Servofloat function

The servofloat function limits the torque and disables position error monitoring for the selected robot axes.

Servofloat is useful for applications with motions where the robot is expected to not reach the target position (e.g. due to tolerances of the workpiece) or where external forces may be applied to the robot (e.g. workpiece being pushed out of an injection molding machine).

Servofloat can be used axis-wise (link servofloat) or cartesian (linear servofloat).

For both of these servofloat types there are eight setting files available in the robot controller. The settings need to be written before any motion takes place (e.g. during startup).

Library components

Availability

Please refer to the function block documentation to see which robot controllers support this function.

Parameters

Below overview shows the relevant parameters for this function.

// Yaskawa parameters (cannot be changed by customer)
FD34      1       // link servofloat
          3       // link & linear servofloat

Restrictions

• A standstill is required to switch servofloat on/off
• Linear servofloat is only supported by selected manipulators
• Servofloat cannot be used for jog commands

Monitoring

Servofloat active status

Certain events (e.g. MLxEnable) cause the servofloat to be switched off. Therefore it is necessary to monitor the servofloat status so that immediate action can be taken in case the servofloat was switched off unintendedly during the process.

This status is not part of the MotoLogix data packet but you can map it to one of the ReadPacket.DigitalInputs bits by adding a few lines in the robot CIO:

// CIOPRG.lst (PART2)
// MLx ReadPacket.DigitalInputs[0].0  <-- servofloat ON status of R1 (see Robot CIO manual)
STR #50510
OUT #00010

TCP feedback data

By default, the actual TCP position red by MotoLogix is the command position. However, when performing servofloat motions the feedback position is more useful:

In below image the same servofloat trajectory is ran twice:

• first cycle shows command data
• second cycle shows feedback data

BlendFactor

The blend factor requires some attention in a program with servofloat motions. Below rules apply to the last motion before activating/deactivating servofloat.

Before activating servofloat:

• BlendFactor=0 results in a smooth transition into servofloat mode
• Other blend factors might reduce cycle time but lead to a rough transition
• Consider using a custom blend factor

Before deactivating servofloat:

• BlendFactor=0 can lead to a unsuccessful fine positioning alarm and should be avoided
• Consider using a custom blend factor

Creating a custom blend factor

You might consider to create a blend factor which is smaller than the default ones. This way you can make a setting which has a narrow positioning window but lacks the long time which comes with BlendFactor=0.

Pick a blend factor which is not yet used in your program and change its setting. Example: change blend factor 8 to a distance of 0.5mm:

Example

A typical use case is placing a workpiece into a fixture:

• The height of the workpiece varies due to manufacturing tolerances
• The workpiece will be pushed with a predefined force into the fixture using servofloat
• Once in position, fixture clamps close and the gripper opens

Below pseudo code gives you an idea of the program structure for this use case.

// write files in startup routine
10  SetServoFloatProperties   // write servofloat settings
                              //  - ServoFloatNumber := 0 (pendant: #1)
                              //  - ServoFloatType := 1 (linear)
// -----------------------------------------------------------------------------

// production
10  SetUserFramePlace         // activate the user frame for the fixture

20  MovePtpApproach           // PtP motion to the approach position
                              // (transition: Sts_PC)

30  FloatOn                   // activate the linear servofloat function with settings #1
                              //  - ServoFloatNumber := 0
                              //  - ServoFloatType := 1

31  MoveLinBottom             // linear motion to the bottom of the fixture
                              // motion performed with limited Z-axis force
    <in fixture>              // product pushed into fixture with the pre-set force
                              //  - actual command position still moving
                              //  - actual feedback position no longer moving

    <motion completed>        // motion completed
                              //  - actual command position no longer moving
                              //  - actual feedback position no longer moving
                              // (transition: Sts_PC)

40  <close fixture>           // close fixture clamps

41  <open gripper>            // open robot gripper

50  FloatOff                  // deactivate the servofloat function
                              //  - actual command position will change to reflect
                              //    the physical robot position

51  MoveLinApproach           // linear motion to the approach position