With the design of the electronics, we are initially focusing on controlling the mechanical axis that will be compressing and releasing the BVM. This includes speed, torque, load and endurance tests.
We have opted for minimal components to speed up assembly and also make it less complicated.
You do not have to use the components that we are within our design and these can be swapped out for lower cost components such as Arduino range of boards. But of course you will need to write your own code.
Control touch screen.
The touch screen that we have chosen is the Nextion NX4832T035 which is a 3.5″ screen with a a resolution of 480*320.
The nice thing is the Nextion range of touch screens is that the screen layouts can be created in a simple to use GUI editor and coded to do do needed calculations and send/receive ASCII commands out via the UART.
We will supplying the design file that we have developed to anyone that wants to edit or simply use on their own projects.
Also we will update with routines for monitoring different sensors that get added in the future.
Full information can be found at their website https://nextion.tech
We have used the these touch screens before in various projects and the picture below shows the 3.5″ version.
The motion controller we have chosen is the Pulse Train Hat that has a On Board Arm Cortex M4 processor running at 100MHz
This controller was originally designed to sit on top of a Raspberry pi, but also can be used in stand alone mode. We will be using it in stand alone mode for this design.
The Pulse Train Hat creates clean, fast and accurate pulses and configured using simple ASCII commands sent to it serially.
These pulses can then be sent out to any stepper/servo motor that has digital inputs for Step, Direction and Enable pins.
As well as the outputting pulse trains, the controller board has I/O’s that can be used for Limit switches, Emergency Stop, ADC sampling and digital I/O’s for other sensors.
Full information can be found at http://www.pthat.com
Closed Loop Stepper Motors
The motors we will be using is the JMC IHSS57-36-20 Closed Loop Stepper Motor.
These motors are great and have built in driver and encoder.
Even though they are only rated at 2.0N-m holding torque, you will struggle to stop these turning and due to the closed loop system, if they ever do miss a step then they will auto correct themselves, so repeatably and accuracy is spot on.
Also the motors have built in alarm outputs that can be monitored if anything goes wrong with them and fed back to the motion controller.
We have used a lot of these motors over the years and have never had one fail.
We are using two medical approved power supplies in the machine.
Both have a wide range AC input voltage 85 – 264 VAC
These comply to a range of standards and the standard is IEC 60601
Here is a really good article explaining main principles of IEC 60601
The two we have chosen are:
The first power supply is the TPP 15-105-J is a 5V 15W supply that is rated at 3amps and will be used for powering the TFT, Controller Board and motor I/O’s
The second power supply is the TPP 100-124 is a 24V 100W supply that is rated at 4.17amps and will be used for powering the motor.
Use of a 5V inductive limit switch will be used for homing the machine.
Typically inductive switches have been 6-36v but now we can source 5v versions and will be using a NPN version on our machine.
Front panel AC power switch
Used to switch power on and off, looking into regulations at the moment.
AC power input
Still debating whether we have fixed flex that comes into the machine or use a IEC fused socket. Need to again look into regulations.
To be used as a redundancy measure in case of AC mains power failure. Again something we need to look into yet.
Below is the wiring diagram.
Future Sensors to be added.
For the initial prototype we have focused mainly on the mechanical axis that will be compressing and releasing the BVM. This includes speed, torque, load and endurance tests.
After we are happy with this, we will then move onto looking at adding other sensors that can be easily incorporated with the existing TFT screen and controller.
The SENSIRION SFM3300-D looks like a good contender and a good price. Shame there is a 8 week lead time at the moment.
This would be used to sense the airflow and can measure up to 200L/min.
Feedback would adjust the pressure/stroke applied to the BVM.
The TE MS5525DSO digital pressure sensor that also does temperature looks like a good candidate for the prototype.
It also has I2C and SPI interfaces for simple interfacing.
The MS5525DSO-SB001GS can measure up to 1psi and Mikroe do a click board which will be handy for using with the prototype.
O2 Flow Sensor