How Can We Reduce the Noise?

Electrical noise interference from motors and drives is affecting our sensor signals and HMI
too often, the sensor signals and even the HMI of our metal-cutting machines go haywire, largely because of electrical noise interference from many old, large, nearby motors and drives that we really can't replace. One solution suggested is to start using fiberoptic cable for signal transmission, but that's not cheap. Alternatively, there also are dozens of power filters, uninterruptible power supplies and other power conditioning devices out there, ranging in price from less than $100 to more than $1,000. We could use some information about both alternatives to help sort out the best course of action.

ANSWERS

Try These First
Following are a few basic design rules that might take care of the problems.

Separation between the low voltage control and high voltage power lines: If those lines need to cross, they should do so at 90° angles.

Use shielded cables and/or shielded cable trays. It is important to note that simply using what is typically sold as shielded sensor cable could actually increase the problem, especially when using cables with M12 connectors. Most such cables do not connect the shield to the coupling nut and, as a result, do not provide a clean path to ground.

Provide for solid machine ground interconnections. This is a must when using shielded cables or the shield will serve to equalize the potential between the different machine sections.

Use ferrite chokes.

Reduce the length of the sensor cables. In this case the sensors are connected to field-mounted I/O modules that communicate over a network with the PLC. For this, AS-Interface is especially suitable due to its simplicity and flexibility. Also, the nature of the design gives this network high inherent noise immunity and bulletproof error detection, combined with automatic data retransmission methods.

Helge Hornis, Ph. D., manager, intelligent systems,
Pepperl+Fuchs

Good Earth Ground

Make sure that all the equipment is grounded to a single point, also known as “star” point. This star point should go back to the power supply ground. This will help to reduce ground loop currents. In conjunction with this, use in-line toroid filters with the power supply lines to each piece of equipment. For sensors, use L-C feedthrough filters between the sensor and controllers or PLC. The filter frequency range should be DC to 50 MHz with attenuation of 30 dB or more. The filter ground must be connected to Earth ground.

Karmjit Sidhu, vice president, business development,
American Sensor Technologies

Electrical Isolation Is the Answer

The need for isolation can take on many forms based on the types of signals and environmental issues in a particular location. Types of isolation can include signal isolation, power isolation and data transmission isolation.

To reduce the effect of electrical noise on signals controlling machinery and other equipment, a combination of two technologies prove to be the most effective. These technologies are optical coupling and inductive coupling.

Generally, signals produced by sensors have low levels and are thus susceptible to capacitive and inductive interference, such as those generated by motors, drives and other processes. One method to protect against noise-related issues is with the use of signal isolators/conditioners that use optical and inductive technologies to isolate signals.

The best signal conditioning solutions use optical isolation between the input and output signals and magnetic isolation of the power supply from the input and output. This is commonly referred to as three-port isolation.
This type of solution can be implemented quickly and easily for between $100 and $150 per point.

An additional way isolation can be improved is effective wiring practices. Depending on length of sensor cabling run, voltage signals are easy to use but provide voltage drops when cable lengths exceed even a few feet and can be expensive if larger conductors are used.

Conversion to current signals could benefit because a shielded twisted pair of conductors can transmit a 4-20 mA signal over very long distances. Earth grounding the shield at one end will help provide protection against electrical noise.

The second type of isolation that can be performed is power-related isolation. An effective method to accomplish this is the use of DC-to-DC converters.

For example, the input signal is converted by means of pulse-width modulation into a frequency signal and demodulated again on the output side to form an analog value. An amplifier then generates a standardized analog signal. A galvanic isolated DC-to-DC converter feeds the input and output circuit with a potential-free supply voltage. It also determines the isolation level through its data, air and creepage distances. Galvanic isolation means the circuit is separated from the signal source in such a way that DC current cannot bridge the connection.

Often a larger power supply is used throughout a machine, electrical panel, process or any equipment. On this shared DC power bus, there are usually devices that produce electrical noise. The noise can interfere with other equipment on the power bus. This noise can be isolated by using a DC-to-DC converter, which often uses optical and magnetic isolation to limit the electrical noise passed along. This type of solution can usually be accomplished for around $250 per power bus.