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Adding preinfusion to a rotary pump machine

Postby ethorson on Mon Feb 01, 2010 1:34 am

My Bezzera BZ35 has a lot of nice features, including a rotary pump, but does not have preinfusion. I am not an expert barista but I think the lack of this feature is adding to my problems with getting good shots. I decided that I would try and add preinfusion by reducing the rotary pump pressure during the first 10 seconds or so of the shot. The pressure produced by the pump is regulated by a relief valve that is loaded by a spring under the pressure adjustment screw. I figured that I could replace the spring and screw with a solenoid that was powered from a timer circuit. During the preinfusion interval the solenoid current would be low, which would allow more bypass flow and a lower pressure. At the end of preinfusion the timer would increase the solenoid current and consequently the force against the relief valve, which would increase the pressure.

I found a Ledex low profile solenoid that would have enough force to get the maximum pressure above 9 bar. I made an adapter to mount the solenoid that screwed into the rotary pump in place of the adjusting screw and spring. Luckily there was enough room for this modification.

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For initial testing I made a simple two-transistor timer circuit to drive the solenoid. It has potentiometers to adjust the preinfusion pressure, brew pressure, and preinfusion time. This circuit probably will need to be refined to improve the pressure stability. The timer and solenoid are powered from +12 volts, which is supplied by an AC adapter that is connected to the pump drive AC power.

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After a few tweaks I was able to get the preinfusion mod working. Here is a video of it in action. Note how the pressure gauge responds.

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Postby Ken Fox on Mon Feb 01, 2010 1:59 am

I don't have first hand experience with this machine and perhaps you have found the best solution for the machine that you own.

Other options for you and others to consider:

(1) Unlike rotary pumps, vibe pumps have a pressure profile which gives you preinfusion, "for free." This is to say that they ramp up to full pressure much more slowly than rotary pumps, taking a number of seconds to get up to pressure. Given a choice of two machines differing only in the type of pump, if one is driven by a vibe pump it will cost less, provide the type of "forgiving" pressure profile that many people seek, and be cheaper to maintain. The downside is noise, however how many shots in a day do you make, anyway? If you can tolerate a few minutes of noise and vibration daily, a vibe pump driven machine is both more economical and will have a pressure profile that all but the most fastidious baristas will prefer.

(2) Lower doses in the portafilter = less problems with the pressure characteristics of rotary pump driven machines which lack preinfusion. There are rotary pump machines that incorporate preinfusion in their designs, however those designs can have their own pitfalls (e.g. La Marzocco's 0.6mm ruby gicleur that has a tendency to clog up). Those updosed shots that give you channeling might well flow without problems if you reduce your dose down to the range that is typically used in Italy, e.g. around 14g. Since your machine was probably designed in Italy, perhaps this explains why the manufacturer didn't think there was any problem with the machine design, because this is how they used it, over there.

(3) Assuming you want to modify your rotary pump machine, an alternate and simpler way of getting controlled preinfusion would be to put in a pressure regulator on the water input side, regulating the input pressure down to around 3 or 3.5 bar. You can then install a "delay on make" timer on the rotary pump itself, so that when you push the button to initiate a shot, the solenoid opens releasing your regulated mains pressure on the coffee cake, and only later (say 6 or 7 seconds later) does the pump itself engage. I put a DOM timer into the electronics box of my Cimbali Junior rotary machine. The DOM timer cost around $25 and the pressure regulator was relatively cheap as well. The whole modification cost well under $100 and did not require any proficiency in electronics to accomplish. I have posted extensively on this modification on HB and it should be easily found using the search function on this board. For the record, this modification was not my idea, but rather the idea of Michael Teahan, one of our resident geniuses who posts regularly here.

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Postby coffee.me on Mon Feb 01, 2010 4:35 am

Eric: beautiful, beautiful, beautiful mod.

How can the controller be changed to achieve full, manual, pressure profilability?
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Postby ethorson on Mon Feb 01, 2010 1:05 pm

coffee.me wrote:How can the controller be changed to achieve full, manual, pressure profilability?


The preinfusion controller is a very simple design that used parts I had on hand. With a little more engineering one could come up with something fancy - programmable profile, pressure feedback, flow feedback, usb port, automatic twitter posting, etc. The basic concept is that the pump pressure is roughly proportional to the solenoid current. On my machine the solenoid requires about 1.5 amps at 8.5 volts DC to produce 9 bars pressure. Other pumps will have different characteristics, mostly determined by the area of the pressure relief valve port.

Here is a schematic of the preinfusion controller, which is a very simple circuit. It is probably not stable with time or temperature (the timing capacitor is electrolytic) but works fine for testing.

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Postby cannonfodder on Tue Feb 02, 2010 1:16 am

Interesting. On my big Faema, I simply added an adjustable delay on make relay inline with the motor. Then I could dial in my delay time and let the mains pressure do my pre-infusion before the pump kicked in. Worked like a champ and help out a lot on that machine. In stark contrast, I have not had the need or desire to modify my Elektra. She works wonderfully out of the box.

Personal opinion, I think the group design, basket, dose and coffee all play a big part in the preinfusion equation. Some machines/coffee's seem to work better with it while others work better without. That is one of the reasons I liked the adjustable relay. I could turn it off to see what, if any, difference the preinfusion made with a particular coffee/dose.

Neat work by the way.
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Postby aindfan on Wed Jan 12, 2011 2:14 pm

Hobbyists and pump experts: could a stepper motor turning the adjustment screw achieve the same effect, albeit with a slower ramp up and down (limited to speed of motor)?
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Postby ethorson on Wed Jan 12, 2011 4:42 pm

A stepper motor with sufficient torque should be fine for pressure profiling. You may not even need a pressure transducer for feedback if you provide a way to move the motor to a home position on power up.

By the way, on my other espresso machine I just put a knob on the pump pressure adjustment screw. I oriented the pump so the knob pokes through a hole in the front panel, which allows me to manually tweak the pressure during a shot. However concurrently watching the extraction, checking the timer, fiddling with the pressure, and steaming milk is a challenge.
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Postby aindfan on Wed Jan 12, 2011 4:53 pm

Pressure transducer? Feedback? You're way ahead of me. Group pressure gauge and manual control are enough for me! Now I wonder where I can find a stepper motor with a flathead screw bit sticking out and a coupling that will hold the motor stationary relative to the pump without it falling off of the adjustment screw...

(Just kidding... I should actually finish my rebuild first.)
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Postby Rosscopico0 on Wed Jan 12, 2011 9:07 pm

Ive been thinking about removing the bypass valve adjustment screw & get a copy turned up on a lathe except that it will directly connect to a stepper motor. Then just need to fashion a bracket with holes that line up with the holes in the motor's base & bolt it all together.
I had been previously thinking of the feedback system, but returning to a home position would be much more cost effective.
Thanks Eric!
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Postby duke-one on Wed Feb 09, 2011 5:43 pm

I wrote to Procon on this since they are the big Kahuna in Espresso rotary pumps asking why don't they build a simple, easy, manual or powered variable bypass. It seems to me that a flexible cable like a bicycle shift or old fashion automobile choke or manual throttle set up could be brought out to the front panel to adjust the bypass with a few $'s worth of parts. Easy, cheap, low maintenance.
Procon did not reply..........
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