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servovalves.md

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DIY servo valves

Servo-front Servo-left Servo-right Servo-far-right Servo-back Servo-top

What do you need to assemble servo valves?

  • four servos in standard size, e.g. TowerPro MG945
  • four ball valves
  • four voltage regulators 5V or 6V,
  • four heat sinks for voltage regulators
  • four endstops
  • four mounting strips 4-pin 5.08mm female + superglue for gluing them to 3D printed case
  • four mounting strips 4-pin 5.08mm male
  • some cables
  • some filament for a 3D printer other than PLA

How strong must the servos used in the project be?

My servos TowerPro MG945 have a capacity of 10 kg per cm with 5V supply. According to my observations, they are barely enough for 1/2 inch ball valves. It's best to use a servo with a higher load than those described above. You should also remember about strong enough voltage stabilizer and an appropriate heat sink. My servo housing with PLA at the stabilizer heat sink becomes soft after frequent switching servo valves in a short period of time, so I advise you to watch this too.

STL files

STL files can be found in the folder assets/stl/servovalve.

Plug

  1. Red: from + 6V to + 32V connected to a 5V voltage stabilizer, servo connected to a 5V voltage stabilizer output
  2. Black: common ground for servo valve power supply and aquarium controller power supply
  3. White: PWM control signal connected directly to the servo
  4. Blue: endstop detecting servo valve closing, if enabled - conducts common ground, not +5V or +12V!

API

The main interface of the servo valve driver is in the file include/control/valves/ValveModule.hpp.

We have two instances of servo valve driver available:

  1. ValveModuleUsingInternalPwm - this driver uses Arduino as a device generating PWM signal. The PWM outputs for this instance are configured in using pinout in the section Valves servos pwm pinout definition. The rest of the pinout is shared with the second driver.
  2. ValveModuleUsingExternalDriver - this driver can be used in case of problems with the PWM generated by Arduino Due. I had to use it. This driver uses Adafruit PWM Servo Driver or any clone supported by the library Adafruit PWM Servo Driver library. Additional when using this module you don't need to worry about the configuration section Valves servos pwm pinout definition, it is not used when configuring this driver.

The main API consists of five methods:

  • bool open(unsigned short valveId);
  • bool close(unsigned short valveId);
  • bool set(unsigned short valveId, bool state);
  • bool isClosed(unsigned short valveId);
  • bool areRemotesAvailable();

Three blocking methods called open, close and set are used to open and close servo valves. As you can see, they return a boolean variable. This is the result of executing the command. If these methods return true, then the controller knows that the servo valve did command correctly. Otherwise, when false is returned, the controller closes all servo valves and will show information that there is a problem with servo valves and you have to look at them and fix them.

The isClosed method, as the name suggests, returns information about current status of the servo valve. It reads this information from the endstop of selected servo valve. If the endstop is closed returns true, otherwise false.

The areRemotesAvailable method is a special method, which should also be implemented if the aquarium is not permanently attached to source of clean water and sewage. It detects if the remote servo valve control cable in another room is connected to servo valves. It can be e.g. a LAN cable in which one wire is mass, next to are used for servo control using the PWM signal, the next two are door detection servo valves and the last one is detection if remotes steering cable is connected. It is shorted simply with mass.

remote-servovalve-connector

The current API works in such a way that the servo valve has 2-3 seconds to open or close, if it does not do it in this time - an error is reported.

The main definition is in include/control/valves/ValveModule.hpp. Individual implementations are in the same folder.

ValveModuleBase

It's a class you can inherit. This is the base for the servo valve driver with special functionality - disconnects the PWM signal of the servos after a specified time. If we use ordinary servos in our servo valves, we can simply implement just two protected methods and do not worry about automatic servo valves detach.

Why disconnect the servo valve control signal?

Built in drivers in servos can cause servo buzzing which means that the servo is trying to move to selected position, even if it is missing just 0.1 degree, so it doesn't use all the power of the servo and uses the engine all the time but can't move to the end. It puts a load to the stabilizers, so that they may not provide the required current to the servo at the required time, which will cause a problem moving it to the selected position.

Implementation:

To implement your own driver inheriting from ValveModuleBase you should overwrite two protected, abstract methods:

  • void moveServo (unsigned short valveId, bool state);
  • void detach (unsigned short valveId);

The moveServo method is used to handle the movement of the servo - opening it or closing depending on the value of the bool state argument. It should not block the program execution.

The detach method is used to interrupt the PWM control signal feed for servo. It is called after a set time. ValveModuleBase automatically memorizes the last time the servo was moved and calls this method after timeout.