Keywords: Control , Comparison of 10 different controller strategies, Bilateral teleoperation; Delayed systems; Force reflection; Intrinsically passive controller; Stability

The possibility of operating in remote environments by means of telecontrolled systems has always been considered of relevant interest in robotics. For this reason, in the literature a number of different control schemes has been proposed for telemanipulation systems, based on several criteria such as passivity, compliance, predictive or adaptive control, etc. In each scheme, major concerns have been on one hand the stability, which may constitute a problem especially in presence of time delays in the communication channel, and on the other the so-called transparency of the overall system. This article aims to compare and evaluate the main features and properties of some of the most common control schemes proposed in the literature, firstly presenting the criteria adopted for the comparative study and then llustrating and discussing the results of the comparison. Moreover, some general criteria will be presented for the selection of the control parameters considering that, due to time delay, a tradeoff between stability and performances

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System under consideration:

Both master and slave are identical simple 1-dof system of which the dynamics are considered to be linear (eg linearized with a proper local controller). (see eq. 1)
The transmission delay is considered to be constant.


Control Schemes:

1) Force Reflection (FR):
position information from master to slave, force information from slave to master
(Note: same as KFF structure in other journals)

2) Position Error (PE)
Forces applied to master and slave depend on the position-difference between them.
(Note: same as PERR structure in other journals)

3) Shared compliance control (SCC)
FR controller with extra compliance termn in slave controller:
Force on master depends on force information from slave
Force on slave depends on both force information from master and position-difference
(Note: same as KFF structure + extra compliance term in slave to modify the desired displacement received from the master site accordingly to the interaction eith the environment. THIS IS NOT P+FF, because position error is only used at the slave not at the master!)

4) Force reflection with passivity (FRP)
FR control scheme with extra damping injection term (at both master and slave side)

5) Intrinsically passive controller (IPC)
Both directions: Force and velocity exchanged through a virtual mass/damper system in the controller (see figure 2)

6) Four channels (4C)
Generic telemanipulation control scheme in which both velocity and force information are exchanged between master and slave.

7) Adaptive motion/force control (AMFC)
Each manipulator has its own local adaptive position/force controller and position/force tracking command are exchanged between master and slave. 4 Channels are used: force and velocity in both directions.

8) Sliding-mode controller (SMC)
A SMC is defined at the slave side in order to acieve a perfect tracking n finite time of the delayed master position while an impedance controller is used at the master side. This controller offers robustness and can deal with time-delay.
4 variables are send from master to slave: delayed position, velocity, force human, force Fedd
1 variable is send from slave to master: force Fed

9) Predictive control (PC)
In controller strategies above: information from the slave is used as feedback to the master. The PC algorithm uses a Smith predictor at the master side to anticipate computation of the delayed information from the slave, whereas a simple PD controller is implemented at the slave. (Very similar to FR, but know force from master computed by both the predictor and force feedback from the slave).

10) Predictive control with passivity (PCP)
Prediction method combined with wave variables.


Comparison criteria:

1) Stability as function of time-delay:
IS (Intrinsically Stable): Scheme is guaranteed to be stable independently of delay T (In this article: also IS if stable when parameters are chosen by using some rules of thumb, not really random choosing)
PS (Possibly Stable)
2) Inertia and damping perceived at the master side when no force is extended on the slave manipulator
3) Traching at the save side without interaction (eg: slave moves in free space) (Steady state error)
4) Stiffness perceived at the maser in case of interaction with a structured environment at the slave
5) Drift between master and slave position in case of interaction at the slave side

Comment:
Only the schemes based on passivity quarantee intrinsic stablility.
Section 5: Tuning of parameters for the various controllers


Consideration:
Each telemanipulation scheme has both positive and negative aspects and should be choosen based on the application under consideration:
- Time delay: how big, constant, variable, etc (permits to choose scheme because of stability aspects)
- Desired performance (tracking and perception of environment)
- Aspects concerning implementation (sensors available, computing power, transmission bandwidth)
- Knowledge of the remote environment structure (stiffness, damping, forces)


Note: Most relevant references in the journal article are conference reports that are not available (yet) on the TU/e.
List with references for the several control schemes