Technical note
Wideband electromechanical stimulator* Abstract--An
electromechanical stimulator with wideband characteristics is described. Nonlinear control techniques have been used to achieve a fast rise time, a speedy settling time, and a very small steady-state error. The device has been used in quantitative investigations on cutaneous receptor units.
Keywords
Electromechanical stimulator, Wideband
Introduction
System description: moving coil motor unit
THE QUANTITATIVEinvestigation of input-output relations of skin mechanoreceptors requires the use of precisely controlled stimuli (ARMETT et al., 1962). The ideal mechanical stimulator should be capable of producing accurately controlled displacements and velocities over a wide range of frequencies, it should be able to operate in any orientation, it should be capable of generating a wide variety of waveforms including square, sine, ramp and pseudorandom; and the mechanical output should be independent of inertial, frictional and spring loads. The mechanical stimulator we have designed consists of two main parts: (i) a control system (Fig. 1) and (ii) a moving-coil motor element with an attached glass cylindrical stylus (Fig. 2). The control system operates in a linear mode for low values of the displacement error and in a nonlinear mode for high values. The instrument operates over a range of 2-3 mm. The response to a command signal is within 0 " 5 ~ of the demanded value. The performance details of the system at 0" 1 mm, 1 mm and 2 mm are as follows: In the time domain, the steady-state errors are 0" 5, 0' 6 and 0" 6 ~ ; the rise times are 0" 45, 1" 50 and 3" 00 ms; the settling times are 0' 75, 2'00 and 3 ' 2 m s ; the delay times are 0"40, 1.15 and 1 975 ms and the percentage overshoots are 4" 0, 0" 6 and 0" 7 ~ . In the frequency domain, the bandwidths are 700, 150 and 95 Hz. A prototype version of this instrument was demonstrated to the Physiological Society (KHALAFALLA, et al., 1973).
The motor unit (Fig. 2) consists of an 11 ~ 38mm. (1" 5") diameter coil suspended in a 16000 Gauss magnetic field. Two corrugated fabric discs are used to suspend the coil. These help to maintain the lateral position of the coil in the gap while permitting 5-6 mm of vertical movement. The electric current, which enters by means of two flexible wires, drives the coil with a maximum force o~ about 2 7 " 4 N / m 2. The coil is wound on a light paper former, this holds an aluminium disc that serves as a mounting for the glass stimulating rod. The mechanical construction is designed for lightness and minimum frictional damping, both of which effect the response time. The main mechanical characteristics of the motor unit are as follows: force/current ratio of 15.7 N m - 2 A -1, spring constant of 1.09kg per metre, effective mass of 10"8 g and a resonant frequency of 50' 7 Hz.
actuating electrical error signal comparator / input ~ reference /~._, \ J .=mpllfier I signal X "~' ~ ) limiter .~//' unlt ]
.I
System description: the control system The control system can be described in terms of four main functional blocks; an optical-displacement transducer, a displacement-signal modifier, a comparator and an amplifier limiter (Fig. 1). Each of these will be described in turn. The optical-displacement transducer system consists of five main parts (Fig. 3); a current generator, an optical link, a reference voltage source, a current-voltage convertor and a razor-edge attachment to the moving coil. The transducer system operates in the following way
/-[,,motor
stylus j~splacement
unlt I
)
displacement, and velocity feedback displacement L signal r modifier
)tical I opti disp=lacement tran msducer
Fig. 1 Block diagram of mechanical stimulator control system
" Firstreceived 15th Septemberandinfinalform 21stNovember 1975
Medical and Biological Engineering
September 1976
585
A current generator (Fig. 4) provides a constant current to a light source; the light source produces a beam of light which is detected by a fiat, linear, photovoltaic cell. The amount of light reaching the photocell is modulated by a razor edge fixed to the moving coil. As the coil moves up and down the razor edge moves with it, this changes
the amount of light reaching the photocell, and hence tile current produced by it. This current, which is proportional to the displacement, is amplified by a currentvoltage convertor (Fig. 5) to yield a voltage proportional to the displacement of the moving coil. A reference-voltage source (Fig. 6) provides a standard
m a g n e t ~
' rf.~n
~ ]
/
d'iU?iniun
..r~ edg
Fig. 2 Diagram of motor unit
Pch~ tO I~bUlcbosure ventilation
extended coil
stimulating rod REFERENCE VOLTAGE SOURCE
CURRENT- toVOLTAGE CONVERTER
optical link --:..-_.---
photo- t - ~ cell ~ I____
CURRENT
,
ig source
GENERATOR
Fig. 3 Block diagram of optical displacement transducer system
MOTOR
UNIT +22v
reference o in
i
R1
0
2.2 k
LAMP
OUT TO o 240mA
~
TIP32A
Fig. 4 Current generator system
R2I 2.7k 586
Medical and Biological Engineering
September 1976
voltage which is used (i) to control the intensity of the light source and (ii) as a reference for the current-voltage convertor. The current drawn by the lamp develops a voltage across the four 82 f~ resistors in the current generator (Fig. 4). This voltage is compared with the reference voltage derived from the reference-voltage source. The comparison is carried out by the transistor 2N 3704 which adjusts the regulating transistor TIP 32A so that the current is kept constant. The current generated by the photocell is compared with a reference current (Fig. 5)--this is derived from the
standard voltage provided by the reference-voltage source--and is then applied to the current-voltage convertor; this circuit has a low input impedance. The output of the current-voltage convertor gives a voltage proportional to the current at the input. The displacement information from the optical transducer system is fed to a displacement-signal modifier (Fig. 7). This circuit operates in such a way as to produce an output, which, compared with the input, has an element of velocity feedback contained in it. This is achieved by means of the transfer characteristics of the RC network preceding the operational amplifier. The
"1 adjusted for zero V0 at zero displacement
~OpF ~fe~nce 8.2k I, I
"+lOv
Fig. 5 Photocell and current--voltage convertor adjust gain
270 f
36,700pF
ram
displacement c transducer
o input
out
,'_~
1
I
3
Fig. 6 Displacement signal modifier
m
5-6v~
3"3k ~_ 2N3702
10v reference out
2N3704 ~25pF
8.2k ~ u
V--
1
0
I
680
470
:N3704
Fig. 7 Reference voltage source
680
~'~'4"7v Medical and Biological Engineering
September 1976
587
output of the device thus consists of two components, a displacement signal and a velocity signal. From this circuit the signal is fed to a comparator where it is compared with a reference electrical signal representing the required stylus movement. The actuating error signal then goes to an amplifier-limiter unit (consisting of a high-voltage-gain amplifier followed by a power amplifier) which drives the motor in such a way as to minimise the error. The power amplifier is self limiting, it thereby controls the maximum voltage that can be applied to the motor. The gain of the amplifier, the design of the limiting circuit and the proportion of the velocity feedback have to be related so that the control loop can operate with a dual mode of action, i.e. relay operated for large values of displacement error, linearly operated for small values. This gives a system with a very small steady-state error and with a very fast response to suddenly applied signals (THALER and BROWN, I960). For the linear mode of operation, fast rise and settling times can be obtained by using a high gain and an appropriate amount of velocity feedback.
IOOmsec Fig. 8 Response of type I mechanoreceptor in the sural nerve of the rat to quantitative mechanical stimulus of 200Fro derived from mechanical stimulator. The response is shown on the top trace
feedback required was initially estimated using classical control theory and then adjusted by trial and error to obtain the best system response.
Results The kind of result that can be obtained by the use of this device is shown in Fig. 8. The record shows the response of a Type l mechanoreceptor in the sural nerve of the rat to a stimulus amplitude of 200/~m.
Acknowledgments--We wish to thank L. Harrison, D. Alpsan and D. Proffitt for advice and help. M. J. O'Connell was a U K Science Research Council scholar and was supported by the Wellcome Trust during the initial part of this work.
References ARMETT, C. J., HUNSPERGER,R. W., GRAY, J. A. B. and LAL, S. (1962). The transmission of information in primary receptor neurones and second order neurones of a phasic system. J. Physiol. 164 395--421. KHALAFALLA, F., LAL, S., O'CONNELL, M. J., TURNER and LINDA M. (1973). An electromechanical stimulator with wide band characteristics. J. Physiol. 229 1-2P. THALER, G. J., and BROWN, R, G. (1960) Analysis and design of feedback control systems. (McGraw-Hill), 433-478. M. J. O'CONNELL F. KHALAFALLA
Department of Electronics Chelsea College London SW3 6LX England S. LAL
The nonlinear mode of operation is caused by the saturation of the error amplifier. Under these conditions a different amount of velocity feedback is required for stable operation of the system. The amount of velocity
588
Department of Physiology Chelsea College London SW3 6LX England
Medical and Biological Engineering
September 1976
Wideband electromechanical stimulator* Abstract--An
electromechanical stimulator with wideband characteristics is described. Nonlinear control techniques have been used to achieve a fast rise time, a speedy settling time, and a very small steady-state error. The device has been used in quantitative investigations on cutaneous receptor units.
Keywords
Electromechanical stimulator, Wideband
Introduction
System description: moving coil motor unit
THE QUANTITATIVEinvestigation of input-output relations of skin mechanoreceptors requires the use of precisely controlled stimuli (ARMETT et al., 1962). The ideal mechanical stimulator should be capable of producing accurately controlled displacements and velocities over a wide range of frequencies, it should be able to operate in any orientation, it should be capable of generating a wide variety of waveforms including square, sine, ramp and pseudorandom; and the mechanical output should be independent of inertial, frictional and spring loads. The mechanical stimulator we have designed consists of two main parts: (i) a control system (Fig. 1) and (ii) a moving-coil motor element with an attached glass cylindrical stylus (Fig. 2). The control system operates in a linear mode for low values of the displacement error and in a nonlinear mode for high values. The instrument operates over a range of 2-3 mm. The response to a command signal is within 0 " 5 ~ of the demanded value. The performance details of the system at 0" 1 mm, 1 mm and 2 mm are as follows: In the time domain, the steady-state errors are 0" 5, 0' 6 and 0" 6 ~ ; the rise times are 0" 45, 1" 50 and 3" 00 ms; the settling times are 0' 75, 2'00 and 3 ' 2 m s ; the delay times are 0"40, 1.15 and 1 975 ms and the percentage overshoots are 4" 0, 0" 6 and 0" 7 ~ . In the frequency domain, the bandwidths are 700, 150 and 95 Hz. A prototype version of this instrument was demonstrated to the Physiological Society (KHALAFALLA, et al., 1973).
The motor unit (Fig. 2) consists of an 11 ~ 38mm. (1" 5") diameter coil suspended in a 16000 Gauss magnetic field. Two corrugated fabric discs are used to suspend the coil. These help to maintain the lateral position of the coil in the gap while permitting 5-6 mm of vertical movement. The electric current, which enters by means of two flexible wires, drives the coil with a maximum force o~ about 2 7 " 4 N / m 2. The coil is wound on a light paper former, this holds an aluminium disc that serves as a mounting for the glass stimulating rod. The mechanical construction is designed for lightness and minimum frictional damping, both of which effect the response time. The main mechanical characteristics of the motor unit are as follows: force/current ratio of 15.7 N m - 2 A -1, spring constant of 1.09kg per metre, effective mass of 10"8 g and a resonant frequency of 50' 7 Hz.
actuating electrical error signal comparator / input ~ reference /~._, \ J .=mpllfier I signal X "~' ~ ) limiter .~//' unlt ]
.I
System description: the control system The control system can be described in terms of four main functional blocks; an optical-displacement transducer, a displacement-signal modifier, a comparator and an amplifier limiter (Fig. 1). Each of these will be described in turn. The optical-displacement transducer system consists of five main parts (Fig. 3); a current generator, an optical link, a reference voltage source, a current-voltage convertor and a razor-edge attachment to the moving coil. The transducer system operates in the following way
/-[,,motor
stylus j~splacement
unlt I
)
displacement, and velocity feedback displacement L signal r modifier
)tical I opti disp=lacement tran msducer
Fig. 1 Block diagram of mechanical stimulator control system
" Firstreceived 15th Septemberandinfinalform 21stNovember 1975
Medical and Biological Engineering
September 1976
585
A current generator (Fig. 4) provides a constant current to a light source; the light source produces a beam of light which is detected by a fiat, linear, photovoltaic cell. The amount of light reaching the photocell is modulated by a razor edge fixed to the moving coil. As the coil moves up and down the razor edge moves with it, this changes
the amount of light reaching the photocell, and hence tile current produced by it. This current, which is proportional to the displacement, is amplified by a currentvoltage convertor (Fig. 5) to yield a voltage proportional to the displacement of the moving coil. A reference-voltage source (Fig. 6) provides a standard
m a g n e t ~
' rf.~n
~ ]
/
d'iU?iniun
..r~ edg
Fig. 2 Diagram of motor unit
Pch~ tO I~bUlcbosure ventilation
extended coil
stimulating rod REFERENCE VOLTAGE SOURCE
CURRENT- toVOLTAGE CONVERTER
optical link --:..-_.---
photo- t - ~ cell ~ I____
CURRENT
,
ig source
GENERATOR
Fig. 3 Block diagram of optical displacement transducer system
MOTOR
UNIT +22v
reference o in
i
R1
0
2.2 k
LAMP
OUT TO o 240mA
~
TIP32A
Fig. 4 Current generator system
R2I 2.7k 586
Medical and Biological Engineering
September 1976
voltage which is used (i) to control the intensity of the light source and (ii) as a reference for the current-voltage convertor. The current drawn by the lamp develops a voltage across the four 82 f~ resistors in the current generator (Fig. 4). This voltage is compared with the reference voltage derived from the reference-voltage source. The comparison is carried out by the transistor 2N 3704 which adjusts the regulating transistor TIP 32A so that the current is kept constant. The current generated by the photocell is compared with a reference current (Fig. 5)--this is derived from the
standard voltage provided by the reference-voltage source--and is then applied to the current-voltage convertor; this circuit has a low input impedance. The output of the current-voltage convertor gives a voltage proportional to the current at the input. The displacement information from the optical transducer system is fed to a displacement-signal modifier (Fig. 7). This circuit operates in such a way as to produce an output, which, compared with the input, has an element of velocity feedback contained in it. This is achieved by means of the transfer characteristics of the RC network preceding the operational amplifier. The
"1 adjusted for zero V0 at zero displacement
~OpF ~fe~nce 8.2k I, I
"+lOv
Fig. 5 Photocell and current--voltage convertor adjust gain
270 f
36,700pF
ram
displacement c transducer
o input
out
,'_~
1
I
3
Fig. 6 Displacement signal modifier
m
5-6v~
3"3k ~_ 2N3702
10v reference out
2N3704 ~25pF
8.2k ~ u
V--
1
0
I
680
470
:N3704
Fig. 7 Reference voltage source
680
~'~'4"7v Medical and Biological Engineering
September 1976
587
output of the device thus consists of two components, a displacement signal and a velocity signal. From this circuit the signal is fed to a comparator where it is compared with a reference electrical signal representing the required stylus movement. The actuating error signal then goes to an amplifier-limiter unit (consisting of a high-voltage-gain amplifier followed by a power amplifier) which drives the motor in such a way as to minimise the error. The power amplifier is self limiting, it thereby controls the maximum voltage that can be applied to the motor. The gain of the amplifier, the design of the limiting circuit and the proportion of the velocity feedback have to be related so that the control loop can operate with a dual mode of action, i.e. relay operated for large values of displacement error, linearly operated for small values. This gives a system with a very small steady-state error and with a very fast response to suddenly applied signals (THALER and BROWN, I960). For the linear mode of operation, fast rise and settling times can be obtained by using a high gain and an appropriate amount of velocity feedback.
IOOmsec Fig. 8 Response of type I mechanoreceptor in the sural nerve of the rat to quantitative mechanical stimulus of 200Fro derived from mechanical stimulator. The response is shown on the top trace
feedback required was initially estimated using classical control theory and then adjusted by trial and error to obtain the best system response.
Results The kind of result that can be obtained by the use of this device is shown in Fig. 8. The record shows the response of a Type l mechanoreceptor in the sural nerve of the rat to a stimulus amplitude of 200/~m.
Acknowledgments--We wish to thank L. Harrison, D. Alpsan and D. Proffitt for advice and help. M. J. O'Connell was a U K Science Research Council scholar and was supported by the Wellcome Trust during the initial part of this work.
References ARMETT, C. J., HUNSPERGER,R. W., GRAY, J. A. B. and LAL, S. (1962). The transmission of information in primary receptor neurones and second order neurones of a phasic system. J. Physiol. 164 395--421. KHALAFALLA, F., LAL, S., O'CONNELL, M. J., TURNER and LINDA M. (1973). An electromechanical stimulator with wide band characteristics. J. Physiol. 229 1-2P. THALER, G. J., and BROWN, R, G. (1960) Analysis and design of feedback control systems. (McGraw-Hill), 433-478. M. J. O'CONNELL F. KHALAFALLA
Department of Electronics Chelsea College London SW3 6LX England S. LAL
The nonlinear mode of operation is caused by the saturation of the error amplifier. Under these conditions a different amount of velocity feedback is required for stable operation of the system. The amount of velocity
588
Department of Physiology Chelsea College London SW3 6LX England
Medical and Biological Engineering
September 1976
