What is Delta Robot Control?

Although a relatively recent development in robot technology, delta robots have gained popularity in high speed pick and place applications due to their novel design which separates the heavier actuator mechanisms (motors and gearheads) from the actuator linkage mechanism resulting in very fast point to point moves.

In addition to three drive motors, each of which controls the position of the robot’s three ‘upper arm’ members, many delta robots have a fourth axes allowing actuation operations at the end effector (also called the platform) such as rotation or torque control. Each upper arm segment connects to a pivoting forearm member which has a special ‘parallel’ linkage design which maintains alignment of the end effector platform with the base plane (the plane where the three drive motors are located.

The most important motion control task for controlling delta robots is managing its complex motion kinematics. Using the dimensions of the delta robot’s upper arm, forearm, base plate, and platform plate, a kinematic transformation is executed to convert motion in the XYZ platform coordinate space to the coordinate space of the drive motors, generally referred to as θ1, θ2, θ3.

An example of this is shown in the diagram below. Executing a simple linear XY move with fixed Z in the end effector space results in very different required profile trajectories in each of the three drive motors. Motion controllers generally achieve this by directly executing the reverse kinematic equations in the controller, or by pre-calculating and storing a 2D or 3D kinematic transform map in memory. In both cases the goal is to convert a stream of desired XYZ positions into the corresponding stream of θ1, θ2, θ3 position commands.

Once the coordinate transform system is in place another key consideration of delta robot control is use of s-curve profiles for point-to-point moves. Compared to trapezoidal trajectories which have instantaneous transitions in acceleration, s-curve profiles introduce a segment to transition from one acceleration to another. A non-zero acceleration transition time is crucial for minimizing the amount of vibrational energy injected into the end effector linkages and load.