Double Sheet/Blank Detection

Double Sheet Detectors

Double Metal Sheet or Double Blank Detectors are instruments used to measure the thickness of flat metal stock and initiate an output device if the thickness is approximately double the normal thickness.

The terms sheets and blanks tend to be used loosely for the same metal piece. Although blanks originate from sheets or coil stock and may be sheared, cut, drilled or tapped in some fashion.

In both cases double detection sensors are installed at the infeed of a metal forming operation. If the double is allowed to enter, the form tooling is damaged or the machine may jam.

There are two different types of Double Detector approaches. Prime refers to these two approaches or categories as single probe detectors and dual probe detectors.

The Single Probe Double Detector is precisely that. It uses only one probe to measure the thickness of metal. Many people think that a metal proximity sensor will detect metal thickness. It will not. Metal proximity sensors rely on the reaction of induction or eddy currents in the surface of metal.

To measure thickness, a penetrating sensing device is needed that measures a reaction that is proportional to thickness. As far as we know now, the only technique with a single probe that is capable of this is by measuring a magnetic field. This approach limits the single probe to metal that reacts to magnetic fields. That is ferrous metals like steel. How they work is covered in another page.

The single probe detector is normally used when metal is detected from one side only. That is true if there is no possibility of mounting an opposing sensor.

The single probe approach has some drawbacks.

1. First, the metal sheet must contact or come within a few micrometers or thousandths of an inch of the probe face.
2. The thickness sensitivity is limited by the magnetic field the probe generates. A metal blank that is normally 5 mm(.125") or greater requires a single probe that approaches 100 mm (4") in diameter.
3. The larger the probe/magnetic field the more difficult it is for the blank to break away from the probe.
4. To overcome the breakaway force electromagnetic probes are used so the field is pulsed or is on for a short time. This requires more electronics and higher cost and circuit sophistication.

Prime has two single probe double detectors. The difference between them is simply the approach to packaging the electronic circuits.

1. First, is our Model DS100, which has an aluminum chassis.
2. Second, the Model DS101 which is fully enclosed in a polycarbonate standalone enclosure.

A Dual Probe Double Detector is a better choice, if it is possible to mount two opposing probes. The Dual Probe Detector consists of a separate transmitter and receiver. They operate when a signal is generated in the transmitter. The signal passes from the transmitter to the receiver. A metal blank that passes between both probes impedes the signal to the receiver. The amount of signal reaching the receiver is inversely proportional to the metal thickness. This approach allows for the detection of ferrous and nonferrous metals. How they work is covered in another page.

Primes' newest detectors have the ability to automatically configure sensitivity to optimally detect different metal. In an effort to simplify selection we do not have a multitude of detectors for different metals, etc. Typically the level of the Prime detector is determined by the level of input and output features required.

For example the Model DS150 is our basic Double Sheet Detector that utilizes dual probes. Its basic operation is just as capable as our most expensive detector. It, however has only one output relay for double control and simple double indicator lamps.

Next up is our Model DS1500. It has two output relays. One is for "Over" (Double) normal thickness. The second output is for "Under" the normal thickness. This detector has the ability to monitor and reject thickness variations in the metal to within 5% of nominal.

The Model DS1510 is the same as the DS1500 with the exception that it is configurable to calibrate automatically. If there is inactivity for a predetermined period of time the detector will go into a standby calibrate mode. The next metal through is the new calibrated value. This is ideal if the metal thickness changes frequently and it is not possible for an operator to change nominal thickness calibration.

The Model DS1520 has all the features as the DS1510 with the exception that it will reject thickness on the order of 5% change. The Model DS1520 also is expandable to store up to ten thickness applications with the attachment of a Model SM10 selector Module.

The Model DS1522 has all the features as the DS1510 with the exception that it will reject thickness on the order of 5% change. The Model DS1520 also is expandable to store up to sixty-four thickness applications with the attachment of a Model SM11 selector Module or a PLC binary output connection