Technology

What is force control?

Force control is the key technology to make robotized grinding and polishing possible.  Surface finishing is a process that starts after the forming phase (milling, molding,
welding,…). The product has already its form and the finishing equipment has to adapt to this form. By controlling the contact force between the grinding media and the product, the result is constantly of high quality, in spite of a variety of disturbing factors.

Why force control?

• To align exactly robot program and workpiece position;
• To machine complex part shapes with simple robot programs;
• To maintain contact during full life time of grinding medium
• To intercept unknown workpiece shapes
• To maintain an accurate contact force regardless orientation

Different types of force control:

Around-the-arm force control &  Through-the arm force control

Around-the-arm force control:

Around-the-arm force control is a method that is based on using the robot arm for positioning and motion only. This method de-couples the force control from the robot controller and servomotors. The force device can be stationary or mounted on the robot. The robot mounted force control device, is an auxiliary tool that adds an additional axis of motion to the system. This axis of motion provides the compliance that is necessary for automated surface finishing.  Around the arm force control is often used for grinding/polishing operations.  Around-the-arm-force-control can be active or passive.

Active force control: 
The force control continuously monitors the force applied and immediately corrects for any detected force variations.  Active closed-loop control provides many additional features not available in the less sophisticated passive systems.  For example the ability to precisely weigh the payload and compensate for gravity induced forces in any orientation. 

Passive force control: 
A passive force device is a simple, low-cost compliant unit for many applications.  Using a low friction cylinder and linear rail technology this device delivers a constant accurate force (also for low force levels).  This type of force control is well suited for applications with grinding forces  remaining the same orientation with respect to gravity.

Through-the arm force control:

Through-the arm force control is accomplished by mounting a six axis force/torque transducer to  the robot wrist plate and then attaching a tool to the transducer. To apply a given force, the robot pushes the tool against the part surface and a force/torque transducer outputs a corresponding electronic signal.  The robot control integrates this signal in a control loop and changes its motion path in order to obtain the desired force.  This type of force control is rarely used for grinding applications since it is slow and still needs a additional compliance between robot and workpiece.