WAIT_STATES (Jul93)       furnace


wait_states --- oven schedule decision parameters


Oven schedules consist of nodes which are composed of a GOALTIME and a GOALTEMP. The node also has a LABEL and some wait conditions which allow the schedules of various zones to be syncronized. The parameters and data for the current node of each active zone schedule are shown in the 'Oven Overview' menu. Additional details for the current node are shown in each of the 'Zone' menus. The full schedule for a particular zone is shown in the 'Zone Schedule' menu. Temperature schedules may be editted by the oven task in the 'Configuration and Status Menu'.

When the V-computer is running a temperature schedule, the clock time is advanced once per minute and the desired or scheduled temperature for each control zone is calculated. The "clock" will continue to advance minute by minute in real time unless it is "stop"ped by the pilot or the schedule encounters a wait condition. If a zone clock arrives at the endpoint of a node with a wait flag, the clock will stop advancing until the wait condition has been satisfied or until the pilot "run"s the clock. Wait flags can require any one or more of three possible conditions (T,D,W). The clock will remain stopped until all of the specified conditions have been met.

The "T" flag indicates a wait for tolerance. The zone clock will stop until the average zone temperature is within one degree C of the scheduled temperature at that node. This condition is normally used to wait until high inertia parts of the oven or glass can catch up with the schedule. Note that the PID algorithm usually holds the oven slightly below the scheduled temperature, so a zone controlling heaters might wait a very long time before reaching tolerance with itself. Also remember that tolerance is 1 degree C and it doesn't care whether the schedule is going up or down. So if the temperature has already passed the wait node, the clock will not run.

The "D" flag indicates a wait for deviation. The zone clock will stop until the scatter among HTMPs in the zone is less than the scheduled deviation. This is normally used when you want the zone temperatures to equilibrate before beginning an annealing cycle.

The "W" flag indicates a wait for another zone(s) to reach a certain node in its schedule. For example, zone R may wait until zone 0 reaches a certain temperature before starting rotation. The "W" flag requires a zone label (Z0,Z1.....Z10,,R or A for all zones) and a node label (a-z). The clocks will stop until the specified zone begins working on the specified node.


Wait on Tolerance (Yes/No)
Should the schedule wait until the average temperature of the zone is within 1 degree of the scheduled temperature?

Wait on Deviation (Yes/No)
Should the schedule wait until the scatter in the zone is less than the scheduled deviation?

Wait on Zone/Node (Yes/No)
Should the schedule wait until some other zone has reached a specified point?

Wait on Zone ID (ID) (Z0-Z18,R,A)
The name of the zone to wait on. (A to wait on all zones)

Wait on Node Label (label) (a-z)
The label of the node to wait on. Labels are not automatically assigned. The pilot is responsible for keeeping them aligned in the schedule.


The major trick to using wait states properly is to set them up so that all zones will have their clocks synchronized when the wait node is passed. The strategy described here is the one we developed for the 3.5 meter castings. Suppose we want to ramp the oven to temperature and hold there for 6 hours after the mold has reached a certain temperature.

First we must put a wait for tolerance (T) in the mold schedule. This temperature needs to be a little bit below the scheduled oven temperature. This is because it would take between 12 and ~100 hours for the mold to come into equilibrium with the outside of the oven.

Mold Schedule (z4)

f    28.0   50.0  843.0 01:23:34   500.0 00:17:51
g    28.0   50.0 1143.0 02:10:17   300.0 00:10:43 T
h    28.0   50.0 1190.0 02:11:57    47.0 00:01:40
i     0.0   50.0 1190.0 02:12:58     0.0 00:00:01 W A i
j     0.0   50.0 1190.0 02:17:57     0.0 00:05:59

Now we want to have the oven (base in this example) stop and wait for Z4 after it reaches its scheduled temperature. To do this we have all the control zones wait on the mold zone to reach the zone after its tolerance wait state. (W Z4 h) This would mean that the mold has reached the proper temperature.

Base Schedule (z3)

f    28.0   50.0  843.0 01:23:34   500.0 00:17:51
g    28.0   50.0 1143.0 02:10:17   300.0 00:10:43
h    28.0   50.0 1190.0 02:11:57    47.0 00:01:40 W Z4 h
i     0.0   50.0 1190.0 02:12:58     0.0 00:00:01 W A i
j     0.0   50.0 1190.0 02:17:57     0.0 00:05:59

In the above example, the oven would start the next node "k" for the cool down 07:40 hours after the mold had reached 1143.

Now we would be done except that the Z4 clock might be anywhere between 00:00 and 01:40 behind the other zones. That would be fine until we get to the ramp down later in the schedule. To correct this delay, we need to add a catch up node which waits on all other zones (W A i). The catch up node must have a duration of only 1 minute. Note that every zone must have the zone labelled "i" at the same clock time for this to work.

Now you're thinking: "How can it possibly take three schedule wait conditions to do this?" Well, you are right. It only takes two wait conditions, but it still takes three nodes. The (W Z4 h) condition is redundant, but we still need that node to keep the ramps matched up. The reason to keep the redundant wait condition is to make it obvious that we are waiting for Z4 (otherwise you have to read the schedule for all nodes to learn this).





schedules, clocks, zones, menus