TEC-B-01 User Manual
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version 1.1
page 28
9.2.
Tuning Procedures for VC Controllers
The initial settings (see
voltage clamp
in chapter 5.5) guarantee only a stable clamp that is not very
accurate and insufficiently rapid for certain types of experiments, e.g. investigation of fast voltage-
activated ion channels or gating currents. Thus, for successful and reliable experiments, it is necessary
to tune the clamp loop.
Only if no intracellular series resistance is considered tuning of the clamp is performed according to
optimization methods. It depends on the type of experiment to which method one should follow (see
below).
o
“
L
inear
O
ptimum” (LO)
with this method only the proportional part of the PI controller is used. The response to a
command step is slow, but produces no overshoot. The response to a disturbance is also slow
with a large deviation of the membrane potential. Clamp accuracy is a maximum of 90-97%
(Finkel and Redman, 1985). Therefore, this method should only be used only if it is very
important to avoid overshoots of the membrane potential.
o
"
A
bsolute
V
alue
O
ptimum" (AVO)
uses the PI controller and provides the fastest response to a command step with very little
overshoot (maximum 4%). The response to a disturbance is of moderate speed and the
amplitude of the deviation is only half the amplitude obtained with LO. It is applied if
maximum speed of response to a command step is desirable e.g. if large voltage activated
currents are investigated.
o
"
S
ymmetrical
O
ptimum" (SO)
uses also the PI controller and has the best performance compensating intrinsic disturbance
signals. The response to a command step shows a very steep rise phase followed by a
considerable overshoot (maximum 43%). The response to a disturbance is fast and the
amplitude of the deviation is in the same range as with the AVO method. The overshoot can be
reduced by adequate shaping of the command pulse by a delay unit (Froehr, 1985; Polder and
Swandulla, 1990; Polder and Swandulla, 2001). This method is preferred for slowly activating
currents, such as those evoked by agonist application.
The upper speed limit for all optimization methods is determined by the maximum amount of current
which the clamp system can force through a given electrode (see chapter 9.3).
Practical Implications
In the following some practical implications of the theory discussed earlier in this chapter are outlined.
It is assumed that you have read the last chapters, that all connections are set up as described in chapter
4.3 and that the system is in VC mode with the initial settings described in chapter 5.5.
Although most of the parameters of the control chain are not known during an experiment it is possible
to tune the clamp controller by optimizing the response to a test pulse applied to the COMMAND
INPUT. The main criterion of tuning is the overshoot seen at the potential output. Since the SO
method provides the tightest control it will be most sensitive to parameter settings and requires much
experience.
Note
: The transitions between the optimization methods are blurred and the tuning procedure is
adapted to the experimental requirements. Often, the adequate tuning of a clamp system can be tested
by specific test signals (e.g. stimulus evoked signals, etc.).
