Continuing my look at master-slave control, this month I'll describe two methods of achieving effective master-slave control. In addition, I'll take a quick look at ramp/soak control in a master-slave setup.
Method 2: Analog Transmission of SetpointLast month, I showed how to achieve master-slave control using back-to-back thermocouples. Now, take the same press, but let each thermocouple be connected only to its own controller (figure 2). Let the master put out an analog DC signal representing its own setpoint, then let the slave receive this signal and assume a setpoint in proportion to it; that is, agreeing with the master.
This retransmission signal, as it is called, is commonly in the range 0 to 10 VDC or 4 to 20 mA, corresponding to the controllers' temperature range. When the operator changes the master's setpoint, the slave setpoint follows without delay, unlike the differential thermocouple method, where the slave setpoint suffers the heatup lag of the master's thermocouple.
The analog transmission method also avoids wiring traps, misconnections and errors inherent in low millivolt thermocouple signals. The method can have some small errors (typically, 0.1 to 0.2%) due to calibration inaccuracies of the DC signal and its interpretation by the slave.
If you have an unusually large number of slave zones, you will probably use a 0 to 10 V signal. Check with the controller supplier to find out how many you can connect to one master controller without pulling the voltage down. Alternatively, calculate the loading effect of the combined resistance of the slaves on the output resistance or current-delivering capacity of the master. The supplier can give you these values.
Make sure the retransmission signal and slave remote inputs are isolated from ground and from the temperature sensors. Isolated means several hundred megohms, including the panel wiring. Don't use a high-voltage insulation tester.
Method 3: Digital Retransmission of SetpointSome manufacturers offer a proprietary digital input/output (I/O) system for communication between instruments. Setpoint is just one of the variables that can be transmitted and received, and typically up to 30 slaves can be connected. With this method, transmission errors are virtually zero, there is no signal degradation, and the digital signal resolution is about one part in 65,000. In comparison, your controller readout resolution is commonly one degree in 1,000.
Ramp and Soak ControlJust as the slaves follow manual settings of the master's setpoint, they will follow any ramp-and-soak program that a master controller/programmer puts out. The actual temperature usually will trail the program temperature, and some may arrive at the soak level a bit too long after the program arrives. This lateness subtracts from the soak time that the program is trying to give to the work. To obtain a guaranteed soak time, you can add a holdback feature: a setting on each controller that halts the program until the sluggish zone catches up and comes within a specified deviation from the program.
There are many PC supervisory systems available for use with controllers that have the EIA-485 communications option. With this setup, you enter the setpoint at the PC and all controllers follow it. The PC also can perform ramp/soak programs. In this case, all controllers are equal, so there is no need to promote one of the zones to a master programmer/controller.
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