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Old January 26th, 2021, 12:48 PM   #1
Mgw1970
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bake time verification

The original programmer wrote the CPT expression as follows.
60/7.08/(CurrentRecipe.Bake_Time/40)*(32767/100)
Dest: is VFD Frequency setpoint.

When they time the oven with stop watch it's off depending on the variety from what I'm told.
Any suggestions would help Thanks!
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Old January 26th, 2021, 01:19 PM   #2
wes.s
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So how far does one revolution of the drive motor advance the conveyor and how long is the baking section?
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Old January 26th, 2021, 01:51 PM   #3
drbitboy
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there is not nearly enough information provided here.


does "variety" mean "recipe?"


is the ratio of the stopwatch-measured back time and the recipe bake time about the same value for all recipes?


and what are the units of CurrentRecipe.Bake_Time? If it's seconds, then the 60 could be part of a conversion to RPM.

and what is the conversion from [VFD Frequency setpoint] to RPM?


also answer wes.s's query (how far does one revolution advance the conveyor, and how long is the baking section?).

Last edited by drbitboy; January 26th, 2021 at 02:31 PM.
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Old January 26th, 2021, 02:50 PM   #4
Mgw1970
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The oven has 34 shelves were pans are loaded onto so how far the conveyor has moved for one revolution is hardly noticeable.
Variety=recipes
the bake time is referenced in minutes/seconds on PV
cpt is the usable formula that is sent to the VFD for its speed reference so the relation I can see is bake time equals to 0-32767 min/max of the drive at 20hz/75hz for example.
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Old January 26th, 2021, 03:05 PM   #5
Drew Ethridge
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Are you saying that the recipe has a bake time of say 45 minutes, but it is taking, say 55 minutes for the actual bake?

If so, the first thing I would look at is the mechanics of the system. It's possible something has changed since the formula was originally written.

Another thing to look at would be the actual speed of the drive versus the commanded speed. It's possible the program is telling the drive to run at 75Hz, but it has a maximum freq of 60Hz.

Has any repair work been done lately? Sometimes sheaves/sprockets get reversed.
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Old January 26th, 2021, 03:07 PM   #6
drbitboy
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that is not helping to get usable answers.


Please look at the recipes. What are the ranges for values of CurrentRecipe.Bake_Time? Pick one CurrentRecipe: what is its .Bake_Time value; what is the expected bake time for that recipe?


What is the actual, stopwatch-measured bake time if the frequency setpoint, which is sent to the VFD, is 60Hz (or any other [stopwatch measured bake time, frequency setpoint] pair)?
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Old January 26th, 2021, 03:09 PM   #7
parky
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I have been involved in Bake/cook times for many years & done many different types, even what is called cook values (these are the minimum time to destroy the pathogens at a given temperature i.e. given a temperature of 90.0 C the cook or residence time (almost the same as Pasteurization) should be x mins, the actual cook time is part of the calculation so as the temperature varies the cook time should = x minutes it is a rather complex system I just did the maths to give the result not design the equation). As ingredients could be added at varying temperatures the system would calculate the cook value and cook on so the minimum cook value was reached, I suggest you run the belt at the slowest speed, measure the time it takes from entering to leaving, then do the same at different levels up to the max speed expected and work out the residence times it should be quite easy, I did a system some years ago where to calibrate the feeder (dry rice) we would run at a known speed for x seconds, capture the product weigh it enter the weight & the system calculated the required speed for a given tonnage per hour. This is standard procedure for checking calibration and the same goes for ovens, for example a given recipe may require a certain temperature for x minutes, this would be set by the process engineers in the recipes based on their trials or known standards, the calibration was up to us, often with the process engineers and then trials were done to check and validate the results.
The Pasteurization or cook value Value corresponds to the duration of the different stages comprising rise in temperature, plateau temperature and cooling, which are the necessary stages to cut the microbial load in the required proportions. It is expressed by the formula: This is the most common settings Tref=70 and Z=10C, however, it will depend on the process what the Tref & Z values are.
Attached Images
File Type: jpg pasteurization-value.jpg (18.6 KB, 183 views)

Last edited by parky; January 26th, 2021 at 03:12 PM.
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Old January 26th, 2021, 03:59 PM   #8
I_Automation
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When I do calculations similar to that I usually use REAL tags instead of set values (32767 & 100 in your example.

It is easier to calibrate by changing a REAL value instead of online edit of the CPT instruction, and they can be calibrated from the HMI
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Old January 26th, 2021, 04:07 PM   #9
drbitboy
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Are there comments on the rung with the CPT instruction?
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Old January 27th, 2021, 04:23 AM   #10
parky
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I_Automation, I agree, have a page on the HMI to calibrate the speed vs time.
It is quite simple just by setting the lowest speed & highest speed measure the time it takes from input to output enter the time & press a calculate button the maths can then use the recipe residence time & calculate the speed required.
There may be a little bit of offsets for example the minimum speed will be governed by the gear ratio as running the motor too slow could overheat (again a lot depends on type of drive, min speed to ensure product does not catch fire etc. one other thing, most ovens like this have a DC drive with batteries so that in the event of a power outage or motor failure the product can be removed from the oven to reduce the likelihood of fire.
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Old January 27th, 2021, 10:58 AM   #11
Mgw1970
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Quote:
Originally Posted by Drew Ethridge View Post
Are you saying that the recipe has a bake time of say 45 minutes, but it is taking, say 55 minutes for the actual bake?

If so, the first thing I would look at is the mechanics of the system. It's possible something has changed since the formula was originally written.

Another thing to look at would be the actual speed of the drive versus the commanded speed. It's possible the program is telling the drive to run at 75Hz, but it has a maximum freq of 60Hz.

Has any repair work been done lately? Sometimes sheaves/sprockets get reversed.
The bake times ranges from 7.5 minutes to 15.00 minutes for this type of oven. As for the mechanics nothing has been changed on it.
But the min / max on the vfd could be a possibility.
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Old January 27th, 2021, 11:08 AM   #12
Mgw1970
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Quote:
Originally Posted by drbitboy View Post
Are there comments on the rung with the CPT instruction?
The programmer didn't document anything it looks like rslogix 500 without descriptions.
I've had to go through and create tags and alias's for the I/O.
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Old January 27th, 2021, 11:44 AM   #13
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Quote:
Originally Posted by Mgw1970 View Post
The bake times ranges from 7.5 minutes to 15.00 minutes for this type of oven. As for the mechanics nothing has been changed on it.
But the min / max on the vfd could be a possibility.



Does 7.5 mean 7m30s (450s) or 7m5s (425s)?


I think I_Automation makes a good point: ensure all constants are REAL by adding a decimal point to them e.g. (32767./100.); with the parens, CPT may evaluate (32767/100) as 328 (or 327); it's only about a 0.1% error, but (CurrentRecipe.Bake_Time/40) could be off by a percent and a quarter.


We still do not have enough information, so it is time for silly guesses.


The formula multiplies and divides by 1 several times.
  • (CurrentRecipe.Bake_Time/40) is s/ft (reciprocal of speed; oven is 40ft long)
  • 60 is s/min
    • so 60/(C...Bake_time/40) is min/ft
  • (32767/100) is output scaling, [output counts]/[% range]
    • output is 32767 for max speed
  • 1/7.08 is [% of range]/[ft/min] i.e. shelves increase speed by 7.08ft/min for every 1% increase in output
Obviously this is wrong because the fastest speed (shortest bake time 7m5s) corresponds to 247 output counts or less than 1% of max speed.




What is the stopwatch-measured bake time when the recipe bake time is 7.5 minutes?


What is the stopwatch-measured bake time when the recipe bake time is 15.00 minutes?




Update:


The silly guess above assumed CurrentRecipe.Bake_time is in seconds; if that was minutes instead:

  • (CurrentRecipe.Bake_Time/40) is min/ft (reciprocal of speed; oven is 40ft long)
  • (32767/100) is output scaling, [output counts]/[% range]
    • output is 32767 for max speed
  • 1/7.08 is [% of range]/[ft/h] i.e. shelves increase speed by 7.08ft/h for every 1% increase in output
  • 60 is min/h
    • so 60/(7.08) is [% of range]/[ft/min]
So at 7.5min, the output is 14810 ~ 45% of max speed reference ... maybe?


(Update: corrected factor of 10 error)

Last edited by drbitboy; January 27th, 2021 at 12:40 PM. Reason: fix typos
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Old January 27th, 2021, 12:25 PM   #14
Mgw1970
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Does 7.5 mean 7m30s (450s) or 7m5s (425s)

Bake time is Real float tag with a range from 7->20 meaning the operator can set bake time at 7.2,7.3,7.4 etc.,etc.
So it's not changing to 8.00 when 7.61, for example.
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Old January 27th, 2021, 12:44 PM   #15
drbitboy
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Quote:
Originally Posted by Mgw1970 View Post
Does 7.5 mean 7m30s (450s) or 7m5s (425s)

Bake time is Real float tag with a range from 7->20 meaning the operator can set bake time at 7.2,7.3,7.4 etc.,etc.
So it's not changing to 8.00 when 7.61, for example.



So if they enter 7.25, they want 7 minutes and 15 seconds, not 7 minutes and 25 seconds. good, thanks.


Can you supply some stopwatch-measured vs. expected-per-setpoint results?
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