Encoder: problems with long cables

By using encoders with long encoder cables, some problems can arise.

The three main problems are not independent of each other

1. Electromagnetic interference

Electromagnetic noise signal influences the signals in the encoder lines and shift the signal levels (high - low). The receiver in the controller may not discriminate the pulses anymore.

Possible solutions:

• A Line Driver is recommended in rough electromagnetic ambient.
• Encoder signal quality can be improved by using paired and twisted (A with /A, B with /B, ...) and/or individually shielded cables.

Remark: The use of an encoder with Line Driver is mandatory for positioning control.

2. Too small driver current

The driver current is too small and the electromagnetic noise influences the signal strongly. The receiver cannot distinguish properly the signal levels. For a correct working of the RS422 interface 20mA per channel is needed. Check the maxon catalog about the driver currents of the respective encoder types.

Possible solutions:

• Use an encoder with Line Driver with sufficiently high current (e.g. 20 mA per channel).
• Mount an additional Line Driver directly at the low driver current encoder with (e.g. DS26LS31 with 20 mA per channel).

3. Too long encoder cable

Voltage drop due to the resistance of the encoder line. The encoder receives a too small a supply voltage. The encoder signals cannot reach the necessary levels.

Possible solutions:

• Reduce the resistance of the encoder cable by selecting a cable with thicker wires.
• Consider the maximum cable lengths.

Besides the line inductance and capacitance, the maximum cable length of an encoder is limited by the total voltage drop U_cable of the encoder supply lines.

The minimum supply voltage and the current per channel of maxon encoders can be found on the catalog data sheets.

As an example let's look at the supply lines of a MR encoder with Line-Driver

U_cable should not exceed 0.25V with a typical encoder supply voltage of 5V from the controller!

The maximum current per channel is 5mA. The maximum supply current is 30mA (Channels A, B, I and inverted channels)

The AWG28 cable has a resistance R_AWG28 = 195 Ohm per 1000m

The maximum permissible cable resistance is: R_{max, cable} = U_cable / I = 0.25V / 30mA = 8.3 Ohm.

Thus the maximum cable length is: L_max = 1000m * (R_{max, cable}/R_AWG28) = 1000m * (8.3/ 195) = 42m.

Since the current must flow both ways between encoder and controller, the distance between encoder and controller must be smaller than 20m in this example.

Please note:

Most probably, the encoder signals will deteriorate at cable length shorter than calculated here, because of the small driver current of the MR encoder.

More information required:

Please note the additional hints provided by the article
"Measures in case of long cables".