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Risk Assessment in the Pre-Clinical Development of Ultrasonic Dental Scalers

Views: 0     Author: Emit Dental     Publish Time: 2024-07-17      Origin: Site

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The risk assessment for the clinical application of ultrasonic scaler device should cover the entire product lifecycle, including pre-clinical (before registration), clinical (post-admission), and post-clinical (after the product exits the market) stages. 


Throughout the product's lifecycle, this risk assessment is a "dynamic" continuous "closed loop."


Pre-Clinical Safety Risks

For ultrasonic scaler device, the primary safety risks to consider in the pre-clinical stage include electrical safety, electromagnetic compatibility (EMC), and biological safety.


1. Electrical Safety

    Companies should create an electrical "insulation diagram" for the ultrasonic scaler device according to the relevant requirements and conduct a safety assessment of the instrument.


2. Electromagnetic Compatibility (EMC)

    The EMC of ultrasonic scaler device is primarily considered during the initial design stage. By adhering to the following design principles in the early development stage, one can effectively suppress and mitigate "electromagnetic interference" in ultrasonic scaler device.


    The principles and sequence for EMC design standards are as follows: dedicated product standards → product standards → general standards. If a product has dedicated product standards, its EMC performance should meet these specific standards. If not, product standards should be used for EMC testing. If product standards are unavailable, general standards are used for EMC testing, and so on.


    The basic method for EMC design of ultrasonic scaler device includes index allocation and functional block design. Firstly, allocate EMC index requirements of the entire product to each functional block, dividing them into product-level, module-level, circuit-level, and component-level indices. Then, design according to the functional and EMC index requirements of each level, implementing necessary protective measures.


Important considerations for effective EMC design of ultrasonic scaler device include:

(1) Concurrent EMC Design During Initial Development

    Experience shows that solving EMC issues during the development stage costs 1 unit, solving them at the finalization stage costs 10 units, solving them during mass production costs 100 units, and solving issues discovered during user use costs up to 1000 units. Therefore, addressing EMC issues early in the development stage can resolve 80%-90% of EMC problems before finalization. Solving them during production or usage stages not only poses technical challenges but also leads to significant wastage of human and financial resources. This underscores the necessity of resolving EMC issues as early as possible for any product.



Chip and printed circuit board 


The selection of active components like chips and the design of printed circuit boards are crucial.


Chip for Ultrasonic Scaler Device


First, attention must be given to the inherent sensitivity characteristics and electromagnetic interference (EMI) emission properties of active components such as chips.

Active components, like chips, can be divided into two types: tuned devices and basicband devices.

Tuned devices act as bandpass components with frequency characteristics that include center frequency, bandwidth, selectivity, and out-of-band distortion response.

Basic band devices act as low-pass components with frequency characteristics that include cutoff frequency, passband characteristics, attenuation slope, and distortion response.

Apart from frequency characteristics, there are also input impedance characteristics, input terminal balance characteristics, and sensitivity characteristics.

The sensitivity characteristics of analog devices depend on sensitivity and bandwidth, while for digital devices, sensitivity is determined by DC noise margin or noise immunity.


Active components such as chips have two types of EMI sources: conductive interference sources and radiative interference sources.

Digital devices are the most common broadband interference sources; the shorter the switching time or rise/fall time, the wider the occupied spectrum.

Transient current is the initial source of both conducted and radiated interference. To reduce transient current, the PCB grounding impedance must be minimized, and decoupling capacitors should be used.

To control differential-mode radiation on a printed circuit board, signal lines and return lines should be kept close together to reduce the loop area of the signal path.

To control common-mode radiation, grounding grids or ground planes can be used, as well as common-mode chokes.

Of course, reducing frequency and signal levels, increasing the signal’s rise/fall time, and choosing grounding points carefully are all important measures to reduce radiation.


Secondly, in designing printed circuit boards (PCBs), it is preferable to use multilayer boards, arranging digital and analog circuits on separate layers. The power layer should be close to the ground layer and positioned beneath it. Interference sources should be placed on a separate layer, away from sensitive circuit layers.

It is important to note that single-sided boards, though easy to manufacture and assemble, are only suitable for general circuit requirements and are not ideal for high-density or complex circuits. Double-sided boards, on the other hand, are suitable for moderate assembly density requirements.


The basic principles of PCB design are as follows:

- 20 “H” Principle: "H" is the distance between two layers. The component plane should be 20 times smaller than the ground plane to reduce radiation.

- 2 “W” Principle: "W" is the trace width, meaning the spacing between traces should be at least twice the width of the trace. Traces should be short, wide, uniform, and straight; rounded corners should be used if turning is necessary. Avoid abrupt changes in trace width, and avoid sudden trace turns.


circuit board for Ultrasonic Scaler Device

To further control radiation, the following principles should be followed when routing the PCB: Signal and power lines should be as close as possible to the ground or return lines to reduce the loop area of differential-mode radiation.


Ground lines between signal lines can help reduce interference. Digital components should be grouped by logic speed and placed relatively close together. High-frequency and high-speed components should be placed close to the PCB connector, and high-level circuits should be isolated from sensitive circuits.


Ground design


Ground design is one of the most critical and challenging aspects of product design. The term "ground" generally refers to the zero-potential reference point in a circuit or system, which can be the product's metal casing or a grounding plane. An ideal grounding plane would have zero resistance, meaning there would be no voltage drop across it when current flows, resulting in no potential difference between grounding points. However, in actual products, such an ideal grounding plane or ground line does not exist. Every ground line has both resistance and reactance, and when current passes through it, a voltage drop inevitably occurs.


There are various grounding types, including floating ground, single-point ground, multipoint ground, and mixed ground. Floating ground is prone to static charge accumulation and discharge, making it rarely used. Since ground lines are not ideal zero-impedance paths, single-point grounding is typically preferred for unit circuits. For multistage circuits, grounding points should be chosen at the input of low-level circuits to minimize the interference caused by ground potential.


common mode interference


To prevent self-excitation in multistage small-signal amplifiers and high-gain amplifiers, shielding is often employed. The shield of an amplifier should be grounded at a single point, ideally at the output end of the ground line. In large, complex ultrasonic dental cleaning devices, which contain various electronic circuits and different motors and electrical components, ground lines should be grouped.


Generally, this involves classifying ground lines into signal ground, noise ground, metal component ground, and chassis ground, a method known as the "four-set method." This approach effectively addresses ground interference.


Thus, the design of the grounding system for large, complex products can be structured in the following steps:

1. Analyze the interference characteristics of various electrical components within the product.

2. Analyze the operating levels, signal types, and anti-interference capabilities of each circuit unit.

3. Classify and group the ground lines.

4. Create an overall layout diagram.

5. Develop a grounding system diagram.


Due to the presence of ground voltage between two different grounding points, multipoint grounding in circuits that have signal connections can create ground loops. This ground voltage will superimpose on the useful signal, affecting the load terminal and resulting in differential mode interference. Additionally, external electromagnetic fields can induce electromotive force in the ground loop, leading to ground voltage and creating differential mode interference as well.


In ultrasonic dental cleaning devices, differential mode interference and common mode interference caused by ground loops—collectively referred to as ground loop interference—must be taken seriously. These devices typically use a metal plate, such as a chassis, as a grounding plane. However, there is inherent impedance between any two points on this grounding plane. When grounding currents pass through, they create ground voltage, which can induce ground loop interference.


The generation of grounding currents can primarily be attributed to several factors:

1. When grounding at two or more points forms a grounding loop, conductive coupling can create grounding currents.

2. The presence of distributed capacitance between circuit components and the grounding plane can result in grounding currents due to capacitive coupling.

3. Electromagnetic induction can generate grounding currents when coils within the circuit are in proximity to the grounding plane.

4. Radiation interference hitting the grounding plane can also induce electromotive force, resulting in grounding currents.


Consequently, there is always some ground voltage present on the grounding plane, and if it overlaps with useful signals, interference occurs. To mitigate this, one approach is to insulate the signal ground from the chassis ground, significantly increasing the impedance of the ground loop. This method allows most of the ground voltage to drop across the insulation resistance, thereby reducing the voltage applied to the conductors.


Another solution is to replace unbalanced circuits with balanced circuits, ensuring that the impedance of the signal line and return line to ground is balanced. This way, common mode currents driven by ground voltage will be equal in both lines, eliminating differential mode interference at the load terminal.


differential mode interference


Furthermore, cutting the ground loop can effectively suppress ground loop interference. For example, inserting isolation transformers, common mode chokes, or opto-isolators between two circuits can yield good results. To further suppress common mode interference, it is advisable to segment the grounding layer of the printed circuit board (PCB) near connectors, creating a dedicated "clean" ground. Each input/output line, including signal and return lines, should be decoupled with parallel capacitors and connected to the "clean" ground, allowing common mode currents on the PCB to be bypassed by the decoupling capacitors before output.


Shielding technology


Shielding technology is used to suppress the propagation of electromagnetic interference (EMI) through space. EMI spreads in the form of electromagnetic fields and waves, and it is typically contained by surrounding the area that needs shielding with metal or magnetic materials. This isolation between the shielded area and the external environment can effectively interrupt the propagation of EMI, achieving effective shielding.



The effectiveness of shielding can be evaluated using the shielding effectiveness (SE), which is the ratio of the electric or magnetic field strength at a measurement point before and after the application of shielding. Electric field shielding aims to mitigate interference caused by electric field coupling between interference sources and sensitive equipment. A metal shield and grounding are essential for effective electric field shielding.



Magnetic field shielding aims to suppress interference from magnetic field coupling. Different measures are needed for magnetic shielding depending on the frequency. For low-frequency magnetic fields, ferromagnetic materials such as iron and silicon steel sheets can be used. The higher the magnetic permeability of the ferromagnetic material, the greater the shielding effectiveness. Increasing the thickness of a single-layer shield can enhance effectiveness, but this is not always cost-effective; using a multi-layer shielding approach is often better.



For high-frequency magnetic field shielding, good conductors like copper and aluminum are recommended. When a high-frequency magnetic field passes through a metal plate, it generates significant eddy currents that create a counteracting magnetic field, thereby canceling the original field. The shielding effectiveness of the shield is related to the magnitude of the eddy currents produced. Additionally, high-frequency currents exhibit the skin effect, flowing only on the surface of the metal, so a thin metal layer can effectively shield against high-frequency fields.

high frequency.jpg



If the shielding body is well grounded, it can simultaneously shield high-frequency electric fields. Electromagnetic field shielding is utilized to suppress interference that occurs at a distance from sensitive devices due to electromagnetic wave coupling. To shield both electric and magnetic fields, good conductive materials are required. When electromagnetic waves strike the surface of a good conductor, they are reflected and absorbed, significantly reducing electromagnetic energy and thus achieving shielding. At high frequencies, absorption losses dominate, while at low frequencies, reflection losses are more significant, and shielding effectiveness generally increases with frequency.



However, good conductors are not very effective for low-frequency magnetic fields in terms of reflection and absorption, making them suitable primarily for high-frequency electromagnetic fields and low-frequency electric fields. Common materials used for shielding enclosures include copper sheets, iron plates, aluminum sheets, and galvanized iron sheets, typically with a thickness of 0.2 to 0.8 mm. If engineering plastics are used for enclosures, they must be mixed with high-conductivity metal powders to form conductive plastics, or a conductive film can be sprayed onto their surfaces.


electric field


Electromagnetic wave leakage can occur through various holes, gaps, and seams in an actual chassis. To improve shielding of these openings, methods such as conductive gaskets, metal mesh, cutoff waveguides, cutoff waveguide ventilation panels, and conductive glass windows can be employed.

Electromagnetic wave


Shielded cables



Shielded cables feature an additional metallic layer, known as the shielding layer, outside the insulated wire. This shielding layer is typically made of braided copper or aluminum mesh or seamless lead alloy, and its thickness is significantly greater than the skin depth. The effectiveness of the shielding layer primarily derives not from reflection or absorption, but from grounding. This means that the shielding layer only functions effectively when it is grounded.

Shielded cables



For instance, interference from a source circuit can couple into the shielding layer of a single-core shielded cable via the coupling capacitance between the source wire and the shielding layer, as well as the capacitance between the shielding layer and the core wire. When the shielding layer is grounded, interference is short-circuited to ground, preventing it from coupling to the core wire, thus providing electric field shielding.



However, to achieve magnetic field shielding, both ends of the shielding layer must be grounded. When interference current flows through the core wire of the shielding cable, mutual inductance exists between the shielding layer and the core wire. If only one end of the shielding layer is grounded, there will be no current flowing through it; the current returns to the source via the grounding plane, rendering the shielding layer ineffective at reducing magnetic radiation from the core wire.


When both ends of the shielding layer are grounded, at higher frequencies, it can be demonstrated that nearly all return current from the core wire flows through the shielding layer back to the source. The magnetic fields produced by the core wire current and the return current in the shielding layer are equal in magnitude but opposite in direction, thereby canceling each other out and achieving effective shielding.


shielding layer



At lower frequencies, however, a significant portion of the return current will flow through the grounding plane, and the shielding layer may still not effectively shield against magnetic fields. Additionally, if there is a voltage difference between grounding points at high frequencies, common mode currents can arise in the core wire and shielding layer, leading to differential interference at the load end. In such cases, dual-shielded cables or tri-axial coaxial cables may be necessary to address these issues.



In summary, for low-frequency circuits, a single-end grounding approach is recommended. For example, if a signal source connects to an amplifier grounded to a common point, the shielding layer of the cable should connect directly to the amplifier's common point. Conversely, if the signal source is grounded but the amplifier is not, the shielding layer should connect to the common point of the signal source.



For high-frequency circuits, the shielding layer of the cable should be grounded at both ends. If the cable length exceeds 1/20 of the wavelength, it should be grounded every 1/10 of the wavelength. When grounding the shielding layer, it is important to ensure that the shielding layer of the cable and the metal housing of the connectors are well soldered or tightly pressed together at 360 degrees. The core wire of the cable should be soldered to the connector pins, and the metal housing of the connector should be tightly connected to the shielding chassis, effectively extending the shielding of the cable into the shielding chassis.


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