Using multiple TECs and multiple controllers to regulate a single load

Date of this version: 12 May 2011

1. Introduction

When an application requires transferring a large amount of heat, it may be desirable to use multiple TECs driven by multiple controllers. This arrangement typically poses difficult control problems, since two (or more) independent drivers have no means of sharing the task equally. They will respond separately to changes in the target temperature, and the entire system will tend to oscillate.

VueMetrix solves this problem by taking advantage of our TEC driver's unique ability to communicate with external devices: in this case, a second VueMetrix TEC driver. This setup is sketched in Figure 1.

The PC is connected in the standard way to one of the drivers, labeled the “master” in the figure. The master connects to a secondary driver through its “passthrough” RS-232 port. Each driver is connected to a separate TEC, but only the first (master) driver has a thermistor.

In this configuration the master performs all the functions of temperature control. It measures the thermistor temperature, performs the temperature adjustment algorithm, and sets the appropriate voltage across the TEC. The master then commands the secondary controller to set its voltage to the same value. In this way there is only one temperature measurement and hence only one temperature servo. The two TECs are guaranteed to be at exactly the same voltage. The total cooling capacity is exactly double that of a single TEC. In principle there can be as many of the secondary controllers as required, limited only by the overhead of serial communication between them.

2. WinVue support

Multiple TECs are supported in WinVue 4.01 or later. Figure 2 shows the TEC setup window, with the “Multiple controllers” tab selected. To enable this feature simply type the number of secondary controllers into the box and press enter. When the servo is engaged the master will tell the other controllers that they are to function as slaves. Slaves do not measure temperature and do not manage a temperature servo; they merely set their voltage in response to commands from the master.

When a controller has been used a slave and you wish to use it in the standard or master configuration, you must configure it to read its thermistor once again. To do this check “Read thermistor.” If you forget to do this, the controller will issue a fault when you attempt to engage the servo.

It is not necessary to configure explicitly any slave controllers. That will be done automatically by the master controller when you engage its servo.

3. Software and interfacing

Beginning with version 10 of the TEC firmware, the following commands have been added to support master mode.

• m x, where is an integer. If x=0 the controller operates in its standard mode as a single controller. If x>0 the controller will operate as a master controller having the specified number of slaves.
• m? Returns the number of slave controllers or 0 if the controller is running as a standard single controller. The factory default is 0.
• tr x If x is 0 the controller will not read its thermistor. The controller will not function in standard mode or as a master. It will work as a slave with no thermistor connected. If x is 1 the controller will read its thermistor. This is the factory default.
• tr? Returns the last value set by the tr command.

The following fault code has been added:

• 7 - Passthrough timeout The controller attempted to send a message to a slave or a passthrough controller, and did not receive a response within the designated timeout interval. If a controller is designated as a master and there is no slave connected, this fault will occur during powerup or when enabling the temperature servo. It will also occur when a passthrough command is sent (i.e., one that is prefixed by a '!') and no reply is received.

The following temperature servo fault codes have been added:

• 8 - Slave timeout During servo operation the controller attempted to set the voltage on a slave controller, and no response was received within the timeout interval.
• 9 - Slave fault One of the slave controllers reported a fault.

These fault codes are retrieved by the “t0s?” command, which returns 100 + the fault code in the event of a servo fault.