Overview
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Intelligent Control of
Robot Manipulators for Performance Enhancement.
The design and production of
high-performance servo-systems for robot manipulators in a cost effective and
timely manner is one of the ultimate challenges to engineers today. Three key
performance criteria for modern robot manipulators are 1) speed, 2) accuracy and
repeatability and 3) vibration suppression. To respond to performance
requirements becoming more and more stringent, the mechatronics approach, which
gives considerations to both mechanical hardware, sensing and actuation devices,
and servo software, must be pursued. The proposed research aims to enhance the
performance of robots with indirect drive mechanism. In indirect drive
mechanisms, the actuator (electric motor) and the link are connected via gears
for speed reduction and torque amplification. The harmonic gear is the most
popular means for this purpose; it is compact and may reduce the speed at the
order of 100 in one stage. This allows the use of high speed and low torque
motors resulting in various advantages such as lower costs and lighter weights.
In the servo control of indirect drive robot joints, the current
practice in industry is based on loop-by-loop gain tuning. In other words, a set
of feedback gains is selected to specify the bandwidths of three servo loops:
current (acceleration) loop, velocity servo loop and position servo loop. One
advantage of this method is that tuning may be performed sequentially from the
inner acceleration loop to the outer position loop. If we have good online
measurements of state variables, we can expect that the closed-loop system will
achieve the desired overall bandwidth with good robustness properties. However,
the motor side position is usually the only measurement in industrial
applications and other states must be estimated by observer or state estimator.
The tuning of control gains and the design of observers is further complicated
because the dynamics of robots depend on configuration. Namely, a set of gains
tuned for one configuration may not work well for other configurations. The
performance may be further limited due to parameter uncertainties and nonlinear
effects such as friction, hysteresi (or backlash) and nonlinear compliance.
We plan to
overcome the above mentioned difficulties in the robot servo systems by applying
intelligent control and sensing methodologies. More specifically, we will study
the following six items over the next twelve month period: 1) the use of robust
control theory for systematic tuning of control gains, 2) the application of 1)
above for selected configurations and establish optimal gain scheduling, 3)
compensation of friction forces, 4) modeling and identification of robots to
study oscillations and vibrations, 5) investigation on vibration characteristics
and suppression of vibrations, and 6) improved design of mechanism and
controllers for reduced vibration. The study will include analysis, simulation
and experimentation. Concrete results are expected from the first three tasks.
The remaining three tasks are related to a long term goal of establishing the
design methodology for mechanism and control algorithms to eliminate or reduce
oscillations and vibrations in trajectory following.
(Prof. Masayoshi Tomizuka, 2005) |
