PEMF : Uniform Rotary Exciter Burst Sine (UREBS)
Index
What about the existing marketed PEMF Products?
PEMF Required Specifications
PEMF Reference Design
How to create a Uniform Rotary Magnetic Field (Homogeneity)
Controller Details
Schematic
C++ Firmware
Gerber Files
Build a Hall Effect Sensor
Coils
Details of Timing
Prototype test results
Changing the Frequency of Operation
Rotary vs Vertical and Horizontal UREBS magnetic field *** Cool Videos of effects on Iron Powder ***
What do the PEMF products out there do? It is hard to tell because they generally list no specifications.
If someone is selling anything with out proper specifications, I would avoid it.
Basically all PEMF devices will take electrical power and convert it to some form of magnetic field.
Any wire with an electrical current thru it produces a magnetic field. When the wire is looped into a coil the magnetic field becomes stronger.
All PEMF devices use that principal and generally speaking there are perhaps 3 types:
a) continuous waveform
b) single pulse, repeated
c) Burst waveform, repeated
The most common specifications listed are:
Magnetic Field Strenght: uT (microTesla) or mT (milliTesla) or sometimes listed in Gauss
Waveform Shapes: Common examples, sine, saw tooth, triangle, square wave, etc.
Frequency: generally in Hertz
They may reference research papers, and use unrecognized claims and even include testimonials.
But a proper product must be defined by it's specifications.
The biggest example of an important specification is the uniformity of the field. For example you may see "full body mats" advertised.
That implies you are getting full body coverage of the magnetic field, which no doubt you are. But at what uniformity?
If you knew the field strenght varied for example 1:10 (0.1 ratio) or 1:100 (0.01 ratio) would you buy it?
When you are prescribed a medicine does your doctor say you can take anything from 1 to 10 pills/day? Definately not. Controlled dosage is important for effectiveness.
The problem with magnetic fields is the field strenght from a single magnet or coil normally drops off rapidly versus distance.
Shown above could be a typical Pulsed ElectroMagnetic Field. This is a visual example of some of the specifications needed.
Here is a list of required specifications to define if a PEMF treatment may be effective.
a. Magnetic Field intensity -in milliTesla (mT) or Gauss (G)
b. Frequency of burst in Hertz (Hz)
c. Repetition frequency/rate of burst in Hertz (Hz) and Pulse ON and OFF times.
d. RATE OF CHANGE of magnetic field in Tesla/second. (T/s)
e. Direction(s) of Magnetic Field, (unipolar, bipolar, horizontal, vertical, rotary).
f. UNIFORMITY of Magnetic Field vs distance (expressed as ratio, where 1=perfectly uniform)
Details on Magnetic Field Intensity Selection
The two categories of ICNIRP recommended limits of interest to us is the "Workplace Exposure", and "Intentional Informed Knowledge Exposure".
We know the ICNIRP workplace recommended limit is 1mTrms. ICNIRP also gives further guidelines for
pulsed exposure:
"One type of waveform is the narrow-band sinusoidal
burst, Fig. 2a. This consists of a series of pulses, each
containing at least five or more coherent cycles of
sinusoidal oscillations with very little distortion (i.e., no
higher order frequency harmonics). This waveform has a
peak value that may greatly exceed the root-mean-square
(rms) value depending on the duty cycle which is the
ratio of the “on” time to the pulse repetition period.
Typical examples of sources of sinusoidal-burst fields
below 100 kHz are some anti-theft devices. The ICNIRP
guidelines can be applied for exposures to phase-
coherent sinusoidal bursts by simply restricting the peak
amplitude below a value obtained by multiplying the rms
reference value valid at the carrier frequency by SQRT of 2 ".
This for a 265Hz signal this would mean 1mT rms * 1.414 = 1.414mT peak (Workplace Exposure recommended limit with x10 safety factor)
Althou we can use the Workplace Exposure recommended limits as a guide, a PEMF device is for intended exposure,
and hence we are operating in "Intentional Informed Knowledge Exposure" range, which limits are not defined by ICNIRP.
Presented here is a reference design for the best possible PEMF device for research purposes. We will call this device a Urebs (Uniform Rotary Exciter Burst Sine)
Start with defining the Specification for such a device:
1. Burst Sine Wave approach, meaning periodic application of magnetic field.
Reason:
- for research purposes sine waves are well defined with no harmonics changing the rise and fall times.
Sine waves are replicable with precision, and fully defined with specification of only the amplitude and frequency.
- many research papers use a burst approach including the historic Bassett non-union bone fractures device(non sine wave),
and the more recent OncoMagnetic device(sine wave), by Houston Methodist Hospital.
- burst approach matches ICNIRP recommendation for magnetic field exposure limits, that states that no time averaging is allowed in exposure limits.
Meaning that a burst could have the same impact as a continuous sine wave for work place exposure.
2.Adjustable burst frequency and level (which together define T/s), and burst time duration and spacing.
Reason:
- to allow repetition of published results.
- adjustable burst time and spacing helps ensure no heating effects.
- adjustable burst time and spacing helps acheive practical power limitations.
- adjustable burst frequency and level to stay within guidelines of exposure limits of T/s.
Design Target:
Frequency: 265Hz
Level: 1mT rms / 1.4mT peak (min desireable)
Tesla/sec: 1.6T/s (computed from 2 * pi * 265Hz * 1.4mTp = 2.3T/s)
(justification is ICNIRP is 2.7T/s for workplace exposure)
3. Burst ON/OFF time duration
- Burst on Time: 40ms (some what arbritary picked becuase adjustable)
- Burst OFF Time: 450ms (some what arbritary picked becuase adjustable)
Overall this results in about a 2Hz Burst repetition frequency.
4. Uniformity (Homogeneity) of magnetic field over range of depth of application.
We will define the objective depth of 30cm to match the approximate depth of the human body when laying down.
Uniformity is defined as a ratio with respect to 1 being the strongest exposure.
For example the OncoMagnetic device varies from 58.3mT@1.4cm to 0.7mT@7cm.
So over a 5.6cm range the uniformity ratio is 0.7/58.3=0.012.
An ideal device would have a uniformity ratio of 1.
Design Target: 0.5 uniformity ratio for 30cm. (meaning over a 30cm distance, the field could vary from 1mT to 0.5mT peak.
5. Type of magnetic field: Rotary (taking advantage of some research papers claiming rotary is superior to directional).
With optional settings for Vertical only field, or Horizontal only field.
One option is to simply rotate a magnet on an axis. The problem with this approach is some pretty huge physical limitations.
To get to a frequency of 265Hz, means a rotation speed of 265 x 60 = 15,900 rotations per minute (RPM), with huge acceleration requirements to support burst operation.
That is very challenging, and only possible with small magnets. The smaller the magnet the more nonuniform the magnet field pattern vs distance.
The solution is to produce the field from electromagnets (coils). However it takes 2 interacting magnetic fields (90 deg apart) to produce a rotating magnetic field.
The classical way to implement a rotating field (for research) is to use 2 pair of Helmholtz coils.
One pair in the X plane, and one pair in the Y plane. Each pair of coils is driven 90 degrees out of phase.
Note that creates a box shape, resulting in problems for human body access.
Here an alternative approach is used, to improve human body access. The rotating field is produced from 2 planes only, instead of 4.
The unique advantage of the 2 plane approach is the magnetic field drop off rate is the same for both the horizontal and vertical fields.
This keeps the rotary magnetic field symmetrical versus distance.
The below coil pairs shows a prototype with a spacing of about 16" (41cm). This distance permits reasonable human body access.
The loop coil pair (2nd one is under the lower spiral coil) generate a vertical magnetic field. The Spiral coil pair generate a horizontal magnetic field.
The above block diagram shows how the coils are driven from the Controller.
Resonance is used to generate the magnetic field, as this is the most efficient method and can provide the greatest level.
The resonance is set up between the total inductance of each series pair, and the corresponding capacitors, using the standard LC resonance equation.
f = 1 / (2π √L C) (Alternately online LC resonance calculators are available.)
It is also necessary that the controller drive the full bridge at the resonant frequency. Typical full bridge driver voltages are 10 to 30V.
However due to the resonance the voltage across the inductors (coils) and capacitors rises to up to 200Vrms or higher.
The driver voltages need to be individually adjustable to allow for adjustment of magnetic field strenght.
The prototype above uses readily available 14/2 electrical wiring. Each wire has 3 conductors each of 14ga.
All 3 wires are connected in parellal to lower the resistance. Some coils have greater than 150m of wire meaning the resistance is about:
14ga wire: 8.3 milliOhm/meter
0.0083 x 150m / 3conductors = 0.42 ohms
Since 2 coils are placed in series the total resistance for a coil pair is about 1 ohm.
At a frequency of 265Hz there is basically no increase in resistance due to the skin effect.
The MCU generates a clock signal at twice the desired resonant frequency, and outputs it to a hardware circuit that divids it by two, and generates 90 deg phase difference.
The enable lines are used to control the burst ON and OFF times. Individual DC/DC converters drive each full bridge circuit to allow individual adjustment of each magnetic field.
The boost line allows the DC/DC convert voltage to be boosted during the start of the resonant burst, to allow a faster rise time on the burst.
Individual Zero crossing detectors, and peak detectors allow feedback to the MCU of the peak resonant voltage and phase of each coil pair.
Since the exact phase at resonant can vary in accuracy, the MCU is capable of adjusting the phase relationship (in a feedback loop) to ensure the resulting produced phase difference is 90 degs.
Shown below is revision 3 of the PCB which has parallel DC/DC converters to allow a high drive level.
(Note that Revision 4 uses 55V supply)
Toroid Coil Construction: 12 1/2 turns of 20ga magnet wire(6 wires in parallel) on a 19.5x33.5x11.1mm yellow/white core
Each wire should be 24" long. The 6 parallel wires yields a series resistance of 3.3mOhms.
Also since the switching frequency is about 200KHz the 20ga wire keeps the skin effect low, keeping the resistance low.
A Capacitor bank is used for each of the two resonate circuits. The resonate frequency is manually tuned for 265Hz (or desired freq) by increasing/decreasing
capacitance for the peak resonance voltage. The 2nd resonance(the one lagging 90 degrees) is also tuned to ensure and approximate 90 phase shift.
The MCU will autotune the phase once the initial manually tuning is down. Ideally 90 degrees matches a duty cycle of 50%. The MCU changes the duty cycle
to fine tune the phase.
The Capacitors should be rated for pulse use and have a high RMS voltage rating. Here the capacitors are the Kemet R75 series with a RMS voltage rating of 600V.
Great caution should be used when working with the high voltage produce by this circuit. Use all caution to prevent electrical shock/death.
Note the Schematic has different pages selected by the tabs.
Detailed Component Selection
Firmware for the MCU
The MicroController is a Texas Instruments MSP430FR5739IRHAT that has 16KB of FRAM.
It can be programmed using the CCS (Code Composer Studio) that is free from TI.
You can program it with an Experimenters board
or a Flash Emulation Tool.
In both case you need to build a custom 2 wire (4 wires with power and ground) programming cable.
firmware download link
Gerber files for the Printed Circuit Board (PCB)
UREBS revision 4 gerber files
can be downloaded from PCBWAY.
Note please review the readme file for instruction on order parameters.
You would also want to order a solder screen, to allow assembly of the board.
All our PCB's are build by hand, using air tool placement, and reflowed in a low cost portable reflow oven.
Thru hole components are then installed.
A part placement guide can be downloaded here .
We commonly print out a few copies and use different colors of highlighters to mark same value of components.
This greatly speeds the placement rate.
You will also note that the below described hall effect sensor is also on the same layout.
If you want to build a hall effect sensor we recommend you cut it out with scroll saw before building it.
If you don't need the sensor, just do not populate that area.
Build a Hall effect Sensor to measure Magnetic Field Strenght
We are going to use an oscilloscope to measure the magnetic field, as then we can view the strength and the phase.
There are 3 common ways to express the level of an alternating field, RMS (Root Mean Square), Peak, or Peak to Peak.
We are going to go with the RMS method on the oscilloscope as it provides for more accurate measurements than a Peak measure.
(Peak measurements are more sensitive to noise). To convert to peak (for a sine wave) just multiply by 1.414.
The Allegro Microsystems ACS70311LKTATN-010B5-C Hall effect sensor
provides for an output of 10mV/G (100mVp/mTp), and a bandwidth of 250KHz. Technically this specification cooresponds to peak values.
However it would be equally valid (for sine wave) to convert the voltage reference to RMS and state 70.7mVrms/1mTp.
We use an op-amp with a gain of 14.14 to convert the 70.7mV RMS to 1V RMS.
That way for each 1V RMS measured on scope would coorespond to 1mT peak.
An edge mount "F" or BNC connector is used to allow the use of a sheild cable directly to the oscilloscope.
A separate wire pair is run for the power.
For convenience we make the sensor powered from the same single voltage supply (30V) available on a connector on UREBS.
Here is a short Video of the rotating phase as a hall effect sensor is manually rotated 180 degrees.
The blue trace is the output from the hall effect sensor amplifier. The Yellow and Purble traces are the 90 degree driving waveforms.
Prototype Bed (Coil Construction) Pictures
This research version has the coils installed in a bed. This potentially would allow for treatment while sleeping.
A timer could be used to control the treatment time.
Details of Timing
The phase relationship is measured by the PCB and automatically tuned to maintain a 90 degree relationship.
This a requirement to create the rotating field.
The phase is measured from the feedback high voltage signal. It is voltage divided down, and feed to the MCU.
The below image shows in blue the signal for the MCU, generated from the yellow high voltage resonance.
The phase will vary slightly through out the burst interval, so the firmware measures the phase at 3 different points in the burst.
(shown below in the blue trace)
The voltage of the resonance is measured twice during the burst (shown below with the blue trace)
These voltages are measures 128 times and averaged from the output of an analog peak detector. The peak detector responce is shown below.
Next there is the voltage boost used at the beginning of the burst timing to decrease the burst rise time.
Test Results: Field Strenght
Revision 3 of the UREBS is capable of Rotary, Vertical, or Horizontal Magnetic Fields, adjustable in 10 steps from 0.4mTpeak to 4mTpeak.
Burst time is 40ms, and the repetion rate of the burst is adjustble from 4 per second(4 Hz), to 1 per 2 seconds (0.5Hz)
However settings above about 3mT may exceed the specifications of the MOSFETs and reduce their lifespan.
As an design for research purposes this is acceptable.
Test Results: Field Uniformity
The uniformity of the magnetic field vs distance is perhaps the most important measurement.
As shown below there is a noticable concave effect to the magnetic field.
This is because the distance used exceeds the distances used in a helmholtz configuration, as we don't need a perfect uniformity. Our goal of 0.5 was deemed acceptable.
Assuming allowance for some practical limitation such as use of a thin 2" mattress for comfort, and upper clearance, the goal is acheive.
If you reduce the treatment area from 18" horizontally to 14" a uniformity ratio of 0.7 is acheived.
Next is the field uniformity in the front to back direction on the test bed. This direction is worse due to it being the spiral coil narrow direction.
So we need to restrict the treatment width to about 10" to acheive similar uniformity. So that results in an oval shaped reasonably uniform area of about 14" by 10" with a height of about 12".
Total cubic inches is 110"(oval) x 12"height = 1310 sq in.
This compared to a device such as the OncoMagnetic device which uses a small permanent magnet, the target area (for uniformity) likely has to be under one square in.
The below scope view is of the highest setting of 9(previous version), which produces a field of 4mTpeak(As of 2024-Sept this is now 5mT peak).
Note the resonant voltage goes to 400V RMS, requiring pulse capacitors rated 600V rms.
Great caution is required at these voltages as it is possible for the voltage to jump through some wire insulation.
(Resulting in possible death)
Also the cyan trace shows the DC/DC converter voltage is not able to keep up, which is why the limit is 4mT peak.
The circuit takes a very high pulse current, so to drive the circuit, it is required to use a 55V (as of 2024 Sept) power supply rated at 5A with proper current limiting.
Proper current limiting means not a hiccup mode type of recovery. This usually means a power supply with an adjustable current limit capability.
Then to handle the pulse current the power supply drives eight 22,000 uF/63V electrolytic capacitors.
When the pulse is generated by the UREBS it takes the necessary pulse current from the capacitors while the power supply goes into current limiting.
Depending on the power setting of the UREBS this could result in up to a 5V voltage drop as the capacitors are discharged.
The capacitors are fully recharged (to 55V) during the off time between bursts.
Changing The Frequency of Operation
The reason for changing the frequency is to change the rate of change of Tesla/sec of the field. Once again caution needs to be expressed,
as it is the rate of change of the field that induces voltages, and possible additional harm to the body. Please review and understand the ICNIRP recommendations, before using any magnetic field.
The frequency of operation of the UREBS can be changed by changing 2 things:
1. The firmware so the frequency of resonance is correct.
2. The Capacitors so the Inductance and Capacitance form the matching resonant frequency.
However lets look at the example of changing from 265 Hz to about 1000Hz. That change is within the practical limits of resonance without changing the inductance of the coils.
Note however that the side effect will be an increase in the resonant voltage. In this case the resonant voltage goes up to about 1400Vrms.
So the capacitors used must be rated to handle this high of voltage. For my test I used the same 600Vrms rated capacitors, but used 3 in series to get a voltage rating of 1800Vrms.
The side effect of using series capacitors is the capacitance drops to 1/3. The increased resonant voltage also is fed back to the control board, so changes in the voltage dividing
is necessary to prevent electrical arcing, and to reduce limits the controll board can handle. In my test case I used 2 external series 470K 3W resistors rated at 750V working voltage each, and placed
these resistors in the feedback line to controller. For safety reasons I placed these resistors close to the connection of resonance, and covered them in heat shrink.
I also changed the divider on the controller to 15K. So I had 470Kx2 and 240Kx4 (all in series) with a 15K resistor to ground.
I placed a X10 scope probe on the 15K resistor and setting the scope to a X1500 probe gave me about the correct readings on the scope.
Reminder that working with high voltage is dangerous, and you also need to take precautions to protect the treatment area from any possiblity of arc'ing, causing electrical shock or death.
Also that these voltages will ark right though electrical insulation. You must provide enough electrical insulation at all points in the circuit.
Rotary vs Vertical vs Horizontal UREBS magnetic field
The UREBS Magnetic Field Exciter, is capable of being switched from a Rotary Field to Horizontal, or Vertical.
It is also capable of alternating the fields during treatment.
It is unknown if alternating field directions has any additional therapeutic effect.
The only reference I could find is the testing of the OncoMagnetic Device.
"We positioned three oncoscillators (3DSeq, Fig. 3A) at right angles to each other and activated them in repeating sequential or alternating cycles compared
to intermittent stimulation with a single oncoscillator (1DSp).
This experiment showed a greater increase in ROS at 2 h in both GBM and DIPG cells with 3DSeq compared to 1DSp stimulation;
however, this difference was not statistically significant (Fig. 3B, C).
The significant increases in ROS levels over control seen at 4 h and 2 h post-stimulation are also not significantly different between 3DSeq and 1DSp (Fig. 3B, C).
This suggests that 1DSp stimulation might be sufficient to produce maximal ROS enhancement given that one activated oncoscillator
sweeps through all angles in a two-dimensional plane."
Below are seperate videos of the different pulsed magnetic Field Effects on Iron Powder.
Rotary Setting: Causes movement away from camera as the Iron Powder "jumps" and rolls away.
Notice also how the Iron Powder has formed vertical towers(flux lines) as well as horizontal flux lines.
Horizontal Setting: forms flux lines, can see some Iron powder "jump" slightly to realign with flux lines
Vertical Setting: All the Iron Powder "jumps" but stays in position.
Alternating Fields Setting:Alternates Rotary, Vertical, Rotary, Horizontal Fields
Added 2024-09-23: Rotating Field Opposite Direction Setting:Rotary Reverse, 4.5mT peak (5mTp not acheiveable in reverse direction)
Note: Paper folded at bottom of video to stop/collect the Iron Powder