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20 Dec 2017, 16:46

Why Wave Distortion Technology is superior

We talked in the past about the benefits of continuous monitors and also introduced the different types (single-wire vs. dual-wire) to you. The focus of today’s post is the technology behind continuous monitors – how they work and how they compare to each other. So, let’s jump right in.

Introduction to Continuous Monitors

While wrist straps are the first and best line of defence against ElectroStatic Discharge (ESD), they must be tested to ensure that they are installed and working properly. On-demand or "touch" testers have become the most common testing method; they complete a circuit when the wrist strap wearer touches a contact plate.

One drawback with on-demand type testers is that they require a dedicated action by the wearer of the wrist strap to make the test. Also, knowing that the wrist strap has failed after the fact may possibly have exposed a highly sensitive or valuable assembly to risk. Continuous monitors eliminate the possibility of a component being exposed to ESD during the time that the wrist strap was not working properly.

If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.

Technologies used for Continuous Monitors

There are three types of wrist strap monitoring on the market today:

  1. Basic Capacitance / Impedance Monitoring
  2. Resistance Monitoring
  3. Wave Distortion Capacitance / Impedance Monitoring

So, let’s look at all 3 types in a bit more detail:

1. Basic Capacitance / Impedance Monitoring

This single-wire technology makes use of the fact that a person can be thought of as one plate of a capacitor with the other plate being ground. The ground and the person are both conductors and they are separated (sometimes) by an insulator (shoes, mats, carpet, etc.) thus forming a capacitor. The combined resistance of the wrist strap and person forms a resistor so that the total circuit is a simple RC circuit.

A tiny AC current applied to this circuit will cause a displacement current in the capacitance to flow to ground providing a simple way to make sure the person (capacitor) resistor (wrist strap) and coil cord are all hooked up. Any break in this circuit results in a higher impedance that can be used to trigger an alarm.

AC capacitance monitors have a few drawbacks:

  1. They do not provide a reliable way to know if the total resistance of the circuit is too low, i.e., if the current limiting safety resistor is shorted.
  2. Simple AC capacitance monitors can be tricked into thinking the person is wearing the wrist strap when they are not. For example, laying a wrist strap and cord on a grounded mat will increase the shunt capacitance, which allows the monitor to show a good circuit even with the person out of the circuit. Forming the cord into a tight bundle or stretching it can also provide false readings.
  3. Since the capacitance and therefore the impedance of the circuit will also vary with such things as the person's size, clothing, shoe soles, conductance of the floor, chair, table mat, the person's positions (standing or sitting), etc., these monitors often have to be "tuned" to a specific installation and operator.


2. Resistance Monitoring

Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. Here again, the concept is simple. By providing a second path to ground (without relying on the capacitor above) we can apply a tiny DC current. It is then simple to measure the DC resistance of the circuit and alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted). Thus, a two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement. The shortcomings with the AC capacitance monitor are eliminated.

Two-wire monitors require two wires to work. This means that the wearer must wear a dual wire two-conductor wrist strap/coil cord which are more expensive than standard single wire wrist straps. There have been some reports that a constant DC voltage applied to the wristband causes skin irritations.

3. Wave Distortion Capacitance / Impedance Monitoring

Wave Distortion Technology continuous monitors feature:

  • low test voltage
  • a low monitor range for 1 megohm of resistance in the operator’s wrist strap and
  • instantaneous detection of an intermittent or failure of the path-to-ground of the operator or work surface that other monitors/technologies miss

Continuous monitors using wave distortion technology apply a continuous test voltage (1.2 volts peak- “Wave Distortion” or vector impedance works by applying a continuous test voltage of 1.2 volts peak-to-peak at 1 to 2 microamperes (0.000002 amperes) to the wrist strap that is connected to the continuous or constant monitor. The test voltage creates a sine wave that the monitor circuit compares to established patterns. By monitoring the “distortions”, or shape of the sine wave, Wave Distortion Technology determines if the monitored circuit is complete - the operator is in the circuit and the total equivalent DC resistance is within specifications. Wave Distortion Technology produces a very fast alarm time (<50 milliseconds) and minimal false alarms.

Comparing Continuous Monitor Technologies

We’ll compare the three different technologies using the following parameters:

  1. Safety Resistor Monitoring
  2. Test Voltage
  3. Banana Jack & 10mm Socket Monitoring
  4. Response Time
  5. In-Use Verification

1. Safety Resistor Monitoring

The purpose of the 1 megohm resistor found in series with wrist straps is solely to provide safety to the human body by limiting the amount of current that could be conducted through the body. The 1 megohm resistor is designed to limit the current to 250 microamps at 250 Volts rms AC. This is just below the perception level (and a bit before the nervous system goes awry) of most people. “Wrist straps have a current limiting resistor moulded into the ground cord head on the end that connects to the band. The resistor most commonly used is a 1 x 106W, 1/4 watt with a working voltage rating of 250 V.” [IEC TR 61340-5-2 User Guide, Clause 4.7.2.5 Current limiting]

  Basic Capacitance / Impedance Monitoring
Resistance Monitoring
Wave Distortion Capacitance / Impedance Monitoring

 

2. Test Voltage

We’ve mentioned further above that some people have reported skin irritations when using resistance monitors which apply a constant DC voltage to the wristband. The problem is that the test voltages of resistance monitors are quite high (up to 16V). You have a similar issue with basic capacitance/impedance monitors (3.5V). Another thing to remember is that higher test voltages increase the risk of damage when handling ESD susceptible devices. Luckily for you, wave distortion monitors only use a test voltage of 1.2 – way below the other two technologies.

  Basic Capacitance / Impedance Monitoring
Resistance Monitoring
Wave Distortion Capacitance / Impedance Monitoring

 

3. Banana Jack & 10mm Socket Monitoring

Coiled cords with banana jack and 10mm sockets are commonly used in the electronics industry. Unfortunately, these cannot be used with dual-wire resistance monitors. As mentioned further above, special dual-conductor wrist straps need to be purchased.

  Basic Capacitance / Impedance Monitoring
Resistance Monitoring
Wave Distortion Capacitance / Impedance Monitoring

 

4. Response Time

Detecting intermittent or complete failures in the path-to-ground of the operator or working surface is the job of a continuous monitor – but, it’s also important to look at how long it takes the monitor to report the issue. What’s the point of using a continuous monitor, if it takes the monitor 5 minutes to tell you there is an issue? All the sensitive devices you handled in the last 5 minutes may have been damaged. An instantaneous detection/alarm is therefore crucial. The slower the response time, the higher the potential impact on sensitive items. Response times for basic capacitance/impedance and resistance monitors is ~1s and ≤ 2s respectively. Wave distortion monitors, on the other hand, have a response time of <50ms.

  Basic Capacitance / Impedance Monitoring
Resistance Monitoring
Wave Distortion Capacitance / Impedance Monitoring

 

5. In-Use Verification

So, imagine this scenario: you received a new constant monitor, you found a nice new home for it, you install it and use it. 12 months down the line, it’s time to verify/calibrate the monitor. You have to remove the monitor from its cosy place, complete the calibration and put it back. What a pain, right? The good news is: the test limits of wave distortion monitors can be verified without removing them from the workstation. Sound like a dream, right?

Basic Capacitance / Impedance Monitoring
Resistance Monitoring
Wave Distortion Capacitance / Impedance Monitoring

 

We’ve created the below table for you to easier compare the different technologies:

Comparison of Continuous Monitors Technologies

As you can see, the latest Wave Distortion Technology provides the most reliable and stable confirmation of an operator’s continuous path-to-ground to ensure ESD sensitive product is protected at all times.

Desco Europe is the newest brand in the Desco Industries family, consolidating our two previous UK-based brands, Charleswater and Vermason. Desco Europe sells the full range of DII products, manufactured in our facilities in the USA and UK, servicing the European market via trained distributors who will sell the Desco Europe value package and complete ESD solution to all ESD users in their territory, leading with hi-end solutions that mark us out from the competition.

20 Dec 2017, 16:46