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Direct-Contact Electromechanical Sensor Considerations



A direct contact electromechanical sensor is a sensor that receives a ground signal by directly contacting the part or strip. The most common example of this type of sensor is the spring probe. These are among the oldest die protection sensors still in use, and because of their low cost and simplicity, are still among the most widely used.

How Do They Work?
For electrical safety reasons, any metal object connected to an electrically powered machine is 'grounded' - this includes the control cabinet, conduit, machine frame, the die, and the coil stock. Ground is equal to zero volts, and direct contact electromechanical sensors take advantage of this fact. Inside the die protection controller, each sensor input is 'pulled up" to low DC voltage through an input resistor (which provides 'impedance'). When a direct contact electromechanical sensor is connected to an input, and the sensor contacts the strip, it 'shorts' the input to ground. This 'short' does not create sparks or damage the electronics because the input resistor limits the current. However when the sensor touches the strip, it causes the voltage on the input to change from a low DC voltage (usually 12 to 24 volts) to 0 volts (ground). The die protection controller detects this change in voltage as an actuation.

The Undesirable Effects of Lubricant on the Sensors
Since a direct contact electromechanical sensor requires physical contact with the strip to complete an electrical circuit, anything that comes between the sensor and the strip - such as the lubricant - will affect the operation of the sensor. Oil is an excellent electrical insulator, so when an oil-based lubricant is used it can partially interrupt the sensor's path to ground when it touches the strip. This causes a weak actuation of the sensor that can be missed by the control if the lubricant contamination is severe enough. One way to avoid this entirely is to use a switch probe rather than a standard spring probe. Instead of depending on the material to provide a ground signal, the switch probe has an internal contact that closes the cable terminal to the (grounded) barrel of the sensor when the spring is moved. The switch probe is not affected by lubricants, and is even able to detect non-conductive and pre-painted material.


The switch probe does not require direct electrical contact with the strip
The switch probe does not require direct electrical contact with the strip

Another way to counteract this is for the control to have high-impedance inputs. Inputs of this type require only a small amount of current flow through the sensor (such as the kind exhibited when the sensor and strip are covered in oil) in order to actuate.

The opposite problem exists when water-based lubes are used. Unlike oil, water conducts electricity. If a direct contact electromechanical sensor is coated with water-based lube, a small amount of current will always be flowing from the sensor, through the lube, to the (grounded) tooling or machine. In some cases, the current flow is enough to trigger the input and cause a false actuation. For a die protection control to properly interpret the signals from direct-contact electromechanical sensors operating in a water-based environment, it must have low-impedance inputs that require current flow in excess of the 'leakage' current that is always present when using water-based lubes.


The Wintriss Die Protection Interface (DSI2) features a mix of high and low impedance inputs
The Wintriss Die Protection Interface (DSI2) features a mix of high and low impedance inputs

The Solution
In addition to providing power and signal conditioning for electronic sensors, the Wintriss Die Protection Sensor Interface (DSI2) was designed to interface properly with direct-contact electromechanical sensors regardless of the type of lubricant that they are exposed to. Inputs 1-4 on the DSI are low-impedance for use with water-based lubes. Inputs 5-8 are high-impedance to function when sensors are used in the presence of oil-based lubes. In addition, all eight DSI inputs are capable of accepting input from both NPN and PNP electronic sensors.

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