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Page 30 of 31 Songthumjitti et al. Intell Robot 2023;3(3):306-36 I http://dx.doi.org/10.20517/ir.2023.20

parameter in the admittance model, . However, the high-frequency oscillation force is greatly reduced com-
pared with a feed-forward system, which means the influence of insufficient structure stiffness is compensated
with feedback.

7. CONCLUSION
In this study, we know that the human-machine system will face an unstable situation when a human operator
is in contact with the machine when the stiffness of the structure is not high enough. Therefore, we have two
methods of adding a compensator to the system that helps improve the stability region of the human-machine
system, so we can use a lighter weight in the admittance model, which means the operator can work in a lighter
load environment.

The first one is a feed-forward compensator, which uses the inverse of the structure transfer function to cancel
out structure characteristics that cause oscillations. The simulation section shows that the stability region is
greatly improved over an uncompensated system, and we can use a lower parameter in the admittance
model. Even though the actual structure parameter is different from the measurement, the stability region
is smaller than the ideal one, but it still shows an improvement over an uncompensated system. To confirm
our simulation, we conduct an experiment with the parameter in the admittance system equal to 2 kg. The
result is as expected, an uncompensated system will oscillate when a human operator is in contact, but a feed-
forward system can remain stable. Therefore, this experiment can confirm that this method of compensation
can improve the stable operation range of the system.

With feed-forward compensation, the stability region is greatly reduced by the difference between the measure-
ment and the actual parameter. So, we tried a feedback compensator with an accelerometer, which is another
method to compensate for structure characteristics. Although in simulation, it shows that the stability region
is not improved from an uncompensated system as much as a feed-forward system. However, we consider
conditions when a structure parameter is different from what we measure. It shows that the feedback system
is greatly improved over the feed-forward system, and it can remain stable when actual structure parameters
change. From the experiment, it is also confirmed that the feedback system has an advantage over the feed-
forward system, with = 1 kg feedback system can remain stable, but the feed-forward system is unstable.
This indicates that simple feedback using accelerometers can compensate for the insufficient stiffness of the
robot structure and greatly enhance the stability of the human-machine system.

Furthermore, we plan to refine the admittance model design because we know that the stability of the system
also depends on human interaction, which causes differences in each movement due to changes in human
impedance, so we will use a dynamic variable in the admittance model. It will be changed according to human
movement and task so the operator can move the object with a light force during a traveling task and can
perform a high-precision task when needed. The feedback compensation studied in this study should increase
the range of variation in the admittance model.

DECLARATIONS
Authors’ contributions
Conducted the experiment, simulation, data acquisition, and data analysis: Songthumjitti N
Conceived and designed the study: Inaba T

Interpreted the results: Songthumjitti N, Inaba T

Availability of data and materials
Not applicable.