how to bumplessly switch between two control signals?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The problem circuit with addition of C1.


Without C1 the input will drop to zero for an instant while the switch is toggled.
The addition of C1 will help but will have to be a largish value on account of the low impedance of the 500 Ω input shunt resistor. This will result in a lag in the system response which may not be acceptable.








simulate this circuit

Figure 2. Conversion to voltage control makes the solution quite simple.

The SEW drive can be configured for 0 - 10 V input control. This will, quite likely, have a high input impedance of > 10 kΩ. By configuring the drive for 0 - 10 V control and adding external shunt resistors you can convert the 4 - 20 mA signals to 0 - 10 V.

With this arrangement C1 will track the selected current source. During the inverval when the switch is thrown C1 just has to maintain the voltage to the high impedance input of the drive. When the switch contact makes again C1 will very quickly settle to the new control voltage due to the low impedance of the 500 Ω resistor now placed in parallel.

Work out your time constants from the usual RC formula.

RS sell a 500 Ω, 0.1% resistor which seems to be designed for this purpose.



Figure 3. The SEW drive supports voltage control input from -10 - 0 - +10 V giving direct forward / reverse control, if required, from a voltage source.



I'm not sure how real your problem is. If you have a non-zero deceleration rate programmed in your drive I would think that you have some protection already.