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Transceivers beef up motion feedback

Transceivers beef up motion feedback

It's been a popular trend among OEM design engineers in the last decade to design autonomous motion control systems with the control electronics and motor in one compact package. But sometimes, the space in the machine for the motor isn't big enough to accommodate an integrated motor and controller. Additionally, engineers may want to separate the motor and controller to avoid the possible undesirable effects control electronics can produce in hazardous work areas. Electronics may not work efficiently in a radioactive or gaseous environment, or in the presence of high heat.

Under some conditions, use of transceivers can be an inexpensive way to shield against EMI/RFI. This is the Faulhaber digital line transceiver series LT-1 from MicroMo Electronics.

However, in a closed-loop motion control system, where the design intent is to get feedback to the controller, distancing the motor from its drive electronics poses two major problems. First, over long distances, attenuation of the encoder feedback signals occurs due to the dc resistance of wire and normal line losses, and the resulting loss or weakening of the feedback signal defeats the purpose of a closed-loop design. Second, the wiring may absorb EMI/RFI noise produced by nearby electrical equipment, further degrading the feedback signals. The longer the distance, the more the chance of EMI/RFI.

There are a number of solutions that help circumvent these problems. Typically, these solutions involve converting the encoder feedback signals to a more robust type of signal. One solution is a fiber optic driver, which can convert the electrical signals from the encoder to pulses of light transmitted over fiber optic cable.

A fiber optic receiver can then be used to convert the light pulses back to the original electrical signals. Since light is not affected by EMI/RFI, and fiber optics allows us to transmit light easily over very long distances, it can be an effective but expensive solution.

One inexpensive solution is a balanced differential driver, also known as a line transceiver. Like the fiber optic driver, the line transceiver (capable of transmitting and receiving) also converts the encoder's electrical signals into a more robust type of signal. In this case, the transceiver produces a "balanced differential" signal (typically 2-6 volts) that appears across a pair of signal lines.

The transceiver converts the encoder channel signal into a pair of signals (called differential signals). These differential signals can then be transmitted very long distances (up to 4,000 ft). Another transceiver can sense the voltage state of the two signal lines, and by examining the "differential" voltage across the pair of signal wires it can output a corresponding data logic state (zero or one). The differential voltage across the pair of signal wires is usually +200 mv and -200 mv, and represents a particular logic state.

Because the transceiver is looking for the differential voltage across an input pair (in essence, acting as a differential amplifier), it has two unique abilities. First, it has the ability to reject EMI/RFI common-mode noise. Second, it can discern logic levels despite attenuation of the signal pairs. Therefore, the use of a line transceiver provides a way to distance your control electronics from the motor, and provides a defense against EMI/RFI interference.

The performance tradeoff for engineers who choose this less-expensive option is bandwidth. Fiber optic devices typically offer higher bandwidths than line transceivers. However, in most motor applications, this bandwidth difference is negligible.

There are practical limits in distancing the motor from its drive electronics. For practical purposes, a typical dc brush motor can be distanced 100-200 feet, more than adequate for most applications. However, there are techniques that can enhance that distance to 1,000 feet if needed for special applications.

To determine when and how to distance your motor from your control electronics, or to determine the right number of transceivers in your next application, and when to use a closed or open loop motion control system, make sure you check with your motion control supplier. They should be able to supply you with an inexpensive and readily available off-the-shelf product or custom solution, and plenty of sage advice.

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