Differential buffer boards do more than reduce electric signal noise between an encoder and a control. They can also reduce wiring costs and installation time, and reduce the number of encoders needed in an installation
Differential buffer boards are a relatively new addition to encoder integration accessories. Each board is an interface assembly that supplies power to and receives output signals from a differential line driver encoder. It measures 6 x 6 x 2 in., and mounts inside a control cabinet. With buffer boards, engineers can:
• Split one encoder signal and send it to eight controllers.
• Filter electrical noise.
• Run signal cables long distances (over 1,000 ft) without a loss in signal voltage.
Buffer boards can simplify encoder wiring where one encoder output is sent to multiple controllers, be they programmable controllers, personal computers, or motion controllers.
Consider a typical 40-ft long material handling system with one master conveyor and four conveyors feeding it, Figure 1. One encoder mounts to the master conveyor and sends a speed reference signal to a drive indicating the speed of the master conveyor. Four other encoders feed speed-reference signals to the feeder drive, one signal for each of the four feeder conveyors. Thus, there are five encoders mounted on the master conveyor. In addition, each of the four feeder conveyors has its own drive and encoder to control its speed. In this example, there are a total of nine encoders — five on the master, and one on each of the four feeders.
However by using one differential buffer board, the single master-encoder signal can be sent to the feeder conveyors, synchronizing them with the master conveyor and eliminating the four redundant encoders on the master conveyor and the associated wiring and termination costs, Figure 2.
Electrical interference, such as EMI/RFI and common-mode noise, on an encoder signal can cause a programmable controller to miscount and lose track of the encoder shaft position or direction of rotation.
Noise can be induced onto encoder signals through the cable (especially where encoder cables are located near ac and dc motor wires), at cable termination points, and through encoder mounts.
Buffer boards help deliver a strong, robust signal from the encoder to the controller by filtering electrical noise from the encoder signal. Part of the filtering comes from the use of differential signals. A differential line driver output channel consists of a signal and its complement, Figure 3 . The encoder generates a differential signal, which runs to a differential line receiver on the buffer board that is mounted in a remote or local I/O cabinet. Differential line receivers respond to the difference in voltage levels between these two complementary signals. Any electrical noise coupled onto the channels is added to both signals, so the differential receiver on the buffer board would not respond to it.
Going the distance
Buffer boards can also help users run encoder outputs greater distances, and reduce the effects that may occur with terminal blocks and line splicing.
Many input modules specify a maximum signal lead length. If the signal is fed more than a specified 250 ft, for example, the I/O circuits can be affected by the capacitance and resistance of the cable and the electrical noise induced over the cable run. Each additional foot of cable makes a control circuit that much more vulnerable to electrical noise. Machines with long cable leads are often the most challenging to design. And, any time a cable run is broken, for example to disassemble a machine or add an I/O rack, the risk of coupling electrical noise to the control circuit increases. This can be a problem for users that need to run the encoder output 1,000 ft or more.
To address this problem, users can mount a buffer board in the control cabinet and run the encoder lead the full 1,000-foot distance to the cabinet. The signal may acquire noise as it travels from the encoder to the cabinet, but once it comes into the buffer board, the board cleans up the signal, and passes it to the controller as if the signal only traveled a distance of a few feet.
Minimizing the effects of voltage drop
A common problem with long signal leads is the loss of signal voltage (voltage drop) over the cable run. Often, control system builders provide a power source for an encoder to compensate for voltage drop. Buffer boards, however, supply power to the encoder and to the signal conditioning circuits on the board, reducing wiring and the effect of voltage drop over long cable runs.
The buffer board regulates its 24-V dc input power, supplying 12 V to the encoder and 5 V to the electronics on the buffer board. The use of a 8 to 24 V-powered encoder also reduces the likelihood of experiencing a voltage drop problem.
From gearing to conduits
Buffer boards can be used almost anywhere encoders are applied, including general motion control, computer numeric control, speed sensing, and feed to length conveyors. Here are a few examples.
One manufacturer had used a mechanical cut-to-length system of one motor to drive two functions in his process. A gearbox split the motor output to turn the feeding and cutting cylinders, which were mounted some distance apart. This system could not measure the material precisely, and gear wear and mechanical strain led to system downtime and excessive maintenance costs.
To eliminate gearing problems and get more precise measurements, this manufacturer replaced the mechanical system with two drives and two motors. One drive powers the feeding cylinder, the other the cutting cylinder. To synchronize drive movements, one encoder mounted on the feeder sends a position reference signal through a buffer board to both drives. The buffer board enables both drives to “see” the same encoder signal.
On many free-standing machines, like CNCs or overhead cranes, encoder signal wires run through cat tracks — flexible conduits that contain multiple electrical cables. Cat tracks prevent the machine from destroying the power cables as it moves. However, power cables in the same conduit can couple electrical noise onto low-voltage control circuits, such as encoders and other feedback devices.
In these applications, shielded cable can provide some protection against electrical noise. But a buffer board can help ensure that this noise won’t disrupt communication between encoders and PLCs on the other end of the cat track. One CNC builder who had this problem put category one conductors, 480-Vac motor power cables, and high-voltage motor starter leads in the same track with 5 Vdc encoder signals. Rather than switch to more expensive fiber optic encoders, or attempt to attenuate the electrical noise on the power circuits, this builder chose to use a buffer board to filter out noise coupled to the encoder signal in the track. In this application, the buffer board provided a low-cost and efficient means to solve the noise problem.
Mike Fox is a senior applications engineer with Allen-Bradley, Co. Inc., Chelmsford, Mass.