Download EE 4BD4 Lecture 28

EE 4BD4 Lecture 28
Electrical Muscle Stimulation
1
Purpose
• Provide muscle control where path from brain
is interrupted
• Provide feedback control during muscle
retraining
• Keep muscle healthier when it has been
dennervated (no alpha motor unit connection)
• Strengthen or maintain muscle when joints
have been immobilized
2
Muscle Stimulation to Maintain
Health
• What happens to muscle after the nerve
supply is lost due to an accident
3
Muscle Structure after 1 Month
• Muscle Cross-sections A – denervated, B – denervated and stimulated, C –
innervated; bar = 100 µm
4
After 3 months + 3 months Nerve
Attached
5
General Instrumentation Layout
Control
Source
Pulse
Generator
Pulse
Shaping
Pulse
Output
Electrodes
6
Control Source
• Generate the pulse timing information and
the pulse amplitude and duration
• Can generate single pulse timings or the start
and stop of trains of pulses at a set frequency
(i.e. the start and number of pulses)
• Can provide constant pulse amplitudes or
shaped trains
• Usually computer or microcontroller
7
What Pulse Shapes Have Been Used?
8
Final Forces Produced by Muscle
9
What Pulse Train Frequency?
10
Pulse Generator
• Can use constant voltage (i.e. current determined
by patient electrode/skin impedances) or
constant current (voltage across electrodes
determined by patient skin/electrode
impedances)
• Stimulation is determined by current flow so
constant current is more modern design
• For surface stimulation and motor nerve close to
skin with 50 μsec pulses get sensation around 10
ma and nerve stimulation around 15 ma
11
Constant Current Designs
• Patient not grounded
12
Howland Current Pump
• Patient grounded
13
Limitations of These Designs
• Voltage compliance (maximum voltage
resulting from impedance) is limited to power
supply voltages)
• With typical load impedances of 1 kΩ and 30
ma stimulating current you already require 30
V, requiring high voltage op amps (and power
supplies)
14
Simpler Designs
Transistor Based
Op amp Added
15
Safety Lockout
• Transistor switch added to stop inadvertent
stimulations
16
Specs for Last Circuit
• Transistor is TIP50
• Op amp is LM358M
• Setting resistor determined by current
required at that input voltage
• VHV Can be high voltage source
17
Overall System
18
+15v
Rat Muscle Stimulator
5V power
Power
5V
Isolator
isolator
PC
USB
Microcontroller
CNTRL 1
High
CNTRL 2
High
CNTRL 2
Low
CNTRL 1
Low
To constant current
source and H-bridge
components
To CCS
Optocoupler
Optocoupler
H-bridge
Inside the H-bridge
Constant
current source
Inside the
constant current
source (CCS)
From H-Bridge
From
H-bridge
Vin
DAC
+
-
From optocoupler
Rsense
Current
through
Vin
______
rat =
Rsense
19
20
Safety Lockout
• Transistor switch added to stop inadvertent
stimulations
21
High Voltage Application
• What do we do if our compliance voltage exceeds
the supply voltage?
• (1) Use a DC-DC converter to give high voltage source
to drive circuit Fig 3.8 slide 21
• (2) Replace load in Fig 3.8 by the input coil of a pulse
transformer
• If output current to electrodes is to be e.g. 40 ma,
the input current must be 480 ma if transformer
ratio is 1:12
• May then replace transistor in Fig 3.8 by a Darlington
pair for current amplification
22
Darlington Pair
• Used as a current amplifier for higher current
loads
• Can be constructed from two bipolar
transistors or implemented with an IC
• If very high currents req’d (amps) use power
transistors
• Disadvantage is there is considerable voltage
drops across the junctions so that the voltage
seen at the input coil is reduced
23
Darlington Pair for Current Amplification
24