Servo Stimulator Upgrades
Variable Parameter Output
A new set of experiments required that stimulus parameters (such as velocity and amplitude) can be changed. The previously described stimulator was therefore upgraded with some new features. First, we replaced the servo motor with a Futaba S9650. This servo features digital control, a coreless motor, and double ball bearings, making it move quicker and smoother than the generic model used in the old version of the stimulator. The servo motor is now mounted on a 3-axis manipulator and connects to the stimulated tissue with a solid pushrod and a small hook (Fig. 1).
Figure 1: The new servo motor is mounted on a XYZ-manipulator and connects to the stimulated tissue with a solid pushrod and a small hook.
Besides these mechanical changes, the control circuit was extended with a D to A converter (Microchip MCP4922) which sends the position signal on a BNC output (Fig. 2). This new output indicates when a stimulus is given during an experiment, and what parameters are used. As it reflects the command given to the servo motor (rather than the position feedback of the servo-control loop) it is however not useful for the accurate recording of the position of the stimulator tip. This would be trickier to build.
Figure 2: The controller was upgraded with a DAC (the piggybacked board) which sends the position command to the BNC output. Also added was
a 4mm grounding socket ()the green socket in the front) to reduce electric noise.
The most severe changes were done to the software loaded onto the controller. As the program is getting increasingly complex (and to facilitate future modifications), it was re-coded in PicBasic Pro, a commercial BASIC compiler for PIC microcontroller. While the two potentiometers now control velocity and amplitude of the stimulus, the second button enters a mode in which a very small stimulus is given in approx. 1 second intervals. The sourcecode can be viewed here.
The new features now allow to control stimulus parameters and to mark the stimulation on a recording during an experiment.
Trigger input and display
The latest incarnation of the stimulator features a TTL trigger input as well as a LC-Display indicating the current settings. The TTL input is used with a Protek B8400 function generator that allows very long intervals between trigger pulses. The current PicBasic sourcecode for the controller can be found here.
