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A stepper motor-driven microelectrode positioner
Journal of Neuroscience Methods, 19 (1987) 235-242
235
Elsevier NSM 00665
A stepper motor-driven microelectrode positioner * K. Schrnid a n d G. B/Shmer Department of Physiology, University of Mainz, Mainz (F.R.G.) (Received 10 April 1986) (Revised 14 October 1986) (Accepted 16 October 1986)
Key words: Cell impalement; Microelectrode; Micropositioner; Stepper motor The mechanical elements and the electronic control system from a stepper motor-driven microelectrode positioner is described. The unit embodies a high-precision small step angle hybrid motor. The compact, rugged and totally concentric design of the mechanic, by a spindle mechanism achieves the necessary precision by translating the stepwise rotations of the motor into steps of linear movement. The system takes advantage of commercially available low friction parts such as ball bearings, ball bushings and axles with hardened surfaces. The related electronic control unit is designed around the most recent integrated circuitry which is both sophisticated and economical. Though the described system is designed to be built in an average departmental workshop it compares favorably with more expensive commercial units and in some aspects outperforms them.
Introduction
The recording of the bioelectrical activity of neurons either in situ or in vitro is a frequently employed technique to explore the functional state of a cell. Extra- or intracellular microelectrode recordings taken from single cells of animals or plants are the most widely used methods. The controlled and precise advancement of the microelectrode, and the quick penetration of the cell membrane without damaging the cell are perhaps the most difficult problems involved in this method. These difficulties are multiplied when simultaneous recordings are made from more than one cell. Mechanically, such multirecording systems are extremely complex (Reitb/Sck and Werner, 1983) and require each electrode to be attached to its own positioning and advancing device. The simultaneous use of several microdrives necessitates that each of them is of miniature size and in addition low cost. This paper describes such a micropositioner.
* Part of a preliminary versior~ of the electronics of the described device has already been published (Schmid and B~hmer, 1985). Correspondence: K. Schmid, Department of Physiology, University of Mainz, D-6500 Mainz, F.R.G. 0165-0270/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)
236 Microelectrode micropositioners have been built using different operational principles. Simple manually driven microelectrode translators have been used since the technique of microelectrode recording was first introduced into electrophysiological research. Nowadays, refined mechanical electrode advancing systems are employed for recordings from freely-moving animals (Deadwyler et al., 1979; Pager, 1984). Devices using a hydraulic approach are still in widespread use (David Kopf; Lutz and Wagman, 1965; Veregge and Frost, 1985). Compact translators using fast-response piezoelectric elements have some advantages but until now they have severe limitations because of their range of total displacement (Nobiling and Btihrle, 1986). A combination of slowly advancing devices (manual or hydraulic) with rapid-advancing piezoelectric or magnetic elements have been used to improve cell penetration (Fish et al., 1971; Van der Pers, 1980; Lederer, 1983; Gutierrez and Salinas, 1984). Most popular positioning devices are motor-driven systems, driven either by DC motors or by stepper motors (Nanostepper, Sonnhof et al., 1982). Unfortunately, mostly these instruments are rather large and costly. For further details and discussion of the pros and cons of the different operational principles of microelectrode translators cf. Brown and Flaming, 1977; Sonnhof et al., 1982.
Design considerations We established that the projected micropositioner for microelectrode work should meet the following minimum requirements: (1) Small outer dimensions, light weight, and easy to mount. (2) Fast and precise advancement in increments as small as 1 #m over an extensive mechanical range. (3) Low cost, using commercially available materials and procedures which can be executed by a departmental workshop.
Mechanical construction Because the stepper motor is a rotating device a conversion of a rotation into a proportional linear translation has to be performed. There is a complete system commercially available (Philips-Valvo: linear actuator) that embodies the stepper motor and the conversion mechanics (Adami, 1984). Unfortunately, for the application in mind this component proved inaccurate. Since the precision conversion mechanics had to be constructed by ourselves, a spindle type mechanism promised to be easiest to construct. The key element of the stepper motor micropositioner is the stepper motor itself. One step of the motor effects in an angle of rotation of ~. The resulting linear translation S at a given spindle pitch P is defined by: S = P- ~/360 Although suitable high-precision miniature stepper motors are hard to find, one
237
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motor (Phytron, Groebenzell, F.R.G., part no. ZSS 19-200-0.6) met our stringent requirements. This hybrid motor is extremely small (19 m m outer diameter, 36 m m length) and very strong. It has a low inertia rotor and thus exerts a rather high s t a r t / s t o p frequency and has a small step angle ( ~ = 1.8"). As long as the step pulse frequency does not exceed the motor's maximum s t a r t / s t o p frequency an open loop system is reliable and error free without the need to sense and control the actual rotor position (Connor, 1973). A concentric, fully coaxial design of the linear translator was preferred (Fig. 1A) since this approach resulted in a sturdy but small mechanical system that is completely self-encapsulated and of high precision. The decision for a coaxial design was mainly influenced by commercially available ball bushings in conjunction with matching steel axles both of which are high precision parts with induction hardened steel surfaces. Because ball bushings of such compact dimensions (Deutsche Star, Schweinfurt, F R G , part no. KB 614-305) usually have no provision against torsion, a guide pin slides in a precisely ground track (tolerance: 2 / 1 0 0 ram) cut into the inner stainless steel housing (Fig. 1B). The stepper motor connected by a flexible suspension (silicone tube) is isolated against vibrations from the inner and outer guide housings. The silicone dam~ing element attached to the motor axle eliminates possible resonant vibrations of the screwed spindle supported by a ball bearing. A precision stainless steel spindle (spindle pitch: e.g. 0.4 turn) by means of a thread in
238
the guide block moves the guiding block (made of bearing metal) within the stainless steel guide housing, the inner surface of which is precision ground. The guiding block carries the guide pin as well as the induction-hardened stainless steel shaft. To shrink the total length of the actuator housing but maintain the travel of the rod at its full extent (40 mm), the screwed spindle is concentric with the shaft. In addition, the drilled centre of the shaft has the additional effect of considerably reducing the moving mass and thus leads to a better dynamic characteristic of the drive. The shaft slides through ball carrying tracks of the ball bushing which for optimal precision is adjusted within the guide housing. A gasket ring protects the ball bushing from dust particles. In Fig. 1C, D two alternative designs of shaft bearings taking advantage of commercially available torsion-free ball bushings are outlined. But it must be mentioned that both designs require special ball bushings and axles (Deutsche Star, Schweinfurt, F.R.G., part no. 06960-12-80 and 0695-21200, respectively) which may not be generally available parts. In either design, the guide housing and the stepper motor are protected by a stainless steel outer housing which also acts as a fixing of the distal end of the stepper motor by means of a silicone tube. The complicated double housing construction ensures a vibration-free movement of the shaft. In addition, it prevents the transmission of harmful vibrations from the motor to the drive carrying mechanism. The extensive use of commercially available mechanical parts of high accuracy to a certain extent eases some of the constraints on fabrication. Nevertheless, it is self evident that the manufacture and assembly of the micropositioner by the workshop demands the greatest possible attention and care.
Electronic circuitry
The electronic control of multiphase stepper motors can become rather complex and requires either a considerable amount of electronic circuitry (Erdman and Zipf, 1982; Neumann, 1982) or computer software (Patil, 1982; Ward, 1982). Recently, suitable integrated circuits have become available. We chose an advanced two chip set that incorporates the complete sequence logic, power drivers and power amplifiers (Baumwolf, 1984). Thanks to the completeness of these integrated circuits, the control logic can be implemented easily as described below. Fig. 2 shows the detailed schematics of the electronic control unit. Since the movement of the electrode is compromised by a sequence of single steps, on a step generator (2 x CD 4510B) the desired number of steps (1-99) can be preset by thumbwheel switches. Upon issuing a forward (FW) or backward (BW) command the correct number of steps are generated and are accumulated (added in forward mode or subtracted when going backwards) by the 5-digit step counter (1/2 74C74; ICM 7217A, Intersil). The step display (NSB 7881; NSN 781, National Semiconductor) indicates exactly the current electrode position. A push-button switch (RS) performs a counter reset. An input debouncing circuitry (MC 14490, Motorola), which in addition delivers the clock pulses for the step generator, assures clean digital signals from the mechanical contacts of the command push buttons. The
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status display ( N S N 781) signals the current direction of electrode movement (direction flip-flop: 1/74C74) and the operational mode of the device which is selected by a doublepole switch (SWA, SWB). In "step" mode (S) the electrode movement is defined by the preset number of steps. In "continuous" mode (C) the electrode continues to move as long as one of the F W / B W buttons is pressed. In this case a blinking "C" that stands for "continuous" is shown on the mode display (N3,N4; MC 14494, Motorola). The L297/L298 chip set (SGS-Ates, 20041 Agrate Brianza, Italy) performs the actual control of the stepper m i t o r rotation. The L297 stepper motor controller can generate the phase drive signals for two phase bipolar as well as for 4 phase unipolar step motors. These drive signals are fed to the L298, a dual full-bridge amplifier that
240 energizes the stepper motor's phase windings. An on-chip (L297) pulse-wide modulator (PWM) permits control of the motor excitation current and allows the implementation of a so called "power-down" circuitry. In idle state, this circuitry automatically reduces the motor current, thus keeping the motor temperature and the heat dissipation of the electronic power circuitry low. In the present application the L297 controller operates in half step mode (HF set to high logic level). This mode reduces the step angle of the 1.8 ° motor to 0.9 ° . The amplifier's full-bridge design allows the stepper motor to be driven in bipolar mode using a single unipolar power supply. The silicone diodes SD (1 N 4004) protect the power amplifiers from destructive reverse voltages induced by the motor windings. The complete electronic control unit including power supply (the latter is not shown) occupies only one small-sized printed circuit board of 100 mm x 160 mm (European-card format). The card and the transformer fitted into a 19' rack 44.6 mm high.
Measurement of performance As for the measurement of performance no suitable commercial equipment was available, two quite simple arrangements were built. The dynamics of the axial movement was measured by a magnetic device based on the Hall effect (Brown and Flaming, 1977). For the detection of movements transverse to the electrode axis an opto-electronic approach (Yates, 1982) was used. To measure the characteristics of the "step" of the electrode holder a permanent magnet was tightly fixed to the end of the micropositioner shaft. The south pole of the magnet (Fig. 3A) was brought close to the internal Hall effect element (H) of an integrated circuit (SAS 231W, Siemens) which, besides other functions, embodies an amplifier for the output signal. The output voltage of this IC is proportional to the induction of an external magnetic field and increases when the south pole of a magnet approaches the chip surface. The output was connected to a digital storage oscilloscope (OS 4000, Gould) equipped with an XY'plotter. Radial vibrations which are the most probable origin of cell damage during membrane penetration, required a more complicated measurement arrangement. For this purpose, an infrared LED (CLED 155-D, Clairex) with built-in lens (a = 15 °) was mounted at the end of the shaft and powered by a constant current source (Fig. 3B). The light beam (~ = 880 nm) was directed onto the surface of a stationary quadrant photodetector (QD 50-5T, Centronic, maximum sensitivity at 830 nm) and centered. The four photodiodes oi~erate in the photoamperic mode with zero bias. A sufficiently low-load resistance, at high-output voltage is achieved by feeding the photocurrent into the virtual earth (inverted input) of the operational amplifier (OP1,2; OP4,5). The output of the operational amplifiers of the corresponding photoelements in x- and y-axis were fed to the respective differential operational amplifiers (OP3,6). The output of OP3 and OP6 (x, y) were connected to an FM-tape recorder. The displacement of the micropositioner shaft in the xy-plane were plotted by an xy-recorder by reducing the tape speed.
241
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Tests were performed after appropriate adjustment of the winding currents to produce maximum velocity without overshoot of the electrode. The step pulse frequency was set to 1000 Hz which is well within the safe operational range of the stepper motor's maximum start/stop frequency. A spindle with a pitch of 0.4 mm and half step mode was used. Thus, a single step resulted in a displacement of 1 #m. The average step velocity was in the range of 1 # m / m s (Fig. 4). For intracellular work the step velocity could be increased to 3 # m / m s (step frequency = 3kHz) substituting the ZSS 19-200 motor by a slightly larger model of the same manufac-, turer (ZSS 22-200:22 mm outer diameter). The motor bridge amplifier (SGS-Ates, L 298) can drive inductive loads at currents up to 4 A. Due to the damping elements, overshoot and oscillation which are the main problems of stepper motor
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B Fig. 4. z-axis displacement. Dynamic characteristics of a one-step (Trace B) or a three-step (Trace A) advancement. The signals are left original (not filtered) and thus show some amount of high frequency noise.
242
driven micropositioners (Fromm et al., 1980) were barely noticeable. Vibrations perpendicular to the electrode axis were extremely small and did not exceed 300 nm. Since this amount of vibration is close to the spatial resolution of the photodetectors, the less attractive plots of the measurements we obtained are not shown. Radial distortion depends to some extent on the stability of the frame carrier of the manipulator. The "floating" mounting scheme for the stepper motor within the micropositioner housing noticeably reduced the need for a sturdy carrying mechanism for the instrument. Carefully manufactured and adequately adjusted, the described microelectrode micropositioning device meets or even exceeds the projected specifications. It competes well with the most expensive commercially available units and can be built for a fraction of the cost (components are about $400). References Adami, H.J. (1984) Der Linear-Aktuator, Elektronik, 33 (8): 89-90. Baumwolf, P. (1984) Schrittmotorsteuerung auf einem Chip, Elektronik, 33 (7):57-62. Brown, K.T., Flaming, D.G. (1977) New microelectrode techniques for intracellular work in small cells, Neuroscience, 2: 813-827. Connor, G.I. (1973) A micropositioner in a close-loop control system, IEEE Trans. Bio-Med. Eng., BME-20: 114-119. Deadwyler, S.A., Biela, J., Rose, G., West, M. and Lynch, G. (1979) A microdrive for use with glass or metal microelectrodes in recording from freely-moving rats, Electroencephalogr. Clin. Neurophysiol., 47: 752-754. Erdman, P.W. and Zipf, E.C. (1982) Simple medium-power stepper motor logic sequencer and phase driver, Rev. Sci. Instrum., 53: 909-910. Fish, R.M., Bryan, J.S., McReynolds, J.S. and P,.ies, J.J. (1971) A mechanical microelectrnde pulsing device to facilitate the penetration of small cells, IEEE Trans. Bio-Med. Eng., BME-18: 240-241. Fromm, M., Weskamp, P., Hegel, U. (1980) Versatile piezoelectric driver for cell puncture, Pfl~gers Arch., 384: 69-73. Gutierrez, O. and Salinas, R. (1984) A simple device to aid impalement of cells using conventional microelectrode drives, Physiol. Behav., 32: 1033-1035. Lederer, W.J. (1983) Piezoelectric translator, Pfli3gers Arch., 399: 83-86. Lutz, A. and Wagman, I.H. (1965) A rolling diaphragm hydraulic micromanipulator, Electroencephalogr. Clin. Neurophysiol., 18: 184-186. Neumann, O. (1982) C-MOS circuit controls stepper motor, Electronics, 55 (17): 165-167. Nobiling, R. and Bi3hrle, C.P. (1986) A piezotranslator with variable movement pattern: experiences with the penetration of very small cells, J. Neurosci. Meth., 16: 201-216. Pager, J. (1984) A removable head-mounted microdrive for unit recording in the free-behaving rat, Physiol. Behav., 33: 843-848. Patil, V.L. (1982) Interface lets microprocessor control stepper motor, Electronics, 55 (15): 119-121. Reitb~ck, H. and Werner, G. (1983) Multi-electrode recording system for the study of spatio-temporal activity patterns of neurons in the central nervous system, Experientia, 39: 339-342. Schmid, K. and Bbhmer, G. (1985) Vorw~.rts im p,m-Bereich, Elektronik, 34 (2): 48-52. Sonnhof, U., F~rderer, R., Schneider, W. and Kettenmann, H. (1982) Cell puncturing with a step motor driven manipulator with simultaneous measurement of displacement, Pfli3gers Arch., 392: 295-300. Van der Pets, J.N.C. (1980) An electromagnetic microelectrode holder for pulsating penetration of brain tissue, J. Neurosci. Meth. 2: 319-321. Veregge, S. and Frost, J.D. (1985) A simple, inexpensive hydraulic microdrive for recording neocortical unit activity in the unanesthetized rat, Electroencephalogr. Clin. Neurophysiol., 61: 94-97. Ward, R. (1982) Interface software form smart stepper controller, Electronics, 55 (24): 84-87. Yates, G.K. (1982) A sensitive optoelectronic displacement transducer for the neurophysiological laboratory, J. Neurosci. Meth., 6: 103-111.
235
Elsevier NSM 00665
A stepper motor-driven microelectrode positioner * K. Schrnid a n d G. B/Shmer Department of Physiology, University of Mainz, Mainz (F.R.G.) (Received 10 April 1986) (Revised 14 October 1986) (Accepted 16 October 1986)
Key words: Cell impalement; Microelectrode; Micropositioner; Stepper motor The mechanical elements and the electronic control system from a stepper motor-driven microelectrode positioner is described. The unit embodies a high-precision small step angle hybrid motor. The compact, rugged and totally concentric design of the mechanic, by a spindle mechanism achieves the necessary precision by translating the stepwise rotations of the motor into steps of linear movement. The system takes advantage of commercially available low friction parts such as ball bearings, ball bushings and axles with hardened surfaces. The related electronic control unit is designed around the most recent integrated circuitry which is both sophisticated and economical. Though the described system is designed to be built in an average departmental workshop it compares favorably with more expensive commercial units and in some aspects outperforms them.
Introduction
The recording of the bioelectrical activity of neurons either in situ or in vitro is a frequently employed technique to explore the functional state of a cell. Extra- or intracellular microelectrode recordings taken from single cells of animals or plants are the most widely used methods. The controlled and precise advancement of the microelectrode, and the quick penetration of the cell membrane without damaging the cell are perhaps the most difficult problems involved in this method. These difficulties are multiplied when simultaneous recordings are made from more than one cell. Mechanically, such multirecording systems are extremely complex (Reitb/Sck and Werner, 1983) and require each electrode to be attached to its own positioning and advancing device. The simultaneous use of several microdrives necessitates that each of them is of miniature size and in addition low cost. This paper describes such a micropositioner.
* Part of a preliminary versior~ of the electronics of the described device has already been published (Schmid and B~hmer, 1985). Correspondence: K. Schmid, Department of Physiology, University of Mainz, D-6500 Mainz, F.R.G. 0165-0270/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)
236 Microelectrode micropositioners have been built using different operational principles. Simple manually driven microelectrode translators have been used since the technique of microelectrode recording was first introduced into electrophysiological research. Nowadays, refined mechanical electrode advancing systems are employed for recordings from freely-moving animals (Deadwyler et al., 1979; Pager, 1984). Devices using a hydraulic approach are still in widespread use (David Kopf; Lutz and Wagman, 1965; Veregge and Frost, 1985). Compact translators using fast-response piezoelectric elements have some advantages but until now they have severe limitations because of their range of total displacement (Nobiling and Btihrle, 1986). A combination of slowly advancing devices (manual or hydraulic) with rapid-advancing piezoelectric or magnetic elements have been used to improve cell penetration (Fish et al., 1971; Van der Pers, 1980; Lederer, 1983; Gutierrez and Salinas, 1984). Most popular positioning devices are motor-driven systems, driven either by DC motors or by stepper motors (Nanostepper, Sonnhof et al., 1982). Unfortunately, mostly these instruments are rather large and costly. For further details and discussion of the pros and cons of the different operational principles of microelectrode translators cf. Brown and Flaming, 1977; Sonnhof et al., 1982.
Design considerations We established that the projected micropositioner for microelectrode work should meet the following minimum requirements: (1) Small outer dimensions, light weight, and easy to mount. (2) Fast and precise advancement in increments as small as 1 #m over an extensive mechanical range. (3) Low cost, using commercially available materials and procedures which can be executed by a departmental workshop.
Mechanical construction Because the stepper motor is a rotating device a conversion of a rotation into a proportional linear translation has to be performed. There is a complete system commercially available (Philips-Valvo: linear actuator) that embodies the stepper motor and the conversion mechanics (Adami, 1984). Unfortunately, for the application in mind this component proved inaccurate. Since the precision conversion mechanics had to be constructed by ourselves, a spindle type mechanism promised to be easiest to construct. The key element of the stepper motor micropositioner is the stepper motor itself. One step of the motor effects in an angle of rotation of ~. The resulting linear translation S at a given spindle pitch P is defined by: S = P- ~/360 Although suitable high-precision miniature stepper motors are hard to find, one
237
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motor (Phytron, Groebenzell, F.R.G., part no. ZSS 19-200-0.6) met our stringent requirements. This hybrid motor is extremely small (19 m m outer diameter, 36 m m length) and very strong. It has a low inertia rotor and thus exerts a rather high s t a r t / s t o p frequency and has a small step angle ( ~ = 1.8"). As long as the step pulse frequency does not exceed the motor's maximum s t a r t / s t o p frequency an open loop system is reliable and error free without the need to sense and control the actual rotor position (Connor, 1973). A concentric, fully coaxial design of the linear translator was preferred (Fig. 1A) since this approach resulted in a sturdy but small mechanical system that is completely self-encapsulated and of high precision. The decision for a coaxial design was mainly influenced by commercially available ball bushings in conjunction with matching steel axles both of which are high precision parts with induction hardened steel surfaces. Because ball bushings of such compact dimensions (Deutsche Star, Schweinfurt, F R G , part no. KB 614-305) usually have no provision against torsion, a guide pin slides in a precisely ground track (tolerance: 2 / 1 0 0 ram) cut into the inner stainless steel housing (Fig. 1B). The stepper motor connected by a flexible suspension (silicone tube) is isolated against vibrations from the inner and outer guide housings. The silicone dam~ing element attached to the motor axle eliminates possible resonant vibrations of the screwed spindle supported by a ball bearing. A precision stainless steel spindle (spindle pitch: e.g. 0.4 turn) by means of a thread in
238
the guide block moves the guiding block (made of bearing metal) within the stainless steel guide housing, the inner surface of which is precision ground. The guiding block carries the guide pin as well as the induction-hardened stainless steel shaft. To shrink the total length of the actuator housing but maintain the travel of the rod at its full extent (40 mm), the screwed spindle is concentric with the shaft. In addition, the drilled centre of the shaft has the additional effect of considerably reducing the moving mass and thus leads to a better dynamic characteristic of the drive. The shaft slides through ball carrying tracks of the ball bushing which for optimal precision is adjusted within the guide housing. A gasket ring protects the ball bushing from dust particles. In Fig. 1C, D two alternative designs of shaft bearings taking advantage of commercially available torsion-free ball bushings are outlined. But it must be mentioned that both designs require special ball bushings and axles (Deutsche Star, Schweinfurt, F.R.G., part no. 06960-12-80 and 0695-21200, respectively) which may not be generally available parts. In either design, the guide housing and the stepper motor are protected by a stainless steel outer housing which also acts as a fixing of the distal end of the stepper motor by means of a silicone tube. The complicated double housing construction ensures a vibration-free movement of the shaft. In addition, it prevents the transmission of harmful vibrations from the motor to the drive carrying mechanism. The extensive use of commercially available mechanical parts of high accuracy to a certain extent eases some of the constraints on fabrication. Nevertheless, it is self evident that the manufacture and assembly of the micropositioner by the workshop demands the greatest possible attention and care.
Electronic circuitry
The electronic control of multiphase stepper motors can become rather complex and requires either a considerable amount of electronic circuitry (Erdman and Zipf, 1982; Neumann, 1982) or computer software (Patil, 1982; Ward, 1982). Recently, suitable integrated circuits have become available. We chose an advanced two chip set that incorporates the complete sequence logic, power drivers and power amplifiers (Baumwolf, 1984). Thanks to the completeness of these integrated circuits, the control logic can be implemented easily as described below. Fig. 2 shows the detailed schematics of the electronic control unit. Since the movement of the electrode is compromised by a sequence of single steps, on a step generator (2 x CD 4510B) the desired number of steps (1-99) can be preset by thumbwheel switches. Upon issuing a forward (FW) or backward (BW) command the correct number of steps are generated and are accumulated (added in forward mode or subtracted when going backwards) by the 5-digit step counter (1/2 74C74; ICM 7217A, Intersil). The step display (NSB 7881; NSN 781, National Semiconductor) indicates exactly the current electrode position. A push-button switch (RS) performs a counter reset. An input debouncing circuitry (MC 14490, Motorola), which in addition delivers the clock pulses for the step generator, assures clean digital signals from the mechanical contacts of the command push buttons. The
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Phase I Kt~nner ,_.
: GND
I
MC14t,%
[ A -r , n ~ , ' - ~ ~
B
C /.?K I
D !: H
L
Fig. 2. Detailed schematic diagram. All of the Nand gates (N1-N4) are CMOS types (1/4 74 CO0). The symbols "L" or "H" mean low (0 V) or high (+ 5 V) logical level, respectively. Power supply circuitry is not shown.
status display ( N S N 781) signals the current direction of electrode movement (direction flip-flop: 1/74C74) and the operational mode of the device which is selected by a doublepole switch (SWA, SWB). In "step" mode (S) the electrode movement is defined by the preset number of steps. In "continuous" mode (C) the electrode continues to move as long as one of the F W / B W buttons is pressed. In this case a blinking "C" that stands for "continuous" is shown on the mode display (N3,N4; MC 14494, Motorola). The L297/L298 chip set (SGS-Ates, 20041 Agrate Brianza, Italy) performs the actual control of the stepper m i t o r rotation. The L297 stepper motor controller can generate the phase drive signals for two phase bipolar as well as for 4 phase unipolar step motors. These drive signals are fed to the L298, a dual full-bridge amplifier that
240 energizes the stepper motor's phase windings. An on-chip (L297) pulse-wide modulator (PWM) permits control of the motor excitation current and allows the implementation of a so called "power-down" circuitry. In idle state, this circuitry automatically reduces the motor current, thus keeping the motor temperature and the heat dissipation of the electronic power circuitry low. In the present application the L297 controller operates in half step mode (HF set to high logic level). This mode reduces the step angle of the 1.8 ° motor to 0.9 ° . The amplifier's full-bridge design allows the stepper motor to be driven in bipolar mode using a single unipolar power supply. The silicone diodes SD (1 N 4004) protect the power amplifiers from destructive reverse voltages induced by the motor windings. The complete electronic control unit including power supply (the latter is not shown) occupies only one small-sized printed circuit board of 100 mm x 160 mm (European-card format). The card and the transformer fitted into a 19' rack 44.6 mm high.
Measurement of performance As for the measurement of performance no suitable commercial equipment was available, two quite simple arrangements were built. The dynamics of the axial movement was measured by a magnetic device based on the Hall effect (Brown and Flaming, 1977). For the detection of movements transverse to the electrode axis an opto-electronic approach (Yates, 1982) was used. To measure the characteristics of the "step" of the electrode holder a permanent magnet was tightly fixed to the end of the micropositioner shaft. The south pole of the magnet (Fig. 3A) was brought close to the internal Hall effect element (H) of an integrated circuit (SAS 231W, Siemens) which, besides other functions, embodies an amplifier for the output signal. The output voltage of this IC is proportional to the induction of an external magnetic field and increases when the south pole of a magnet approaches the chip surface. The output was connected to a digital storage oscilloscope (OS 4000, Gould) equipped with an XY'plotter. Radial vibrations which are the most probable origin of cell damage during membrane penetration, required a more complicated measurement arrangement. For this purpose, an infrared LED (CLED 155-D, Clairex) with built-in lens (a = 15 °) was mounted at the end of the shaft and powered by a constant current source (Fig. 3B). The light beam (~ = 880 nm) was directed onto the surface of a stationary quadrant photodetector (QD 50-5T, Centronic, maximum sensitivity at 830 nm) and centered. The four photodiodes oi~erate in the photoamperic mode with zero bias. A sufficiently low-load resistance, at high-output voltage is achieved by feeding the photocurrent into the virtual earth (inverted input) of the operational amplifier (OP1,2; OP4,5). The output of the operational amplifiers of the corresponding photoelements in x- and y-axis were fed to the respective differential operational amplifiers (OP3,6). The output of OP3 and OP6 (x, y) were connected to an FM-tape recorder. The displacement of the micropositioner shaft in the xy-plane were plotted by an xy-recorder by reducing the tape speed.
241
Current ~~] Source
Permanent Mognef
¥
LED
Vref
J--~ O
Out t
Zero Gain UC GND +12V Fig. 3. Test arrangements (simplified). A: arrangement for the measurement of axial movement. B: arrangement for the detection of axial vibrations.
Tests were performed after appropriate adjustment of the winding currents to produce maximum velocity without overshoot of the electrode. The step pulse frequency was set to 1000 Hz which is well within the safe operational range of the stepper motor's maximum start/stop frequency. A spindle with a pitch of 0.4 mm and half step mode was used. Thus, a single step resulted in a displacement of 1 #m. The average step velocity was in the range of 1 # m / m s (Fig. 4). For intracellular work the step velocity could be increased to 3 # m / m s (step frequency = 3kHz) substituting the ZSS 19-200 motor by a slightly larger model of the same manufac-, turer (ZSS 22-200:22 mm outer diameter). The motor bridge amplifier (SGS-Ates, L 298) can drive inductive loads at currents up to 4 A. Due to the damping elements, overshoot and oscillation which are the main problems of stepper motor
I IJm
10ms
B Fig. 4. z-axis displacement. Dynamic characteristics of a one-step (Trace B) or a three-step (Trace A) advancement. The signals are left original (not filtered) and thus show some amount of high frequency noise.
242
driven micropositioners (Fromm et al., 1980) were barely noticeable. Vibrations perpendicular to the electrode axis were extremely small and did not exceed 300 nm. Since this amount of vibration is close to the spatial resolution of the photodetectors, the less attractive plots of the measurements we obtained are not shown. Radial distortion depends to some extent on the stability of the frame carrier of the manipulator. The "floating" mounting scheme for the stepper motor within the micropositioner housing noticeably reduced the need for a sturdy carrying mechanism for the instrument. Carefully manufactured and adequately adjusted, the described microelectrode micropositioning device meets or even exceeds the projected specifications. It competes well with the most expensive commercially available units and can be built for a fraction of the cost (components are about $400). References Adami, H.J. (1984) Der Linear-Aktuator, Elektronik, 33 (8): 89-90. Baumwolf, P. (1984) Schrittmotorsteuerung auf einem Chip, Elektronik, 33 (7):57-62. Brown, K.T., Flaming, D.G. (1977) New microelectrode techniques for intracellular work in small cells, Neuroscience, 2: 813-827. Connor, G.I. (1973) A micropositioner in a close-loop control system, IEEE Trans. Bio-Med. Eng., BME-20: 114-119. Deadwyler, S.A., Biela, J., Rose, G., West, M. and Lynch, G. (1979) A microdrive for use with glass or metal microelectrodes in recording from freely-moving rats, Electroencephalogr. Clin. Neurophysiol., 47: 752-754. Erdman, P.W. and Zipf, E.C. (1982) Simple medium-power stepper motor logic sequencer and phase driver, Rev. Sci. Instrum., 53: 909-910. Fish, R.M., Bryan, J.S., McReynolds, J.S. and P,.ies, J.J. (1971) A mechanical microelectrnde pulsing device to facilitate the penetration of small cells, IEEE Trans. Bio-Med. Eng., BME-18: 240-241. Fromm, M., Weskamp, P., Hegel, U. (1980) Versatile piezoelectric driver for cell puncture, Pfl~gers Arch., 384: 69-73. Gutierrez, O. and Salinas, R. (1984) A simple device to aid impalement of cells using conventional microelectrode drives, Physiol. Behav., 32: 1033-1035. Lederer, W.J. (1983) Piezoelectric translator, Pfli3gers Arch., 399: 83-86. Lutz, A. and Wagman, I.H. (1965) A rolling diaphragm hydraulic micromanipulator, Electroencephalogr. Clin. Neurophysiol., 18: 184-186. Neumann, O. (1982) C-MOS circuit controls stepper motor, Electronics, 55 (17): 165-167. Nobiling, R. and Bi3hrle, C.P. (1986) A piezotranslator with variable movement pattern: experiences with the penetration of very small cells, J. Neurosci. Meth., 16: 201-216. Pager, J. (1984) A removable head-mounted microdrive for unit recording in the free-behaving rat, Physiol. Behav., 33: 843-848. Patil, V.L. (1982) Interface lets microprocessor control stepper motor, Electronics, 55 (15): 119-121. Reitb~ck, H. and Werner, G. (1983) Multi-electrode recording system for the study of spatio-temporal activity patterns of neurons in the central nervous system, Experientia, 39: 339-342. Schmid, K. and Bbhmer, G. (1985) Vorw~.rts im p,m-Bereich, Elektronik, 34 (2): 48-52. Sonnhof, U., F~rderer, R., Schneider, W. and Kettenmann, H. (1982) Cell puncturing with a step motor driven manipulator with simultaneous measurement of displacement, Pfli3gers Arch., 392: 295-300. Van der Pets, J.N.C. (1980) An electromagnetic microelectrode holder for pulsating penetration of brain tissue, J. Neurosci. Meth. 2: 319-321. Veregge, S. and Frost, J.D. (1985) A simple, inexpensive hydraulic microdrive for recording neocortical unit activity in the unanesthetized rat, Electroencephalogr. Clin. Neurophysiol., 61: 94-97. Ward, R. (1982) Interface software form smart stepper controller, Electronics, 55 (24): 84-87. Yates, G.K. (1982) A sensitive optoelectronic displacement transducer for the neurophysiological laboratory, J. Neurosci. Meth., 6: 103-111.