Med. & Biol. Eng. & Comput., 1979, 17, 271-274
Technical note Morphognostic coils: a technique for transmitting radio signals through the same space 1 Introduction ALL multielectrode neurological prostheses made in the past within this Unit have had many radio channels for conveying the stimulating signals. Either there has been one channel per electrode (BRINDLEY and LEWIN, 1968; FENXON et al., 1977) or, in visual prostheses (DONALDSON, 1973), a matrix coding has used (m-t-n) channels to excite (m • n) electrodes. In both cases, the large number of channels has made it necessary to have an array of small receiver coils in the implant, each inductively coupled to a corresponding transmitter coil of equal diameter outside the patient. Crosstalk between channels depends on:
several nearfield
(c) the useful selectivity would be constrained by the signal bandwidth (d) the total available band would be limited by radio energy loss in the surrounding tissue at high frequencies (SCHUDER et al., 1976).
2 Special coil shapes A n alternative is to use two pairs of coils with zero, or in practice small, mutual inductance between channels,
JL
Char~nelJ L
(a) the mutual inductance between corresponding transmitter and receiver coils (b) the mutual inductance between transmitter and adjacent-channel receiver coils (c) the receiver selectivity (d) the frequency spacing of channels. All the coils in the transmitter array must be held in alignment laterally with a tolerance of approximately one coil radius; the tolerance on axial separation is also of this order and may not be exceeded due to misplacement of the transmitter array or the growth of subcutaneous fat, if the implant is to operate correctly. The use of time division multiplexing increases the ratio of the number of electrodes to the number of radio channels substantially. We are at present developing a 32 electrode implanted nerve stimulator that uses only two channels, one for stimulating information and one for synchronisation and power supply (to be published). This note is concerned with the coil design for such an implant. The larger the coils used, the larger is the absolute tolerance on the placement of the transmitter with respect to the receiver (FLACK et al., 1971). Therefore, in a small implant it is desirable that the coil surrounds the whole. If a second radio channel is required, can its receiver coil be juxtaposed with the first coil without crosstalk between channels? In principle, the receivers could be made sufficiently selective but
/ v.~hannel~. Transmitter coils coils 1
Receiver
Fig. I
Two pairs of coils used
Channel Channel Tran~lllt:er(~ ~ @ 1
2
coils
(a) the receivers would be more complex (b) the resonant frequency drift in the receiver as the potting material absorbs water would cause greater detuning
First received 25th March and in final form 14th May 1978
0140-0118/79/020271 +04$01-50/0 1979
Fig. 2 Coupling diagram showing the anticipated mutual inductance between the four coils shown in Fig. 1
9 IFMBE:
M e d i c a l & Biological Engineering & C o m p u t i n g
M a r c h 1979
271
and large mutual inductance along channels, Fig. 1 shows how this can be done and Fig. 2 illustrates the anticipated mutual inductances of such an arrangement. The channel 2 coils are wound topologically like a figure 8. In principle, the coils could be of equal diameter but in practice there are two reasons to make one coil smaller: (a) together the coils are flatter in this configuration which happens to be convenient (b) the mutual inductance between the two coils at the transmission, or the reception side was found not to be zero. The measured ratio of self to mutual inductance approximately halved when the nominal diameters were made different; then if both coils had two turns (a 'turn' on a number 2 coil is one circuit of the figure 8), the ratio Lch,,,ez t/Lck,,,ez 2 / M = 99/128/1. For this measurement, no particular care was taken to form the coils accurately. The diameters of the coils used were 50 and 62.5 ram. Owing to the very low mutual inductance, no interaction was noticed between the two transmitter circuits.
4 Measurements To test the independence of real coils, three transmitter coils and four receiver coils were made, all wound on a 62-5 mm former. Each transmitter had four 'turns' and was centre tapped; each receiver had two turns. The transmitter coils were connected one after another to a push-pull p.a. stage that was fed from a 7 MHz oscillator. The receiver coils were connected by a • 10 probe to an oscilloscope, and so were loaded by 10 M[2 shunting 11 pF. The receiver coils were placed on a horizontal polystyrene sheet, which was fixed parallel to the plane of the transmitter coil so that the coils were always parallel with a perpendicular separation of 15 mm.
@@ @@ NO. 1
N,O. 2
NO. 3
No. 4
Fig. 4 First four members of the morphognostic-coil family
Fig. 3 Prototype nerve-stimulating implant showing the two receiver coils Fig. 3 shows a prototype implant with the coils described. The diametric part of the smaller coiI is partially hidden under flatpack leadouts. 3 The family of morphognostie coils
The principle demonstrated by the coils of the twochannel system can be extended. Fig. 4 shows the first four members of a family of morphognostic ('shape recognising') coils, i.e. coils where no member has any mutual inductance with any other member so long as they are coaxial. Each one is made by winding successive segments round the perimeter, alternately clockwise and anticlockwise, as shown for number 3 in Fig. 5. Here again the coils can have any diameter since it is only the number of segments that characterises each channel. Careful choice of the diameter may be desirable in order that the mutual inductance between coils alonga channel should not change very rapidly with displacement (FLAcK et al., 1971), or to maximise the efficiency (Ko et aL, 1977). 272
Fig. 5 Winding of coil number 3
Medical & Biological Engineering & C o m p u t i n g
M a r c h 1979
For each transmitter coil, the drive was adjusted so that there was 17.2 V peak-peak across it. Tables 1 and 2 show the e.m.f.s (peak-peak) induced in the receiver coils for all combinations of coils. Where a dash is recorded, signals were less than 10 mV. Table 1 shows the interaction when the coils are ideally placed, parallel and coaxial.
The number 2 coil receiver drives c.m.o.s, logic, the number 1 coil receiver produces an analogue-stimulating information signal for demultiplexing. With the circuitry used, the tolerance on position of the number 2 transmitter is shown in Fig. 6. Beyond these limits, at which the c.m.o.s, fails to operate, the e.m.f, from the receiver,
Table 1. Received e.m.f, in volts: coil interaction in coaxial position T r a n s m i t t e r coil
1 Receiver coil
1
2
3
2.1
--
0.02
2
--
0.99
--
3
--
--
0"52
4
--
0'04
--
Table 2. Received e.m.f, in volts: coil interaction in position of maximal voltage T r a n s m i t t e r coil
1 1 Receiver coil
2.1
2 0.51
3 0.18
2
1.08
0.99
0.32
3
0-48
0.43
0.52
4
0.01
0.07
0.08
The values in Table 2 were found by trial to be the maximal voltages induced in the receiver in any position on the parallel plane. They represent the worst crosstalk possible by misalignment of the coils.
Fig. 6 Maximal position error of number 2 transmitter coil: each distance is for pure movement in the direction indicated (mm)
10 9 8
5 Discussion 7
It can be seen from the first table that if the transmitter coils are correctly placed over the receiver coils, the channels can operate relatively independently due to their spatial selectivity. The reduction in received e.m.f, along each channel as the order is increased (2" 1, 0" 99, 0" 52V) is due to the increasing convolution of the higher fields which reduces flux linkage, and hence the coupling coefficient k, at fixed range. Because the turns ratio is constant for like pairs, the voltage ratio is proportional to k. Table 2 shows that although there are large crosschannel e.m.f.s induced for some misaligned positions, they are never much greater than the maximal alongchannel e.m.f.s, at alignment. A separation of 15 m m was chosen for these measurements because it was typical. Symmetry requires that crosschannel mutual inductance should be approximately zero irrespective of range when the coils are coaxial. However, it was observed that the coil pairs did remain specific whatever their coaxial separation, but, as noted above, k is a different function of range for each member pair. In the 2-channel implant, different frequencies (7 and 10 MHz) are used to separate the channels further. Medical & Biological Engineering & Computing
V 6
5 4 3 2 1 0
0
20 Eccentricity,
40
60
m m
Fig. 7 Normalised voltage on number 1 receiver coil
when unloaded and without voltage regulation, has fallen from its maximum of 45V to 8" 5 V. The voltage in the number 1 coil receiver varies with eccentricity, as shown in Fig. 7. With these results together, one can stipulate the positional tolerance of the transmitter coils. March 1979
273
6 Conclusions
A family of coils have been described, which, by having no mutual inductance, can be used to enable several inductively-coupled radio channels to be superimposed without recourse to frequency selection. The method has been applied to a 2-channel implant and the tolerance on the relative positions of the receiver and transmitter coils are recorded. N. DE N. DONALDSON M R C Neurological Prostheses Unit I Windsor Walk London SE5 8BB, England
References
BRINDLEY, G. S. and LEWIN, W. S. (1968) The sensations produced by electrical stimulation of the visual cortex. J. Physiol (Lond.) 196, 479-493.
274
DONALDSON, P. E. K. (1973) An experimental visual prosthesis. Proc. IEE 120, 281-298. FENTON, G. W., FENWICK, P. B. C., BRINDLEY, G. S., FALCONER,M. A., POLKEY, C. E. and RUSHTON,D. ]31. (1977) Chronic cerebellar stimulation in the treatment of epilepsy: a preliminary report. In J. KIFHN PENRY (Ed.) Epilepsy; the eighth international symposium, pp. 333-340. Raven Press, New York. FLACK, F. C., JAMES, E. D. and SCHLAPP, O. M. (1971) Mutual inductance of air-cored coils: effect on design of radio frequency coupled implants. Med. & Biol. Eng., 9, 79-85. SCHUDER,J. C., GOLD, J. H., STOECKLE,H. and HOLLAND, J. A. (1976) The relationship between the electric field in a semi-infinite conductive region and the power input to a circular coil on or above the surface. Ibid. 14, 227-234. Ko, W. H., L1ANG, S. P. and FUNG, C. D. F. (1977) Design of radio frequency powered coils for implant instruments. Ibid. 15, 634-640.
Medical & Biological Engineering & Computing
March 1979
Technical note Morphognostic coils: a technique for transmitting radio signals through the same space 1 Introduction ALL multielectrode neurological prostheses made in the past within this Unit have had many radio channels for conveying the stimulating signals. Either there has been one channel per electrode (BRINDLEY and LEWIN, 1968; FENXON et al., 1977) or, in visual prostheses (DONALDSON, 1973), a matrix coding has used (m-t-n) channels to excite (m • n) electrodes. In both cases, the large number of channels has made it necessary to have an array of small receiver coils in the implant, each inductively coupled to a corresponding transmitter coil of equal diameter outside the patient. Crosstalk between channels depends on:
several nearfield
(c) the useful selectivity would be constrained by the signal bandwidth (d) the total available band would be limited by radio energy loss in the surrounding tissue at high frequencies (SCHUDER et al., 1976).
2 Special coil shapes A n alternative is to use two pairs of coils with zero, or in practice small, mutual inductance between channels,
JL
Char~nelJ L
(a) the mutual inductance between corresponding transmitter and receiver coils (b) the mutual inductance between transmitter and adjacent-channel receiver coils (c) the receiver selectivity (d) the frequency spacing of channels. All the coils in the transmitter array must be held in alignment laterally with a tolerance of approximately one coil radius; the tolerance on axial separation is also of this order and may not be exceeded due to misplacement of the transmitter array or the growth of subcutaneous fat, if the implant is to operate correctly. The use of time division multiplexing increases the ratio of the number of electrodes to the number of radio channels substantially. We are at present developing a 32 electrode implanted nerve stimulator that uses only two channels, one for stimulating information and one for synchronisation and power supply (to be published). This note is concerned with the coil design for such an implant. The larger the coils used, the larger is the absolute tolerance on the placement of the transmitter with respect to the receiver (FLACK et al., 1971). Therefore, in a small implant it is desirable that the coil surrounds the whole. If a second radio channel is required, can its receiver coil be juxtaposed with the first coil without crosstalk between channels? In principle, the receivers could be made sufficiently selective but
/ v.~hannel~. Transmitter coils coils 1
Receiver
Fig. I
Two pairs of coils used
Channel Channel Tran~lllt:er(~ ~ @ 1
2
coils
(a) the receivers would be more complex (b) the resonant frequency drift in the receiver as the potting material absorbs water would cause greater detuning
First received 25th March and in final form 14th May 1978
0140-0118/79/020271 +04$01-50/0 1979
Fig. 2 Coupling diagram showing the anticipated mutual inductance between the four coils shown in Fig. 1
9 IFMBE:
M e d i c a l & Biological Engineering & C o m p u t i n g
M a r c h 1979
271
and large mutual inductance along channels, Fig. 1 shows how this can be done and Fig. 2 illustrates the anticipated mutual inductances of such an arrangement. The channel 2 coils are wound topologically like a figure 8. In principle, the coils could be of equal diameter but in practice there are two reasons to make one coil smaller: (a) together the coils are flatter in this configuration which happens to be convenient (b) the mutual inductance between the two coils at the transmission, or the reception side was found not to be zero. The measured ratio of self to mutual inductance approximately halved when the nominal diameters were made different; then if both coils had two turns (a 'turn' on a number 2 coil is one circuit of the figure 8), the ratio Lch,,,ez t/Lck,,,ez 2 / M = 99/128/1. For this measurement, no particular care was taken to form the coils accurately. The diameters of the coils used were 50 and 62.5 ram. Owing to the very low mutual inductance, no interaction was noticed between the two transmitter circuits.
4 Measurements To test the independence of real coils, three transmitter coils and four receiver coils were made, all wound on a 62-5 mm former. Each transmitter had four 'turns' and was centre tapped; each receiver had two turns. The transmitter coils were connected one after another to a push-pull p.a. stage that was fed from a 7 MHz oscillator. The receiver coils were connected by a • 10 probe to an oscilloscope, and so were loaded by 10 M[2 shunting 11 pF. The receiver coils were placed on a horizontal polystyrene sheet, which was fixed parallel to the plane of the transmitter coil so that the coils were always parallel with a perpendicular separation of 15 mm.
@@ @@ NO. 1
N,O. 2
NO. 3
No. 4
Fig. 4 First four members of the morphognostic-coil family
Fig. 3 Prototype nerve-stimulating implant showing the two receiver coils Fig. 3 shows a prototype implant with the coils described. The diametric part of the smaller coiI is partially hidden under flatpack leadouts. 3 The family of morphognostie coils
The principle demonstrated by the coils of the twochannel system can be extended. Fig. 4 shows the first four members of a family of morphognostic ('shape recognising') coils, i.e. coils where no member has any mutual inductance with any other member so long as they are coaxial. Each one is made by winding successive segments round the perimeter, alternately clockwise and anticlockwise, as shown for number 3 in Fig. 5. Here again the coils can have any diameter since it is only the number of segments that characterises each channel. Careful choice of the diameter may be desirable in order that the mutual inductance between coils alonga channel should not change very rapidly with displacement (FLAcK et al., 1971), or to maximise the efficiency (Ko et aL, 1977). 272
Fig. 5 Winding of coil number 3
Medical & Biological Engineering & C o m p u t i n g
M a r c h 1979
For each transmitter coil, the drive was adjusted so that there was 17.2 V peak-peak across it. Tables 1 and 2 show the e.m.f.s (peak-peak) induced in the receiver coils for all combinations of coils. Where a dash is recorded, signals were less than 10 mV. Table 1 shows the interaction when the coils are ideally placed, parallel and coaxial.
The number 2 coil receiver drives c.m.o.s, logic, the number 1 coil receiver produces an analogue-stimulating information signal for demultiplexing. With the circuitry used, the tolerance on position of the number 2 transmitter is shown in Fig. 6. Beyond these limits, at which the c.m.o.s, fails to operate, the e.m.f, from the receiver,
Table 1. Received e.m.f, in volts: coil interaction in coaxial position T r a n s m i t t e r coil
1 Receiver coil
1
2
3
2.1
--
0.02
2
--
0.99
--
3
--
--
0"52
4
--
0'04
--
Table 2. Received e.m.f, in volts: coil interaction in position of maximal voltage T r a n s m i t t e r coil
1 1 Receiver coil
2.1
2 0.51
3 0.18
2
1.08
0.99
0.32
3
0-48
0.43
0.52
4
0.01
0.07
0.08
The values in Table 2 were found by trial to be the maximal voltages induced in the receiver in any position on the parallel plane. They represent the worst crosstalk possible by misalignment of the coils.
Fig. 6 Maximal position error of number 2 transmitter coil: each distance is for pure movement in the direction indicated (mm)
10 9 8
5 Discussion 7
It can be seen from the first table that if the transmitter coils are correctly placed over the receiver coils, the channels can operate relatively independently due to their spatial selectivity. The reduction in received e.m.f, along each channel as the order is increased (2" 1, 0" 99, 0" 52V) is due to the increasing convolution of the higher fields which reduces flux linkage, and hence the coupling coefficient k, at fixed range. Because the turns ratio is constant for like pairs, the voltage ratio is proportional to k. Table 2 shows that although there are large crosschannel e.m.f.s induced for some misaligned positions, they are never much greater than the maximal alongchannel e.m.f.s, at alignment. A separation of 15 m m was chosen for these measurements because it was typical. Symmetry requires that crosschannel mutual inductance should be approximately zero irrespective of range when the coils are coaxial. However, it was observed that the coil pairs did remain specific whatever their coaxial separation, but, as noted above, k is a different function of range for each member pair. In the 2-channel implant, different frequencies (7 and 10 MHz) are used to separate the channels further. Medical & Biological Engineering & Computing
V 6
5 4 3 2 1 0
0
20 Eccentricity,
40
60
m m
Fig. 7 Normalised voltage on number 1 receiver coil
when unloaded and without voltage regulation, has fallen from its maximum of 45V to 8" 5 V. The voltage in the number 1 coil receiver varies with eccentricity, as shown in Fig. 7. With these results together, one can stipulate the positional tolerance of the transmitter coils. March 1979
273
6 Conclusions
A family of coils have been described, which, by having no mutual inductance, can be used to enable several inductively-coupled radio channels to be superimposed without recourse to frequency selection. The method has been applied to a 2-channel implant and the tolerance on the relative positions of the receiver and transmitter coils are recorded. N. DE N. DONALDSON M R C Neurological Prostheses Unit I Windsor Walk London SE5 8BB, England
References
BRINDLEY, G. S. and LEWIN, W. S. (1968) The sensations produced by electrical stimulation of the visual cortex. J. Physiol (Lond.) 196, 479-493.
274
DONALDSON, P. E. K. (1973) An experimental visual prosthesis. Proc. IEE 120, 281-298. FENTON, G. W., FENWICK, P. B. C., BRINDLEY, G. S., FALCONER,M. A., POLKEY, C. E. and RUSHTON,D. ]31. (1977) Chronic cerebellar stimulation in the treatment of epilepsy: a preliminary report. In J. KIFHN PENRY (Ed.) Epilepsy; the eighth international symposium, pp. 333-340. Raven Press, New York. FLACK, F. C., JAMES, E. D. and SCHLAPP, O. M. (1971) Mutual inductance of air-cored coils: effect on design of radio frequency coupled implants. Med. & Biol. Eng., 9, 79-85. SCHUDER,J. C., GOLD, J. H., STOECKLE,H. and HOLLAND, J. A. (1976) The relationship between the electric field in a semi-infinite conductive region and the power input to a circular coil on or above the surface. Ibid. 14, 227-234. Ko, W. H., L1ANG, S. P. and FUNG, C. D. F. (1977) Design of radio frequency powered coils for implant instruments. Ibid. 15, 634-640.
Medical & Biological Engineering & Computing
March 1979
