the load becomes more difficult and the dynamics may be changing continuously at the same time.
Amazingly, all of these challenges have been met by custom controllers designed for use in robotic applications. In the case of systems like the Stewart platform, robots can package candy and cookies at rates of 300 placements per minute. Seeing these machines operate in real life is amazing.
A true mechatronic challenge that engineers have been successful with for years.
Filed Under:Actuators,Mechatronic Tips,Linear motion • slides,Robotics • robotic grippers • end effectors
