Watlow Electric Revision 5 - Manual (Page 52)

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For temperature control, the most useful and easiest to use entry is the PB in
actual degrees for the SYSTEM 32. The nominal setting of the PB can be
between 5-20% of the SP. Thus, a SP of 300 oF may require a PB of 15-60
oF. To start use 10% of the SP.
A good way of establishing a PB setting is to start at a wide PB and then to
keep decreasing the PB [increasing gain] until the process cycles about the
SP. Take note of the PB at this point and double the figure. PB should be set
at this number. Reset should be set to a low value such as .3 [or set integral
to zero: off] and derivative should be at zero [off] before tuning PB.
The PB of 30 oF with a SP of 300 oF specifies that the output from the
controller will change proportionally from 0 to 100% over 30 degrees. The
output will be at 0% or 100%, if the PV is outside the PB of 30 oF from the
SP of 300 oF. Below 270 oF [greater than a 30 o error] the output will be at
100%, at 285 oF it will be at 50% and at 300 oF and above it will be at 0%.
All PID control functions take place within the PB, otherwise the controller
output is full on or full off with no Integral or Derivative action.
8.1.6 PROPORTIONAL AND INTEGRAL CONTROL [PI]
The Integral mode is also known as RESET action. Reset is the older of the
two terms and is descriptive of the control action that takes place. The
primary reason for for integral control is to reduce or eliminate steady state
errors, but this benefit typically comes at the cost of reduced stability.
With proportional only control the output will be zero when the PV is at SP
[error is zero]. Thus, in a heating system for example, the stable
temperature will always be below the setpoint. When the PV is stable at a
point above or below the SP, the deviation from the SP is known as
OFFSET. The control action that corrects for this offset, is integral or reset.
Reset is only active when the PV is not equal to the SP. The unit of Reset
most often used is called Repeats/Minute (R/M). This expresses the number
of times the PB response is repeated in one minute. This means with one
repeat per minute, that the control output would be exactly double the
proportional band [repeats the proportional band] only, if the error [SP-PV]
remained steady for the full minute. As long as reset is active, it will repeat
the PB response until the output has reached 0% or 100%.
MANUAL RESET is a manual biasing of the output, so that when the PV is
at SP, the output will be at the proper level to hold SP. It is more common
on older type controls. The newer controllers including the SYSTEM 32
provide AUTOMATIC RESET as described above. AUTOMATIC RESET
automatically makes the correction for offset errors, but the R/M value must
be set for the process.