The BARTH lococube® mini-PLC STG-800 makes a great cup of coffeeFollow article
The BARTH EspressoMaker project
Can you make good Espresso at home, with equipment for under 400€ and without suffering 3rd-degree burns too? Yep, you can do it. But how? With ESE pads!
And don’t let disappointing experiences with generic pad machines fool you, because all pads are not alike. Well, apart from consisting of biodegradable filter paper and containing about 7g of coffee. But when comparing coffee pads side by side, the differences become apparent immediately:
With a diameter of 44mm, the Easy Serving Espresso pad is considerably smaller than its generic counterpart coming in at 70mm. The height difference is remarkable as well with the 10mm high ESE pad dwarfing its 5mm thin opponent. The fine grind of the coffee within the premium pad (0.3mm grain size) is another remarkable difference, as it increases the contact area between water and coffee.
But making good coffee is more involved than just picking the right pad: Because the volume of an Espresso should always be 30ml the water pressure must be adjusted accordingly, so that the water is in contact with the ground coffee for a certain amount of time. This is because extracting for too long will create a bitter taste while too short leaves you with a boring, bathwater-type flavour. It's recommended that 20 to 25 seconds is the ideal timeframe for brewing espresso.
The seasoned barista may use this basic knowledge of hot concentrated extraction to their advantage and create a “Ristretto”, that is mild-tasting and easy on the stomach, this is done by cutting extraction time in half.
Since 30ml of coffee is barely visible in a standard size mug an Espresso can be diluted with hot water to create a more traditional coffee. An Italian would call the resulting beverage, with a total volume of 100ml, “Americano”.
All of the above requires a water pressure of about 10 Bar. Most commonly it is created by a magnetic armature that vibrates within a cylinder due to an externally applied magnetic field.
This motion drives water through a one-way valve into the brewing unit, where it is heated up to 92°C by a cartridge heater. The brewing unit will often be milled from a sizeable piece of brass that provides the thermal mass necessary to keep temperature stable during extraction.
Luckily some manufacturers have introduced machines with the right hardware for ESE pads to the consumer market lately. But what will 270€ get you? Instead of trusting “real customer reviews” we took one apart and were pleasantly surprised: Hidden under its brushed, stainless skirt are a name brand “ULKA” pump, the aforementioned brewing unit out of a massive piece of brass as well as a sturdy toggle joint mechanism for clamping the pad.
Despite being so well made, this machine in factory condition will never reach its full potential. This is due to its reliance on an extremely complicated regulating device: The user can switch the pump on by pressing a button on the top of the housing. The amount of water dispensed by feel will rarely be correct, therefore we decided to automate the process with one of our PLCs (type: STG-800). Therefore, we replaced the mechanical thermostat with a 10kΩ NTC resistor.
An analogue input reads the voltage drop over this resistor so that it can be used in the program. If the temperature drops below the desired value, a solid-state relay switches the heater on.
The buttons at the front of the device are each connected to an input. A different dispensing time will be set depending on the chosen beverage. A press of the “Americano”- button, for example, will provide the right amount of hot water to dilute an espresso. Dispensing will only happen when the water is at the correct temperature, the heating element will switch on if the brewing temperature falls below a pre-defined level. Dispensing resumes when the temperature target is reached and the water volume is correct.
Description of program operation for STG-800 EspressoMaker:
The voltage drop over the NTC-resistor is read by IN1. The value of the variable “Temp_Raw” is measured in Volts.
Converting the value to Kelvin was deemed unnecessary, as only three setpoints are required. If the voltage rises above 410mV (temperature falls below 92°C) “Heater_State” changes to TRUE. As a result, the PLC sets OUT1 and switches the heater on. Pressing a button will only start a dispense cycle if the temperature is high enough. If all conditions are met pressing a button will set the corresponding flip-flop.
IN3, for dispensing espresso, must be handled differently from IN4 (for ristretto) and IN5 (for hot water) since it is an analogue input: The result is compared to a value of 12V first. Since different data types are involved in the comparison, we need to convert the FLOAT data type of the analogue value into a WORD data type with a converter block first. Comparing the value to half of the supply voltage will create a clear TRUE state if the supply voltage is applied to the input by a button. The TRUE state at the output of the comparator will then set the flip-flop and start a dispensing cycle for the espresso.
This will set “Dispensing_in_Progress” which enables the heater and the pump to simultaneously prevent an excessive drop in the temperature during dispensing. The heater will only activate if the temperature of the brewing group is below 95°C (which corresponds to a 380mV drop across the NTC).
While the pump is active “Dispense_Time” is incremented once every second. When the time target is reached, the corresponding flip-flop is reset and dispensing ends. With this “Dispensing_in_Progress” becomes false and resets the counter. As mentioned before, dispensing hot water will be interrupted if the temperature falls below 86°C (corresponds to 500mV or more across the NTC). Since the user may not expect the machine to stop and start on its own, two LED modules flash during operation to show the current state of operation.
Attached below, you will find the complete parts list and STG-800_EspressoMaker program.