COMBINED STIMULATION AND MEASUREMENT SYSTEM

FOR ARRAY ELECTRODES

Markus Valtin

1,2

, Thomas Schauer

, Carsten Behling

, Michael Daniel

and Matthias Weber

Control Systems Group, Technische Universit¨at Berlin, Straße des 17. Juni 135, Berlin, Germany

HASOMED GmbH, Paul-Ecke-Straße 1, Magdeburg, Germany

Keywords:

Multi-channel transcutaneous electrical stimulation, Array electrode, Electromyography.

Abstract:

Array electrodes have the potential to signiﬁcantly advance Functional Electrical Stimulation (FES) perfor-

mance and patient compliance by optimizing the electrode position. To evaluate the potentials and for research

purposes, an universal stimulation system for array electrodes has been developed. The system additionally

features volitional EMG recording from the array electrodes during active stimulation. Multiple devices, one

stimulator and at least one demultiplexer, are synchronized to deliver up to 10 stimulation pulses per stimula-

tion cycle at a frequency of ≈ 420Hz. A typical stimulation cycle period is 50 ms. The real-time controllable

array electrode can include up to 60 elements for the active electrode and up to 4 elements for the indifferent

electrode. A small switch module permits placement near the array electrode, eliminating extensive wiring.

The stimulation system is fully controllable from a PC via USB interfaces.

1 INTRODUCTION

For selective transcutaneous Functional Electrical

Stimulation (FES) it is important to place one smaller

active electrode over the motor point of the target

muscle to achieve optimal results. Motor points

are areas where muscles are most likely to be acti-

vated. Finding these motor points requires some ex-

pertise and often another placement point achieves

better results because of patient speciﬁc character-

istics (O’Dwyer et al., 2006) (Popovi-Bijeli et al.,

2005). In FES, usually biphasic, charge balanced,

stimulation pulses are applied. The larger indiffer-

ent electrode is placed away from any motor point to

avoid that the compensating pulse generates an action

potential.

Array electrodes have the potential to simplify the

electrode placement because any number of array el-

ements can form a “virtual electrode”, which can dy-

namically change position and size. This, together

with intelligent control software, allows automatic

optimization towards the distinctive, real motor point

of the target muscle, even for imprecise placement of

the array electrode (Keller et al., 2006). Array elec-

trodes also allow a much more precise muscle activa-

tion in areas like the forearm, where a lot of different

muscles are close together.

A number of research projects used array elec-

trodes because of the above mentioned advantages.

The presented studies demonstrated that a selective

stimulation of the forearm muscles by array elec-

trodes can be realized to induce precise ﬁnger and

wrist-joint movements (Popovi-Bijeli et al., 2005)

(O’Dwyer et al., 2006). A selective correction

of a drop-foot in hemiplegics was investigated in

(Azevedo-Coste et al., 2007). The largest applied

electrode array comprised 60 elements (Keller et al.,

2006). Most stimulation systems presented in litera-

ture serve only one array electrode and do not provide

an open and ﬂexible PC interface for real-time control

of the switch conﬁgurations.

2 CONCEPT

The developed system utilizes an already existing 8

channel stimulation device and extend it for use of

array electrodes. This requires the development of a

demultiplexer and the ability to synchronize the dif-

ferent devices.

The stimulation is controlled by a 20Hz top level

control loop, implemented on a designated com-

puter in Scilab/Scicos (http://www.scilab.org) or Mat-

lab/Simulink (http://www.mathworks.com). Every

50ms, the stimulator will generate a sequence of up

345

Valtin M., Schauer T., Behling C., Daniel M. and Weber M..

COMBINED STIMULATION AND MEASUREMENT SYSTEM FOR ARRAY ELECTRODES.

DOI: 10.5220/0003786303450349

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 345-349

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

to 10 stimulation pulses that are distributed to normal

stimulation channels or channels with a demultiplexer

(arrays). Therefore, the top level control loop trans-

mits a sequence conﬁguration to the stimulator and to

the demultiplexer(s) every 50 ms.

The sequence conﬁguration consist of up to 10 in-

dividual conﬁgurations for the stimulation pulses and

switch settings. This way the stimulator receives up to

10 pulse conﬁgurations which deﬁne the pulse width

and stimulation current of each biphasic stimulation

pulse as well as the information to which channel the

pulse has to be delivered. Each demultiplexerreceives

a corresponding number of switch conﬁgurations for

the stimulation pulses as well as one optional EMG

switch conﬁguration, if an electromyography (EMG)

signal shall be recorded from the array electrode. A

switch conﬁguration deﬁnes which of the available 64

switches are active.

As soon as stimulator and demultiplexer(s) have

received there conﬁgurations they synchronize them-

selves as explained in section 5. The computer on

which the top level control loop runs is not involved

in the synchronization. If applicable the EMG switch

conﬁguration is set after all stimulation pulses of that

stimulation cycle have been sent.

Figure 1 shows an example with a stimulation se-

quence for only one demultiplexer (array) where 4

distinctive muscles are activated.

1 2

5 6 7 8 9 10

comm. (seq. conﬁguration)

one pulse

sequence lenght

500

t [ms]

1 2

5 6 7 8

one switch setting

500

t [ms]

5 EMG 11 EMG

Demultiplexer

Stimulator

1 2

Figure 1: Example for a sequence of 4 pulses for the stim-

ulator device and one demultiplexer. The largest possible

sequence is indicated in gray.

3 SYSTEM OVERVIEW

The core system consists of a RehaStim

stimula-

tor and at least one demultiplexer. A demultiplexer

distributes one stimulation channel over an array of

small, especially designed electrodes. Up to 8 demul-

tiplexer can be connected to one stimulator but stim-

ulator channels can also be used without the demulti-

plexer.

Figure 2 depicts the entire system with all possi-

ble components. For the sake of simplicity, we as-

sume in the following, that only one demultiplexer is

connected to the stimulator.

multi−channel EMG amplifier

USB

to Demux 2

real time controller

(PC with Linux/RTAI)

Ch1 Ch2 Ch3

to EMG

USB

(de−multiplexed current source)

multi−channel stimulator

synchro−

nisation

amplifier

discharging

EMG

plexer 2

covers

of small

muscles

an area

virtual active electrode

virtual indifferent electrode

switches A

protection &

line B1

Array Electrode

covers one muscle

Standard

Pair of

Electrodes

Ch1 Ch2 Ch3

USB to PC

reference

electrode

demulti−

demultiplexer 1

switches B

line A1

Figure 2: System overview.

A standard PC or laptop controls the entire system

by implementing a top level control loop. A real-time

operating system like RTAI (https://www.rtai.org) is

advised but not crucial since the top level control loop

only runs with about 20Hz, providing the next se-

quence conﬁguration for the stimulator and all con-

nected demultiplexer(s). The conﬁguration data is

sent via an USB link to the respective device. No spe-

cial safety precaution is needed since all devices have

galvanic isolated USB ports.

The system uses the certiﬁed, current-controlled

8 channel stimulator RehaStim

available from the

company HASOMED GmbH. The stimulator outputs

a biphasic stimulation pulse with a stimulation cur-

rent from 2mA to 130mA and a pulse width from

10µs to 500µs. The RehaStim

device can be di-

rectly controlled by an external device, preferable a

personal computer. The RehaStim

implements a

special operation mode which accounts for the need

of synchronization and pulse sequences.

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

346

The hardware design of the demultiplexer, the syn-

chronization process and the EMG measurement in-

cluding EMG ampliﬁer protection are described in

more detail later on.

The demultiplexer can support a width range of

array electrodes with two exclusive sets of array el-

ements. The ﬁrst set is used to build the virtual ac-

tive electrode and may consist of up to 60 elements.

The second set with maximal 4 elements is used to

build the virtual indifferent electrode. To minimize

losses and to prevent a short circuit due to miscon-

ﬁguration the array elements related to the virtual in-

different electrode must be placed on a different gel

layer than the array elements belonging to the virtual

active electrode.

4 DEMULTIPLEXER DESIGN

The demultiplexer is divided into two parts, a power

module and a switch module. This segmentation al-

lows the switch module to be placed very close to the

array electrode, with a minimum of necessary cables.

Power Module. The demultiplexer is powered from

an external 12V power supply or battery. The power

module hosts the galvanic isolated USB port and the

galvanic isolated SYNC port.

The SYNC port safely connects the demultiplexer

to the RehaStim

stimulator. The signals “Demulti-

plexer Ready” (DMR) and “Stimulation in Progress”

(STIM), which are essential for synchronization, are

routed through the SYNC port. The synchronization

process is further explained in Section 5.

All demultiplexer functions, including communi-

cation, synchronizationand switch operation, are con-

trolled by a Cypress programmable system-on-chip

(PSoC). The 8-bit MCU core uses a 24MHz system

clock, providing sufﬁcient execution speed to ensure

peak performance and minimized stimulation delays.

The power module communicates with the switch

module over a 24MHz differential data link. This

high speed forward channel includes a CRC check to

ensure data integrity. Any error on the switch module

is reported back over a low speed backward channel

and is processed by the PSoC MCU.

Switch Module. The switch module includes the re-

ceiver for the 24MHz differential data link, the ac-

tual switch matrix and the EMG protection circuit.

The switch matrix consists of 64 high voltage CMOS

analog switches and makes a compact design of the

switch module possible.

One of the eight stimulator channels is connected to

the switch module. The stimulation channel’s active

electrode line is connected to 60 switches, allowing

up to 60 array elements to form the virtual active elec-

trode. The stimulation channel’s indifferent electrode

line is connected to 4 additional switches. A short

circuit due to misconﬁguration is therefore impossi-

ble. The stimulation voltages can rise up to ±150V.

Therefore two 300V rated 34 pin IDC connectors are

used to connect the demultiplexer with the array elec-

trode.

5 SYNCHRONIZATION

Since the system uses independent devices for gen-

erating the stimulation pulses and controlling the

switches, these devices need to be synchronized. The

demultiplexer must set the switches before any stim-

ulation pulse is generated and move on to the next

conﬁguration as soon as the stimulation pulse is com-

pleted. Therefore the demultiplexer needs to know

when a stimulation pulse is generated and the stimu-

lator needs to know when the switch conﬁguration for

the next stimulation pulse is established. This infor-

mation is encoded into the two synchronization sig-

nals STIM (Stimulation in Progress) and DMR (De-

multiplexer Ready).

Figure 3 shows the handshake and the synchro-

nized execution of a stimulation sequence with two

pulses sent via the demultiplexer while no EMG

switch conﬁguration is applied. The pulse frequency

within a sequence is ﬁxed to 420Hz.

Stimulator

Demultiplexor

Communication (seq. conf.)

• reconﬁgure switches to

’Switch Conﬁguration 1’

• set DMR high

• wait for DMR high

• set STIM low = active

• execute

’Pulse Conﬁguration 1’

• set STIM high= inactive

• wait for STIM low

• set DMR low

• wait for STIM high

• reconﬁgure switches to

’Switch Conﬁguration 2’

• set DMR high

wait for next Sequence Conﬁg.wait for next Sequence Conﬁg.

• wait for DMR high

• set STIM low = active

• execute

’Pulse Conﬁguration 2’

• set STIM high= inactive

• wait for STIM low

• set DMR low

initial state STIM = high (inactive)

initial state DMR = low (not ready)

time

Figure 3: Execution of a sequence where two stimulation

pulses are generated and distributed over different switch

conﬁgurations.

COMBINED STIMULATION AND MEASUREMENT SYSTEM FOR ARRAY ELECTRODES

347

If more than one demultiplexer is connected to the

stimulator, the DMR signal will be set high by the

slowest demultiplexer indicating that all demultiplex-

ers are ready. The STIM signal is shared by all de-

multiplexer devices.

6 EMG MEASUREMENT

The measurement of EMG from a stimulated muscle

is of interest in order to detect residual volitional mus-

cle activity. The latter might be used to control the

stimulation. The use of array electrodes for stimula-

tion does not allow the placement of additional EMG

electrodes close to the virtual active electrode due to

the larger size of the array electrode. Therefore, EMG

measurement must be performed from virtual EMG

electrodes formed by elements of the stimulation ar-

ray. As shown in (Shalaby, 2011), EMG can be di-

rectly measured from the stimulation electrodes dur-

ing stimulation if the EMG ampliﬁer is protected and

the electrodes are periodically discharged.

The EMG protection circuit consists of multiple

PhotoMOS relays and a high pass ﬁlter. The required

timing diagram for the PhotoMOS switches is shown

in Figure 4. During delivery of stimulation pulses two

last switch conﬁg. for stimulation

EMG switch conﬁguration

next sequence

f ilter_connected

f s

EM G_valid

PhotoMOS 1 on

PhotoMOS 2 on

1 21 2

Figure 4: EMG protection and discharging timing, t

. . . t

depend on sequence length and array setting.

PhotoMOS switches are used to mute the EMG mea-

surement (PhotoMOS 2 off). After the stimulation is

completed, another PhotoMOS relay short circuits the

electrodes over which the EMG signal is measured for

an user deﬁned time interval to eliminate any resid-

ual charge on the electrodes (PhotoMOS 1 on). The

electrodes then are connected to the high-pass ﬁlter

(PhotoMOS 2 on) which reduces low frequency dis-

turbances. The actual EMG measurement however is

performed by an external standard EMG ampliﬁer.

The duration of the short circuit t

is between 7

and 8ms. The high-pass ﬁlter needs the time t

to set-

tle. Thus it is best to activate PhotoMOS 2 as early as

possible to maximize the valid EMG recording time

EMG

valid

. The stimulation-induced EMG response

(M-wave) falls together with the ﬁlter transients and

is excluded from the EMG measurement. Basically,

only volitional muscle activity is captured in the time

interval t

EMG

valid

. An EMG recording of the wrist

extensor under stimulation is shown in ﬁgure 5. Parts

of the EMG recording which are disturbed by ﬁlter

transients are already blanked in the shown EMG sig-

nal.

41.0 41.5 42.0 42.5 43.0 43.5

−1000

−500

500

1000

1500

time [s]

EMG [uV]

EMG under stimulation

Figure 5: EMG recording of volitional muscle activity

from the stimulated wrist extensor (pulse width 100µs,

I=15mA).

7 CONCLUSIONS

The system enables researchers and health profes-

sionals to use array electrodes without large effort.

The developed software simpliﬁes the setup of com-

plex stimulation patterns and permits a straightfor-

ward integration of array electrodes into existing

stimulation setups. Parameters like pulse widths,

stimulation currents or demultiplexer conﬁgurations

can be adjusted in real-time. The demultiplexer sup-

ports array sizes up to 60 elements for the active elec-

trode and up to 4 elements for the indifferent elec-

trode. The small switch module can be placed near

to the array electrode, avoiding extensive wiring. The

ability to measure volitional EMG signals from a pair

of virtual electrodes makes array electrodes usable for

diagnostics or control applications. In future work,

the remaining problem of ﬁlter transients must be

solved by introducing digital ﬁlters and including the

EMG ampliﬁer into the switch module.

REFERENCES

Azevedo-Coste, C., Bijelic, G., Schwirtlich, L., and

Popovic, D. (2007). Treating drop-foot in hemi-

plegics: the role of matrix electrode. In 11th Mediter-

ranean Conf. on Medical and Biomedical Eng. and

Computing 2007, pages 654–657.

Keller, T., Lawrence, M., Kuhn, A., and Morari, M. (2006).

New multi-channel transcutaneous electrical stimula-

tion technology for rehabilitation. In Conf Proc IEEE

Eng Med Biol Soc., volume 1, pages 194–197.

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

348

O’Dwyer, S. B., O’Keeffe, D. T., Coote, S., and Lyons,

G. M. (2006). An electrode conﬁguration technique

using an electrode matrix arrangement for FES-based

upper arm rehabilitation systems. Med Eng Phys,

28(2):166–176.

Popovi-Bijeli, A., Bijeli, G., Jorgovanovi, N., Bojani, D.,

Popovi, M. B., and Popovi, D. B. (2005). Multi-ﬁeld

surface electrode for selective electrical stimulation.

Artiﬁcial Organs, 29(6):448–452.

Shalaby, R. (2011). Development of an EMG detection sys-

tem for the control of FES in neurological rehabilita-

tion. PhD thesis, Technische Universit¨at Berlin.

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349