PROGRAMMABLE CFE PULLER

Transcription

1 PROGRAMMABLE CFE PULLER Manual Pulling of PE tubing is a critical step in CFE fabrication. Getting constant shapes in CFE is difficult and to achieve a high success rate in pulling CFE requires patience and a lot of practice. We have devised an automatic puller that aims to obtain reproducible CFE shapes. Two major goals in pulling PE tubing for CFE in patch amperometry are: 1) Small bulb in the initial retraction of PE tubing that allows easy insertion of the CFE into the patch pipette. 2) Appropriate length of the CFE shank; sufficient to reach the pipette tip, but not too long to prevent curling of the CFE inside the patch pipette. The programmable puller described here is easy to build and allows reproducible shapes of CFE for patch amperometry. This Puller is based on a microprocessor (BASIC Stamp HomeWork Board, Parallax Inc., USA) programmed with the BASIC Stamp editor (Parallax Inc. USA). The puller operates by heating a filament of a glow plug by applying a constant current (1-1.5 A) and then pulling in a highly controlled manner the velocity and position of two independent servomotors by a fully programmable microprocessor. Parts List: 1 BASIC Stamp HomeWork Board (microprocessor BS2) 2 servo motors (Parallax Inc.) modified for continuous rotation (common Hobby or R/C servos) 1 transistor for switching (M734 or equivalent) 1 relay (5 volt switching voltage, 2 A max. peak current, Finder model ) 1 resistor (2.2 KΩ)) 1 resistor (8 Ω, power dissipation up to 20 watts) 1 glow plug (1300 ma; common glow plug for radio control engines) 2 Translation stages (Type BSR1540SL, RS code , RS-Amidata, Spain) 1 Metal Box Assorted cables 1 battery 9V for Basic Stamp HomeWork Board 4 batteries AA (2300 ma, 1.5 V - alkaline) or Power supply (6 V, 2 A) 2 pieces of lucite 20 x 10 x 8 mm) Assorted screws and nuts (3 mm)

2 Procedure Mechanical assembly: Step 1. Screw the two translation stages onto the top panel of the metal box. Leave a separation of 25 mm between the stages. Screw the two pieces of Lucite onto the translation stage. The two Lucite pieces were perforated and a screw with a retractable spring on one end was used to trap the PE tubing without pressing it too much. Step 2. Make a notch in the metal surface near the other end of each translation stage for the cantilever to move the stages. Screw the two servos from the inside of the box and let a cantilever from each servo protrude through its notch towards the top part of the metal box to move the stages when the servos are activated. Step 3. Connect the two translation stages to the cantilever of the servos with a small chain or metal bar. See Figures P1 and P2 below. Step 4. Drill a 8 mm Ø hole in the center between the lucite pieces for insertion of the glow plug. Before inserting the glow plug gently extract the glow filament and make a nice and round ring. A B Figure P1. (A) CFE Puller top view, (B) View of servo motor underneath.

3 Servo motors are controlled by a train of pulses (1-2 ms long) at 50 Hz. Pulses of 1.5 ms will maintain the servo still, shorter pulses will rotate in one direction; pulses longer than 1.5 ms will rotate in the opposite direction. Pulses of 1 or 2 ms will move servos at the fast rate. The PULSEOUT command from the BASIC Stamp editor control the pulse duration applied to each servo. The schematic for connecting the two servos is shown in Fig P2. Figure P2. Schematic of servo motor wiring. Pulses are generated at pins #12 and #13 of the BASIC Stamp HomeWork board. Vbp = 6 V. Vss = GND. See BASIC program below. The filament is heated by passing a constant current of 1.5 A through the glow. This current is critical to get sufficient heat at the PE tube. Use a different resistor to limit the current (3-6 Ω). The schematic of this circuit is shown in Fig P3. Figure P3. Circuit controlling the current flowing through the filament.

4 Figure P4. Puller circuit built onto the BASIC Stamp HomeWork board.

5 Programming. Programming is done using the BASIC Stamp Editor. A simple program that heats and pulls a PE tube is shown below: Adjust Heat ON and Delay before start pulling to obtain an appropriate CFE. Source code of Puller1.bs2 program: '{$STAMP BS2} 'Program: Puller1.BS2 'Purpose: Switch a Transistor connected to BASIC Stamp & Pull two Servo motors ' 'independently to make a CFE: HeatFilament VAR OUT0 'Heat on/off control HeatCtrl VAR DIR0 'Heat i/o pin control counter VAR Byte ' Counts the number of cycles Heat is ON On CON 1 Off CON 0 Initialize: HeatCtrl=%1 ' Configure PIN 1 as OUTPUT Start: HeatFilament = On PAUSE HeatFilament = Off PAUSE 5000 ' Filament ON ' Wait 20 seconds ' Filament OFF ' Wait 5 seconds before pulling FOR counter = 1 TO 75 'Start pulling procedure PULSOUT 12, 758 ' Pin 12, Pulse of ms Servo # 1 PULSOUT 13, 742 ' Pin 12, Pulse of ms Servo # 2, opposite direction PAUSE 20 ' Pulse frequency of 50 Hz for servos NEXT PULSOUT 12,750 ' Stop Servo # 1 PULSOUT 13,750 ' Stop Servo # 2