TEC-B-01 User Manual
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version 1.1
page 29
Very important
: All parameters that influence clamp performance (microelectrode offsets, capacity
compensation, etc.) must be optimally tuned before starting the PI controller tuning procedure (see
chapter 5). Always activate the OSCILLATION SHUTOFF unit.
The tuning procedure involves the following steps:
Again
: The main criterion of tuning is the amount of overshoot seen at the potential output.
Tuning of the proportional gain
Use the command input without smoothing and apply adequate, identical pulses to the cell (e.g.
small hyperpolarizing pulses).
The controller is in P-mode (proportional only). Watch the potential output and rise the GAIN
so that no overshoot appears (LO method). The response to a command step is slow and has no
overshoot (potential output). The response to a disturbance, e.g. synaptic input or an activating
channel, is slow and has a large deviation.
Since the integral part of the controller is disconnected a steady state error in the range of a few
percents will be present.
Tuning the integrator (SLOW mode)
Reconnect the integrator to form the complete PI controller by setting the voltage clamp control
mode switch to SLOW.
Apply adequate test pulses without filtering.
Adjust the integrator time constant (#
19
) to achieve the overshoot of the selected optimization
method (4% with the AVO method and 43 % with the SO method). With the AVO method the
response to a command step is very fast with 4% overshoot (potential output). The response to
a disturbance, e.g. an activating channel is slow and has a slight deviation. With the SO method
the response to an unsmoothed command step is fast with 43% overshoot (potential output).
The response to a disturbance, e.g. an activating channel, is very fast and has a slight deviation.
Now the steady-state error must disappear.
Note
: If the SO is used, an external command input filter can be used to smooth the command signal
and consequently reduce the overshoot according to the requirements of the experiment (see also
Figure 6).
Tuning the series resistance compensation (FAST mode)
The optimization methods mentioned above cannot be applied if series resistance is present and has to
be considered.
Generally, 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. In experimental situations
where very high clamp speed is desirable (e.g. recording of gating currents), the clamp speed can be
improved additionally by optimizing the position of the electrodes and using SERIES RESISTANCE
COMPENSATION (see also Greeff and Polder, 1998; Greeff and Kühn, 2000).
