Hardware
Subsystems
Valves
For smooth pressure control, I chose the EFB-1DV-24-L proportional fill/bleed valve assembly from Clippard Minimatic. Each valve in the pair can handle high flows at reasonable operating pressures (100 liters/min at 50 PSI). They run at 24 V, but can only handle 60 PSI of back-pressure before they crack.
For the input shutoff valve, I chose to use the E215E-2C024 2-way Valve from Clippard. The valve can withstand high back-pressure with reasonable flow rates (150 PSI with 70 L/min). These also run at 24 V.
To drive all the valves, I am using one mosfet per valve as switches. The valves are then modulated in a smooth way using pulse width modulation (PWM).
Sensors
Main Control Sensors
The pressure control system features 10 channels, requiring 10 pressure sensors, each of which needs to be capable of measuring pressure up to ~ 35 PSI, and vacuum down to -14.5 PSI (full vacuum).
After a lot of searching I found the Honeywell SSCSNBN030PDAC5 differential pressure sensor. This sensor measures ±30 PSI over a voltage range of 0.25 V ~ 4.25 V, and we can extend the top end of the range slightly since we can measure up to 5 volts (the accuracy just gets worse as we move past the top of the measurement voltage range). The sensor is abundant and inexpensive at the time of writing, but has a HUGE footprint on a PCB.
Input Shutoff Sensor
An addition to the pressure control system in v3.4 is an input shutoff valve for the high-pressure line. The main control valves can only take ~ 60 PSI of back pressure before they break, so we need to prevent anything higher from being supplied. The input shutoff utilizes a positive-pressure sensor to measure the input pressure and shut off the flow if it gets too high.
For this, I chose the Honeywell SSCDANN150PGAA5 vented gauge pressure sensor. It can measure up to 150 PSI (more than we should ever be supplying), and has the typical 0.5 V - 4.5 V output range.
Microcontroller
I used the Teensy 3.5 MCU because it’s 32 bits with a built-in floating point unit for super fast floating point math (good for real-time control). It also has enough pins
Custom Printed Circuit Board
I built a custom circuit PCB to breakout signals to my various sensors and solenoid drivers.
General Design
This board is designed to be relatively space-efficient while still being modular. It also has everything set up to break out every signal from the Teensy 3.5 to potentially be used later if necessary. This results in a relatively large board, but it was fairly cheap to get it fabricated by OSH Park.
Electrical Components
All of the electronic components used on this board can be found on Digikey.
| Part Description | Digikey P/N | Qty | Price Each |
|---|---|---|---|
| Motor Drivers | |||
| Power Barrel Connector Jack 2.10mm ID, 5.50mm OD | CP-059AH-ND | 1 | $0.89 |
| Slide Switch SPDT Through Hole | 679-1877-ND | 1 | $4.39 |
| N-Channel 40V 450mA (Ta) 1W (Tc) Through Hole TO-92-3 | TN0104N3-G-ND | 20 | $0.90 |
| MCU Power | |||
| Teensy 3.5 32-Bit MCU Eval Board | 1568-1443-ND | 1 | $26.25 |
| DC-to-DC Converter, Input 8V - 36V, Output 5V 1A | 102-5018-ND | 1 | $2.96 |
| Green 572nm LED Indication - Discrete 2.1V 1206 | 516-3233-1-ND | 1 | $0.53 |
| 150 Ohms ±1% 0.125W, 1/8W Chip Resistor 0805 | RHM150AHCT-ND | 1 | $0.17 |
| 4.7 kOhms ±1% 0.125W, 1/8W Chip Resistor 0805 | 311-4.70KCRCT-ND | 2 | $0.10 |
| Sensors | |||
| Pressure Sensor ±30PSI Differential, 0.25 V ~ 4.25 V 4-SIP | 480-5507-ND | 10 | $27.50 |
| Pressure Sensor 150PSI Vented Gauge, 0.5 V ~ 4.5 V 8-DIP | 480-5203-ND | 1 | $23.82 |
| 10 kOhms ±0.1% 0.25W, 1/4W Chip Resistor 0805 | P20708CT-ND | 11 | $0.30 |
| 20 kOhms ±0.1% 0.25W, 1/4W Chip Resistor 0805 | P20740CT-ND | 11 | $0.30 |
| Connectors | |||
| Through Hole 2 pin 0.100” (2.54mm) | WM4800-ND | 2 | $0.84 |
| Receptacle 2 pin 0.100” (2.54mm) | WM2900-ND | 2 | $0.27 |
| Through Hole 3 pin 0.100” (2.54mm) | WM4801-ND | 10 | $0.80 |
| Receptacle 3 pin 0.100” (2.54mm) | WM2901-ND | 10 | $0.23 |
| Tactile Button | CKN9085CT-ND | 3 | $0.31 |
| Pin Headers (For Teensy) | |||
| Surface Mount 6 pins (2x3) 0.100” (2.54mm) | 609-5381-1-ND | 1 | $0.35 |
| Surface Mount 8 pins (2x4) 0.100” (2.54mm) | A121623-ND | 1 | $0.46 |
| Surface Mount 10 pins (2x5) 0.100” (2.54mm) | 609-5381-1-ND | 1 | $0.50 |
| Through Hole 24 pins (1x24) 0.100” (2.54mm) | S1012E-24-ND | 4 | $1.28 |
| Pin Headers (For Board) | |||
| Through Hole 12 pins (2x6) 0.100” (2.54mm) | S2012E-06-ND | 1 | $0.60 |
| Through Hole 6 socket (2x3) 0.100” (2.54mm) | S7106-ND | 1 | $0.62 |
| Through Hole 10 socket (2x5) 0.100” (2.54mm) | S7108-ND | 1 | $0.71 |
| Through Hole 2 socket (1x3) 0.100” (2.54mm) | S7000-ND | 1 | $0.32 |
| Through Hole 3 socket (1x3) 0.100” (2.54mm) | S7036-ND | 1 | $0.37 |
| Through Hole 4 socket (1x4) 0.100” (2.54mm) | S7002-ND | 1 | $0.45 |
| Through Hole 5 socket (1x5) 0.100” (2.54mm) | S6103-ND | 1 | $0.47 |
| Through Hole 24 socket (1x24) 0.100” (2.54mm) | S7057-ND | 2 | $1.00 |
| 2-pin Jumper | S9001-ND | 11 | $0.10 |
| Crimp pins | WM2512-ND | Lots | $0.09 |
| Crip sockets | WM2517CT-ND | Lots | $0.06 |
Mounting Components
Store-Bought Components
All of the mounting components used on this board can be found on McMaster-Carr.
| Part Description | McMaster P/N | Qty | Package | Price Each |
|---|---|---|---|---|
| Narrow Head Slotted Screws, Plastic, M3 , 16mm Long | 96295A711 | 1 | Pack of 100 each | 9.63 |
| Narrow Head Slotted Screws, Plastic, M3 , 20mm Long | 96295A712 | 1 | Pack of 100 each | $9.69 |
| Nylon Unthreaded Spacer 6 mm OD, 2 mm Long, for M3 Screw Size | 93657A001 | 12 | Each | $1.07 |
| Nylon Plastic Washer for M3 Screw Size, 3.2 mm ID, 8 mm OD, Black | 95610A530 | 1 | Pack of 100 each | $3.60 |
| Nylon Unthreaded Spacer 4.5 mm OD, 1 mm Long | 93657A501 | 6 | Each | $1.46 |
| Aluminum Female Threaded Hex Standoff 40mm Long, M3 | 95947A500 | 4 | Each | $1.76 |
| Aluminum Male-Female Threaded Hex Standoff 40mm Long, M3 | 98952A401 | 4 | Each | $2.97 |
| Aluminum Male-Female Threaded Hex Standoff 20mm Long, M3 | 98952A118 | 4 | Each | $1.90 |
| Aluminum Female Threaded Hex Standoff 20mm Long, M3 | 95947A016 | 4 | Each | $0.91 |
| Nylon Hex Nut M3 x 0.5 mm Thread, Black | 93800A400 | 1 | Pack of 100 each | $13.82 |
Custom-Made Components
In addition, I laser-cut a flat frame to place everything together in a neat package.
Download the DXF - Coming Soon
Issues
There are many issues (both anticipated and unanticipated) with the current PCB Design (v3.4).
Major
- Pressure sensors coming off both sides cause mounting and soldering issues.
- makes the board hard to solder (end up soldering underneath other sensors)
- makes the board hard to mount (needs 40mm standoffs)
- Onboard 5V regulator can’t handle spikes in current
- If you turn on the 24V input switch, turn it off, and turn it on again, the 5V regulator will burn itself up.
- I just had to removed it, so the Teensy is can ONLY be powered through the USB port.
- Mounting holes are crowded and not shielded
- I placed components too close to the mounting holes
- It’s hard to get bolts thought the holes without shorting components, so I ended up using plastic screws (bad solution)
Minor
- Extra headers for overriding onboard pressure sensors don’t fit
- headers are poorly placed - interfere with sensor seating and mounting
- Headers are hacky
- “Robot” and “Ext” headers don’t fit the Molex connectors they were designed for.
- I just mis-calculated the width of these connectors
- USB port for teensy isn’t supported and has poor placement
- the USB cord just hangs off the teensy board in the middle of the main board
- usb cord also covers the power switch.