System description

The Pendubot is a planar under-actuated robot with two rotational joints. The two links are connected by a passive rotational joint and the first link is directly connected with the motor axis. Next figure shows two perspective images of the Pendubot.



Next figure shows a photographic image of the Pendubot together with the power state, the UPM module (universal power module).

Both the two links have a mobility of 360 degree. This is possible thanks to 6 slipring contacts set at the base of the first link, as shown in the figure above. The slipring contacts allows the signals transmssion without requiring a fixed link. Nevertheless, the slipring contacts cause a non negligible fricition.
The first link has a flattened shape, to offer a higher stiffness, needed for a faster transmission of the motion to the non actuated link.
Every joint has an icremental encoder. The motor encoder has a resolution of 1/8192 of total angle, while the encoder on the passive joint has an half resolution (1/4096 of total angle). The angular velocities of the links are obtained from the position informations.

The actuator is a direct current motor with permanent magnet (Pittman 14205), drived in current by the UPM module. The power module modulates the power flow using a control signal, and send it to the power source of the motor. The signal applied to the motor is a PWM signal (pulse width modulation). The PWM control is useful to improve the throughput of the power states (greather than 0.9) and to obtain higher gains.

Driving the motor with a current signal instead of a voltage signal we obtain a system characterized by a lower dependence from the motor parameters (like the torque constant, the motor resistence, etc.), which often depend from the operating conditions.

In front of the Pendubot we have installed a fixed webcam, that allows to record the experimental activities. To have better light contrast and cleaner images we put white panels behind the system. This solution also improves the MPEG compression for the digital video.

The presence of a non actuated link, give more interest to the postion control experiments with the second link in the up position.

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