Saturday, August 6, 2016

FZ-94 (2) – Pushing the Drum Heat Limit Controller

That third MaxWell MTA-48 temperature PID, all the way to the right on the front of the FZ-94, plays a dual role. Besides reporting the actual temperature of the roasting drum, it is wired to control the heaters based on the set temperature limit.

That 3rd PID runs in simple binary control mode (it is not set to implement the p-i-d control loop). It  switches the heaters OFF if the configured temperature limit is reached and ON again if the drum temperature falls below that limit modulo some hysteresis parameter. Note that it always switches all heating elements as they are configured using those 3 manual switches. So if all 3 manual switches are ON, the PID switches all heaters on and off. If only one or two manual switches are ON, the PID only switches those one or two heaters on and off.

Artisan v1.0 provides some interface elements that can be used to control external PIDs connected via MODBUS. We can configure a slider in Artisan to modify the temperature limit, often called set value (SV) in standard PID terminology.

First we have to enable that PID control interface by ticking the "Control" flag in the Device Assignment dialog (menu Config >> Device), next to the meter popup.

Now a new blue "Control" button shows up on the main window that opens the PID Control dialog.

In this PID Control dialog we can tick the "SV Slider" flag and define an appropriate range (min/max) for our SV slider.
With those settings, the slider shows up on the left of the main window in logging mode, or if forced by pressing the 's' key.

Now we still have to define the MODBUS command to send out on changing that SV slider such that our control PID changes its heat limit. Checking the Communication Manual of the MTA-48k we find that the Set Value (SV) register is 0005H (Hex), which corresponds to (Dec). We put that number as SV register number in the MODBUS configuration (menu Config >> Serial Port, MODBUS tab) and as slave ID a 12 (the MODBUS slave ID of that drum temperature PID as configured by Coffee Tech). We also have to configure the SV Multiplier to 10 as the MTA-48k interprets a value of 1205 as 120.5.

NOTE: Please, study the linked manuals of the devices to understand the implications of any modifications you undertake as well as possible. Remember that you yourself are responsible for making sure the modifications are understood correctly and carried out precisely with care and common sense. We prepared our posts as careful as possible, but do not take any responsibility for remaining errors, mistakes or bad consequences that may result from you modifying any device based on the information given here.