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PMAC configuration
20-sim configuration

Igus robotic arm


We are currently doing a project that is named "Igus robotic Arm". The main goal of this project is to develop a low-cost robotic arm that can be used in several different applications. The project is being done at Fontys University "Kenniscentrum Mechatronica".


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Igus


Igus is a German supplier for bearings and cable management systems and so on. Currently they're also producing parts for the realisation of robotic arms. Examples of these parts can be seen in figure 1 & 2. The joint in figure 1 has 2 degrees of freedom (DOF), pivoting and rotating. The joint in figure 2 has 1 degree of freedom, rotating.
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Figure 1: RL-50-001-WS

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Figure 2: RL-50-TL1-WS














The build of the system at Fontys can be seen in figure 3.
2 joints with 2 DOF and a shoulder joint is used. The total sums up to 5 DOF.
  • Rotating movement (DOF2 en DOF4) -> ±270 degrees from center
    • Total angle = 540 degrees -> 1,5 rotation
  • Pivoting movement (DOF1 en DOF3) -> ±90 degrees from center
    • Total angle = 180 degrees
  • Rotating movement shoulder joint (DOF5) -> ±90 degrees from center
    • Total angle = 180 degrees -> 0,5 rotation
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Figure 3: Setup robotic arm


Each DOF has an incremental encoder attached. The resolution for the pivoting and rotation is different:

  • Pivoting
    • 31 pole pairs
    • 40 pulses/polepair
    • 160 positions/polepair
    • 1240 pulses/revolution
    • 4960 positons/revolution
  • Roterende beweging
    • 29 pole pairs
    • 40 pulses/polepair
    • 160 positions/polepair
    • 1160 pulses/revolution
    • 4640 positions/revolution
  • Accuracy
    • Pivoting -> 360/4960 ≈ 0,0726 graad
    • Rotation -> 360/4640 ≈ 0,0776 graad

Information encoder

See also:
http://www.igus.de/Blog_DE/category/robolink-blog.aspx

Mechanical design

The mechanical design is dependent from a few elements;
  • Tensioning wheel
  • Motors
  • Bearing
  • Coupling
  • Rope management

Tensioning wheel

For realising the movement of the joint it's necessary that the ropes can be driven. At the current setup this is done through drivewheels from Igus. The advantage of these wheels are:
- ropes can be tensioned
- drivewheel size equals joint size (easy math)
The drivewheel can be seen in figure 4.
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Figure 4: New tensioning wheel


User manual tensioning wheel (German)


Motors

Maxon motors with Maxon Motor Controllers (4-q-dc) are used in the new assembly. The torque is an important aspect of these motors.

Because 2 different movements are made, it is important to look at the need torque and forces. At igus the following data is available:
Pivoting movement -> 12[Nm] at force 600[N] on the rope
Rotating movement -> 5[Nm] at force 300[N] on the rope

The diameter from the motor driving wheel equals the diameter in the joints, therefore:
Pivoting movement -> 2*12[Nm] = 24[Nm]
Rotating movement -> 2* 5[Nm] = 10[Nm]

The motor selection delivered the following;
  • Torque pivoting movement ±27,88[Nm]
  • Torque rotating movement ±15[Nm]





Bearings

Because the radial forces on the drive shaft are to high (max. radial forces exceed max. allowed radial forces gearbox), bearings are used. In this way the radial forces don't exceed the maximum allowed radial forces on the gearbox.
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Figure 5: Concept bearings

The used bearings are of the type:6201-2Z & 6301-2Z:
SKF 6001-2Z PDF
SKF 6301-2Z PDF


Coupling

The output of the gearbox must be coupled to the drive shaft, therefore we fitted a coupling. This coupling has a high torsional stiffnes and it can handle a small misalignment.
MCOCGWK38-12-12 product page


Rope management


The shoulder joint has a rope management system, in this way the ropes get seperated.
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Figure 6: Guiding of ropes



Total concept


All of the above is used in the total concept.


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Figure 7: drivetrain (motor to drivewheel)



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Figure 8: Simulation of forces on the design


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Figure 9: Final concept in 3d



The drawings from the designed parts can be seen here:






Motion controller


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Figure 10: Motion controller schematic



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Figure 11: Control system PMAC


20-sim

The possibility to implement the motion controller in 20-sim is currently investigated. The goal is to simulate the robotic arm and it's controller.

FRF motors
The Frequency Response Function from the motors be seen in figure 12 & 13.
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Figure 12: FRF motor pivoting movement

igus_frf_bode_rot.jpg
Figure 13: FRF motor rotating movement


In 20-sim a torque actuator with an inertia and a damper is used to simulate the used motors.
igus_modelvorming_motor.jpg
Figure 14: Model motor



Sources

http://igus.de/_wpck/pdf/global/robolink_04_12_s.pdf
http://www.maxonmotor.com/maxon/view/content/