installation and control in polyscope on universal...

EZGripper Dual Robotic Gripper Control in Universal Robots PolyScope v.2 May 2017

EZGripperTM Dual 1st and 2nd Generation Robotic Gripper

Installation and Control in

PolyScope on Universal Robots

Figure 1: EZGripperTM Dual 2nd Generation Robotic Gripper


Contents List of Figures ................................................................................................................................................ 2

Downloads .................................................................................................................................................... 3

Overview ..................................................................................................................................................... 11

EZGripper Range of Motion and Action ...................................................................................................... 11

Python Control ............................................................................................................................................ 13

Calibrate ...................................................................................................................................................... 13

Goto_Position(Effort, Position) ................................................................................................................... 13

Position: ...................................................................................................................................................... 13

Effort: .......................................................................................................................................................... 13

List of Figures

Figure 1: EZGripperTM Dual 2nd Generation Robotic Gripper ........................................................................ 1

Figure 2: PolyScope – Setup Robot ............................................................................................................... 3

Figure 3: Open URCaps ................................................................................................................................. 4

Figure 4: Add URCaps File ............................................................................................................................. 4

Figure 5: Navigate to the .urcap file and Open it .......................................................................................... 5

Figure 6: Restart the Robot ........................................................................................................................... 5

Figure 7: Test the EZGripper installation: Calibrate, Open then Close the EZGripper robotic grippers ....... 6

Figure 8: New Empty Program ...................................................................................................................... 6

Figure 9: Add BeforeStart Sequence. This is important to calibrate the EZGrippers before running the

robot program ............................................................................................................................................... 7

Figure 10: Insert program lines ..................................................................................................................... 7

Figure 11: Select URCaps tab ........................................................................................................................ 8

Figure 12: Insert EZGripper instances into BeforeStart and Robot Program ................................................ 8

Figure 13: Insert Move command into robot program ................................................................................. 9

Figure 14: Select the first EZGripper in set to Calibrate ............................................................................... 9

Figure 15: Instance of EZGripper showing Calibrate command ................................................................. 10

Figure 16: Set the other EZGrippers to settings shown. ............................................................................. 10

Figure 17: EZGripper Dual Gen2 Robotic Grippers with Links, Joints and Range of Motion ..................... 11

Figure 18: EZGripper Gen1 robotic grippers in various state of position from fully open to fully closed . 12

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Downloads The EZGripper drivers for Universal Robots PolyScope can be downloaded at

https://github.com/SAKErobotics/ezgripper-ur

Copy the ezgripper-ur-1.3.2.urcap onto a USB stick.

Stick the URCap USB stick into the Universal Robots Teach Pendent

Figure 2: PolyScope – Setup Robot

https://github.com/SAKErobotics/ezgripper-ur

https://github.com/SAKErobotics/ezgripper-ur/releases/download/v1.3.2/ezgripper-ur-1.3.2.urcap


Figure 3: Open URCaps

Figure 4: Add URCaps File


Figure 5: Navigate to the .urcap file and Open it

Figure 6: Restart the Robot


Figure 7: Test the EZGripper installation: Calibrate, Open then Close the EZGripper robotic grippers

Figure 8: New Empty Program


Figure 9: Add BeforeStart Sequence. This is important to calibrate the EZGrippers before running the robot program

Figure 10: Insert program lines


Figure 11: Select URCaps tab

Figure 12: Insert EZGripper instances into BeforeStart and Robot Program


Figure 13: Insert Move command into robot program

Figure 14: Select the first EZGripper in set to Calibrate


Figure 15: Instance of EZGripper showing Calibrate command

Figure 16: Set the other EZGrippers to settings shown.


Overview Controlling the EZGripper robotic grippers is the same across generation 1 and generation 2 grippers and

in Dual, Quad or higher order configurations. The control software is available with support for Python

on Universal Robots PolyScope, Python on Linux, Python in ROS and in C# for windows. This document

focuses on the PolyScope control and related Python interface.

The EZGripper Dual robotic grippers (gen1 or gen2) are an under-actuated architecture composed of

two opposing fingers with 2 links each, all controlled by a single servo. The servo provides position

control and torque control over the 2 fingers. Each finger is composed of 2 links which rotate around

joint 1 and joint 2 shown in .

The EZGripper Dual grippers are the base

unit for control. The EZGripper Quad

(gen1) is composed of two EZGripper

Dual (gen1) grippers. The Quad can be

controlled as a single synchronized

gripper or two independent grippers.

Furthermore, groups of EZGrippers can

be assembled and controlled as

individual or synchronized grippers.

EZGripper Range of Motion

and Action The EZGripper Dual uses a single Robotis

Dynamixel MX-64AR servo to control the

two opposing fingers. The fingers are

under-actuated. As shown in Figure 17,

the fingers rotate approximately 110

degrees around both the first joint and

second joint. When the 1st link of a

finger comes into contact with an object,

the 2nd link begins to rotate around the

2nd joint thus initiating a wrapping grasp.

If 1st links never come into contact with

an object, the finger continues to rotate

until the 2nd link comes into contact with

an object (or the opposite finger). The

2nd link will tend to wrap if the object

contacts the link near the knuckle. The

2nd link will tend to stay straight if the

object comes into contact towards the tip

Figure 17: EZGripper Dual Gen2 Robotic Grippers with Links, Joints and Range of Motion


of the link. While the two opposing fingers are controlled by the same servo, they can wrap

independently around an object.

The grippers have full position control and torque

control. The gripper range of motion has a resolution

of 2500 points over the range of full open (2500) to

full close (0). This value is currently scaled within the

EZGripper driver interface to a range of 0 to 100

representing a range of 100%. The grippers can move

to any point within this range under program control.

The EZGripper fingers are pulled closed by a tendon

and are pulled open by springs. The result are fingers

that are compliant when opening which is particularly

useful when pulling the grippers out of narrow spaces

after letting go of an object. On closing, the

compliance of the EZGripper is totally under the

control of the servo via SW control.

Figure 18: EZGripper Gen1 robotic grippers in various state of position from fully open to fully closed


Python Control At the most fundamental level, the grippers have 2 basic commands:

- Calibrate

- Goto_Position(Effort, Position)

Calibrate When the EZgrippers are first turned on, the grippers must be calibrated. This should be done at startup

and can be done on command anytime the robot desires. Calibrate is quite simple. When a

calibrate(servo) command is issued, the grippers move to the closed position which resets the zero

position of the grippers. The maximum range of the grippers is covered by an ~60% rotation of the

servo (or 2500 points in an MX-64).

Resolution from open to close – 2500 units. This is scaled in the GotoPosition command to the range 0

to 100.

Goto_Position(Effort, Position)

Position: The position can range between 0 (closed) to 100 (open)

When position is 1 to 100

• The fingers move to that position

• This is the best setting when wanting to move the fingers to a specific position

When position = 0

• The fingers want to close continuously

• 0 is the best setting to use when grasping an object

Effort: The effort can range between 0 (no torque) to 100 (full torque)

When effort = 0

• The servo torque is turned off and the fingers can be easily back driven

• This is the default state after calibration.

When effort 1 to 100 AND position = 0


• The servo uses “Torque Control Mode”, an operating mode of the Dynamixel MX-64 which is

based on current sensing of the motor load (torque).

• This produces the best torque for holding objects

When effort 1 to 100 AND position > 0

• The servo uses “Position Mode”, an operating mode of the Dynamixel MX-64 which is based

on PWM control of the voltage to the motor (should be proportional to torque)

• This produces the best torque for moving the fingers1

The Effort has an automatic back-off algorithm that:

- Maintains the grasp

- Minimizes power and therefore temperature rise in the servo

The back-off algorithm is simple, automatic and applies to all effort settings.

- Close grippers until fingers stop moving (motor stops)

o Drop applied torque to 10% torque (this keeps fingers from back-driving thus holding

grasp)

- To refresh requested effort, repeat the goto_position() command call with your desired effort

Examples:

Goto_position(0, 25) # close with torque only mode with 25% requested effort – current sensed

- Will continue to apply effort no matter the finger position. This is good to maintain a grasp on

an object.

Goto_position(50, 50) # move grippers to 50% open position using 50% torque – PWM controlled

- Torque stops when the gripper is at the target position. However, any movement away from

the position runs against a torque that keeps the gripper in that position.