First demonstrated at the Chicago Sensor Expo 2012, Plessey has now developed a heart rate monitor using its award-winning EPIC™ sensor technology, which can be used in a wristwatch. The technology means that it does not require a chest strap or second sensor at the end of a cable that can be easily lost or damaged.
The watch has a sensor electrode embedded in the back, which remains in permanent contact with the wrist while the second electrode is positioned on the front. This means that by touching this top electrode with a finger from the opposite hand it enables the device to collect the heart signals and obtain an ECG. This information can then be transmitted via Bluetooth to a smart phone or PC. The data gathered is actually accurate enough that it can provide detailed ECG signals with the appropriate signal processing, which could be done on a mobile device or PC via a Bluetooth link.
EPIC is an acronym for "Electric Potential Integrated Circuit" but the term has become synonymous with the integrated circuit technology, the sensor itself, and, in a wider context, the physical principles of operation of the device within a system.
EPIC is a noncontact electrometer, meaning that there is no direct DC path from the outside world to the sensor input, a condition that is somewhat analogous to the gate electrode of an MOS transistor. The electrode is protected by a capping layer of dielectric material to ensure that the electrode is isolated from the body being measured. The device is AC coupled with a lower corner frequency (-3dB) of a few tens of MHz and an upper corner frequency above 200 MHz. This response is adjustable and can be tailored to suit a particular application. Such an electrometer cannot be DC coupled because the Earth's electric field close to the surface is ≈100-150 V/m.
In single-ended mode the device can be used to read electric potential; used in differential mode it can measure the local electric field; or it can be deployed in arrays to provide spatial potential mapping (locating a conducting or dielectric material placed within free space).
Figure 1 shows a basic block diagram of the EPIC sensor . The size of the electrode is somewhat arbitrary and depends on the input capacitance required for a particular application. For bodies placed close to the electrode, the electrode's size is important and the device operation can be understood in terms of capacitive coupling. For devices that are several meters away, the coupling capacitance is defined only by the self-capacitance of the electrode and the device's response is largely a function of the input impedance as it interacts with the field. This is rather counterintuitive but is a function of the very small amount of energy that EPIC takes from the field in active mode.
The input resistance to the device can be boosted by using bootstrapping techniques while the input capacitance can be reduced using guarding techniques. The input capacitance can be driven as low as 10-17F with the input resistance being boosted to values up to around 1015Ω, thus keeping the interaction with the target field to an absolute minimum and ensuring that all currents are small displacement currents only.
A better understanding of the feedback mechanisms can be obtained by considering the input buffer of the amplifier and its associated impedances as shown in Figure 2. The resistors RG1 and RG2 are used to set the gain of the first stage, which is nominally unity. Cin and Rin represent the input capacitance and resistance native to the amplifier, respectively, and include any parasitic components due to layout or substrate issues. The capacitor Cext models the capacitive coupling to the measurement target.
For close coupling (Cext >> Cin) this is usually defined as
a = the equivalent shared electrode/target area
d = the distance between target and sensor
ε0 = the permittivity of free space
εr = the relative permittivity of the dielectric in which the sensor is operating
For loose coupling (Cext << Cin) we have the limiting case (self-capacitance) shown as
Where r is the diameter of the sensor plate.
Analysis of the circuit shows us that we have a classic single-pole transfer function shown as
The Bode plot for this is shown in Figure 3.
The corner frequency (Fc1) can be expressed as
By applying the bootstrapping techniques mentioned earlier, we can control the values for Cin and Rin to give effective values, allowing us to control both the gain plateau and the corner frequency (Fc1 moves to Fc2). The response of the sensor can be further controlled by the design of subsequent stages and positive feedback loops. Thus we have a sensor that can be tailored to suit the particular application at hand.
Figure 4 shows a pair of Plessey EPIC sensors and the associated control box. The control box is an amplifier/filter combination and is used for demonstration purposes only. The electrodes shown here have been tailored for contact ECG measurement but can also be used for remote sensing and other applications.
A great amount of interest has been generated within the medical community where the primary focus is on using EPIC for surface body electrode physiology applications such as electrocardiograph (ECG), electromyograph (EMG), electroencephalograph (EEG), and electrooculargraph (EOG).
The EPIC sensor can be used, for example, as a replacement technology for traditional wet-electrode ECG pads, because it requires neither gels nor other contact-enhancing substances. When the EPIC sensor is placed on (or in close proximity to) the patient, an ECG signal can be recovered. The sensor is capable of both simple 'monitoring' ECG as well as making more exacting clinical diagnostic measurements. In the latter application it can be used as a replacement for the traditional twelve-lead ECG, in which electrodes are placed on the limbs and torso (each pair of electrodes is called a lead and each lead measures the electrical activity of the heart from a slightly different perspective) to achieve a clearer picture of how the patient's heart is working. An array of EPIC sensors placed on the chest can be used to recreate the lead required with resolution as good as or better than that achieved using traditional systems. Figure 5 shows a comparison between the results using EPIC and using traditional wet electrodes for leads II and aVL . These two leads are important in the diagnosis of conditions such as coronary artery occlusion.
The sensor can also be used for recovering other physiological signals such as those caused by the electrical activity of the eye muscles as one looks left, right, up, or down. These signals have unique signatures; an EOG can be used to track the position of the eyes and therefore produce targeting information for military and gaming applications, for example. Perhaps the most exciting application in the medical field is that of electroencephalography (EEG) where the electrical activity of the brain is recorded. Application of the EPIC sensor to this field is still in its infancy but the potential ability to record identifiable signals against known thought patterns opens up possibilities that currently only exist in science fiction.
Because of EPIC's mode of operation, it can be used to detect any disturbance in the local electric field at distances of up to several tens of meters. The human body, because it acts as a large container of conducting/polarizable material, causes a large perturbation in the electric field and so presents an easily detectable target for the sensor. Sitting a few meters away from the sensor, one has only to raise the sole of one's foot to create a strong signal. Arrays of sensors can be used to provide spatial resolution and therefore the location of a target. Such arrays can also distinguish between humans and quadrupeds because the time signature of the response is a direct function of cadence. Such a system of sensors could perhaps be used for border security in remote areas.
The ability of EPIC to resolve signals unique to various muscles or groups of muscles presents opportunities for improved man-machine interaction. For example, a quadriplegic who currently depends on either a unicorn stick or a suck/blow tube to issue commands to equipment within his or her local environment could achieve a faster and more efficient interaction using EPIC for eye tracking and detection of activity in any muscle groups still under voluntary control. Alternatively, because EPIC can assign a unique signature to the use of certain muscle groups, it opens up many possibilities for interfacing with and controlling prosthetic limbs.
EPIC is also a useful tool in the microscopic domain. Small sensors scanning a microchip, for example, can show areas of high or low potential, allowing the user to map the current distribution within metal tracks and other circuit elements. Faults in dielectric materials can also be detected either by passive means (by detecting piezoelectric effects) or by identifying leakage paths in an active circuit.
Recently a ≈6 µm sensor has been used to reveal a human fingerprint left on an insulating PTFE material (Figure 6) and to characterize its decay over time . The advantage to the forensic scientist of being able to date a fingerprint is obvious. The technique is nondestructive and leaves no chemical residue, which means that DNA samples can be taken at a later date.
The release of EPIC technology into the wider commercial environment has been talked about as being disruptive. The technology is certainly novel in its operation and opens up a wide range of fields in which EPIC may be applied to provide solutions to diverse engineering problems. In this article we have only touched on a small subset of these. Other potential applications could include building and vehicle health, communications, and seismology. The future for EPIC is an exciting and challenging one. It is my opinion that in years to come, the introduction of this technology will be seen as marking a milestone in sensor technology development.
 C. J. Harland, N. S. Peters, et al., "A compact electric potential sensor array for the acquisition and reconstruction of 7-lead ecg without electrical charge contact with the skin," Physiol. Meas. 26 (2005) 939–950, doi:10.1088/0967-3334/26/6/005
This page contains summary information and documented details for the currently developed end applications.
See the end of this page for links to detailed application notes.
|Comparison of EPIC electrodes with Ag/AgCl for ECG||17-May-2012|
|Application Note # 291491 - Single arm ECG measurement using EPIC||13-Jan-2012|
|ECG sensor in a SmartPhone||05-Dec-2011|
EPIC datasheets are available for the EPIC product family, defined as:
|EPIC PS25405B advance datasheet||16-Sep-2013|
|Preliminary Datasheet PS25451 EPIC Ultra High Impedance Movement Sensor||21-May-2013|
|PS25251 EPIC Ultra High Impedance ECG Sensor||21-May-2013|
|PS25201A / B EPIC Ultra High Impedance Electrophysiological Sensor||22-Mar-2013|
|PS25203B EPIC Ultra High Impedance Electrophysiological Sensor||22-Jan-2013|
|PS25401A / B EPIC Ultra High Impedance Movement Sensor||22-Jan-2013|
|PS25102 EPIC Ultra High Impedance ECG Sensor Advance Information||11-Jun-2012|
|EPIC PS25014A2, PS25014B2 Application Boards for EPIC sensor PS25402||05-Mar-2012|
|PS25014A1, PS25014B1 Application Boards for EPIC sensor PS25401A||05-Mar-2012|
|PS25012A1, PS25012B1 Application Boards for EPIC sensor PS25201A||27-Feb-2012|
|PS25204 - EPIC Ultra High Impedance ECG Sensor - Advance Information||21-Feb-2012|
|EPIC PS25101 Ultra high impedance adance information||21-Feb-2012|
|PS25012A4, PS25012B4 Application Boards for EPIC sensor PS25204||02-Feb-2012|
|PS25012A3, PS25012B3 Application Boards for EPIC sensor PS25203||02-Feb-2012|
|PS25012A2, PS25012B2 Application Boards for EPIC sensor PS25202||02-Feb-2012|
EPIC sensors are available in can or board form
Included in the demo kits are: -
Currently the sensor is available with mid-range (flat-band) voltage gains of x10 or x50. This corresponds to around 20dB and 34dB.
The PS252xx and PS254xx family (square compact) require a bipolar supply of between
±2.5V and ±4.5V.
The PS25012x family of application boards generate the bipolar supply from a single supply, and so require only a unipolar supply of between +4 and +8V.
The PS251xx family of sensors require only a unipolar supply between +4.75 to +8.0V.
The PS252xx and PS254xx family (square compact) of sensors draw a supply current of around 2.5mA per sensor.
Lower current versions of the compact sensor are in development.
The PS251xx draws higher supply current − around 4.5mA per sensor.
The output of the PS251xx will swing between ±2.5V full scale. For the PS252xx and PS254xx sensors it will swing between the supply rails.
From around 50 to 100 Ohms.
The EPIC sensor detects electric fields and, because the strongest electric field in the
vicinity of the sensor is often from the 50Hz or 60Hz mains electricity supply that ambient
noise is probably what will dominate the sensor's raw output. On 50x sensors that are not
in contact with a subject, the mains frequency signal can swing from rail to rail.
For sensors with electrodes that are designed for contact sensing, the mains pick-up will reduce significantly when both the electrode and the system ground are touched.
Touching just the electrode with no ground will usually increase the mains noise as it couples through the body onto the sensor electrode. System ground contact is made by touching the metal case of PS25101 sensors or the plate on the back of the PS25012 application boards. Where sensors are used without applications boards, some means of touching the GND terminal should be provided if this method of reducing mains pick-up is being used. An alternative approach to reducing mains pick up is to employ a Driven Right Leg technique, as described in our application note Non-contact ECG measurement using EPIC.
As long as the signal is not limited by the supply rails, the required signals (e.g. ECG) can be extracted by a combination of filtering and common mode rejection (by differentially amplifying the output of two sensors). Mains frequency swings from rail to rail do not affect the ability of the sensors to detect movement, as motion will still cause a change in the measured signal. Suitable filtering should still be applied. In very noisy situations, or for measurement of non-contact electrophysiological signals, or for more sensitive movement sensing, the use of 10x gain sensors is recommended.
No. It senses through 360°, which makes it ideal for motion sensing. It is best viewed as something akin to a uni-directional microphone. A (very) small amount of forward sensitivity enhancement (really rear sensitivity reduction) can be obtained by placing a grounded metal plate behind the sensor.
The sensors are AC coupled (although the lower frequency point can be made almost arbitrarily low − typically a few tens of mHz) and so will only sense changes in the electric field. As long as one or more of the object, the sensors or the field (i.e. AC field), are moving then the object can be detected. There is no analogy to a DC baseline measurement for static field conditions.
Yes. Currently we can accurately position a human hand placed between two sensors with an accuracy of better than 10% in one dimension and around 10% in two dimensions. We can track the movement of the hand in real time, which gives the option for recognising gestures with appropriate software. See also the next question, regarding tracking moving targets, which deals with movement and gestures in larger areas.
Yes they can. The latest developments in software allow a single target to be tracked in
real-time whilst moving around an area covered by four EPIC sensors. For example a room with
one sensor placed in each of the upper corners. As the EPIC sensors track what is effectively
the "centre of charge" it is not possible at the moment to track more than one object. With an
array of sensors however we believe that this is possible.
More recent work suggests that monitoring the extant 50/60 Hz field may be a better approach for such applications (security related) and this is currently under development.
No. A highly charged (electrically) child will look like a lowly charged adult. Moreover a child close to the sensors will appear similar to an adult standing further away.
This depends on the charge on the target, the rate of motion and the local environmental conditions.
It is best to think of the EPIC sensor as being characterized by its input referred noise and its gain
rather than trying to describe a range. Adult humans can carry anywhere between zero and several thousand
volts, depending on clothing and their surrounding environment. This can mean that a moving human target
is detectable from say a metre worst case, up to several metres best case.
Recent developments suggest that maybe it is best not to measure perturbations in the quasi DC field but to use already existing AC fields that surround us in our normal day to day life. For example 50/60 Hz signals Using changes in the detectable signal strength amongst an array (or even between a pair) of electrodes it may be possible to locate targets at a greater range and with more reproducibility.
Yes − in some circumstances. If the sensor is placed close against an interior wall then it is possible (but not guaranteed) that electrical activity can be detected on the other side. If the wall contains metal or is in itself conducting then this may act as a Faraday Shield and render detection impossible.
No. The upper frequency detected by currently available sensors is in the tens of kHz range. RF signals, including those from mobile phones are much higher than this and so do not disrupt the EPIC signal.