6.6 Temperature monitoring

The Thermonom monitors the temperature of every channel in three mutually independent ways. They serve different purposes, and they differ in what they trigger.

Stage	Reference	Reaction
Warning band	relative to the setpoint	Warning. The channel keeps heating and counts as not operational.
Temperature limit	absolute	Error. The channel switches off and stays off until the error is acknowledged.
Sensor diagnosis	measuring signal	Error with an adjustable reaction – switch off, follow, hold or manual output mode.

Figure 21: Up to the marker the channel is heating up – it lies below the warning band but reports no alarm. Only after that is a deviation a deviation alarm, here the excursion upwards.

The warning band

The warning limit is a distance from the setpoint, 10 K ex works, and applies equally upwards and downwards. If the actual temperature leaves this band, the channel is no longer operational – this already applies while heating up, because a channel at half temperature lies far below the band.

A deviation alarm only arises from this once the channel has previously reached its setpoint, that is has been inside the band for at least two seconds. This distinction is the reason why a normal heat-up does not produce a flood of alarms, while a drop during operation is noticed immediately.

What the band indicates is usually not a sensor problem but a thermal one:

Permanently too cold at a high output level: the heater is too weak, has partly failed, or the heat dissipation is too great – open tool, cold material.
Permanently too hot at a low output level: heat input from outside, for example from a neighbouring zone or from the melt.
Oscillating around the setpoint: the control parameters do not match the zone, see Section 6.9, “Controller tuning and control parameters”.

The absolute temperature limit

This limit protects the tool and the material, not the control. It is entered per channel as an absolute temperature, 600 °C ex works. If the actual temperature exceeds it for longer than three seconds, the Thermonom triggers an error and switches the channel off.

Two properties are important:

The error is latching. It does not disappear when the temperature falls again but has to be acknowledged. This is the only way an overtemperature that occurred during night operation and disappeared again by itself remains findable.
The main contactor stays switched on. Only the affected channel is switched off; the others keep running. If you want the tool to stand still as a whole, use “On error group OFF”, see Section 6.10, “Groups, substitution and failures”.

NOTICE

Set the temperature limit to a value that the material and the tool can withstand – not to the highest one the device permits. The factory setting of 600 °C is above the decomposition temperature of most plastics; it protects the device, not the production.

Sensor diagnosis

A thermocouple can become unusable in several ways, and the Thermonom distinguishes them because the remedy differs:

Finding	Meaning and remedy
Sensor break	No continuous measuring circuit – cable detached, connector not plugged in, thermocouple broken. The channel cannot be operated under temperature control.
Sensor polarity	The measured value runs downwards while heating: the two wires have been swapped. Instead of rewiring, the polarity can be reversed in the channel settings.
Sensor short circuit	The wires touch each other in front of the measuring point. The sensor then delivers a plausible but wrong value – usually too cold. This is detected by the fact that a lot of energy has flowed into the channel over a longer period without the temperature having risen.

NOTE

The short circuit detection is switched off ex works. It infers from the ratio of energy introduced to temperature rise – with very sluggish tools this inference can be wrong and switch off an intact channel. If you switch it on, watch the behaviour during the first heat-up.

What is to happen in the event of a sensor fault can be set: switch off, follow another channel, hold the last output level, or change to manual output mode with the nominal output level. The last three keep production going, see Section 6.10, “Groups, substitution and failures”.

WARNING

With the reactions “Hold” and “Manual output” the channel keeps heating without temperature measurement. There is then no monitoring that could detect overheating – not even the absolute temperature limit, because it lacks the measured value. Only use these settings on channels whose heating power matches the zone, and watch the channel afterwards.