DIY Piezo-Electric Touch Probe with high sensitivity

I built this mechanical piezo-electric touch probe recently as an extension to a CNC machine to sense the depth of surfaces. It works with the grbl CNC controller and probably others.

The goal was to flip a TTL (5V) signal whenever the probe was touching a surface mechanically. Unlike classical mechanical switches which have moving parts, this probe is ‘solid state’ and must emit a signal as early as possible when there is mechanical contact, even when the probe is touched ever so slightly, and even on non-conducting materials.

The idea of piezoceramic touch probes is not new. They are based on the bending of piezo ceramic, which is constructed behind the surface of the probe. The benefit of using a piezo touch switch is its ability to interact with surfaces of virtually any type of material.

The cheapest professional touch probes cost several hundred Euros, and upwards. That was out of the question for my purposes. However, with a bit of ingenuity, I managed to construct a probe from scrap metal and electronic parts that cost less than 10 euros.

The heart of the sensor is a piecoceramic loudspeaker/beeper. However, it also can act as a “microphone”. When the piezoceramic surface is deformed mechanically (either via sound or by direct pressure), voltage is generated (up to 2 or 3 Volt).

The tricky part is that upon pressing, charge is generated, which dissipates quickly via the piecoceramic material itself. This means that upon releasing the mechanical pressure, charge of the opposite sign is generated. This negative voltage is a problem: One cannot simply feed this signal into a TTL-level microcontroller (µC) without risk of damaging it. Also, slight touches would only generate Millivolts, below the threshold voltage of a µC pin. So, electronics have to be made that act as an analog-to-digital converter. Specifically, I implemented an inverting Schmitt-Trigger (see below).

Here is the piecoceramic loudspeaker/beeper that I used (less than 1 Euro):

Piezoceramic beeper

 

I simply removed the plastic back plate from the beeper to expose the piezoceramic element, and, in its center, glued a short wood screw on it with a bit of epoxy glue.Then I bent some metal and drilled some holes to create a stable holder for the beeper (this will be mounted directly on the CNC machine). The nice feature about this particular beeper is that the piezoceramic element is suspended by its plastic packaging, which means that I could squeeze it between the metal holder without deforming the piezoceramics:

Piezoceramic beeper as touch probe

 

Now that the mechanical part is done, on to the electronics!

Electronics

Goal: The electronics will receive the analog (positive or negative) voltage of the beeper, and output a well defined TTL level between 0 and 5V that can be directly and safely be consumed by a microcontroller. The values of the electrical components have to be chosen such that the slightest mechanical deformation of the piezo element will immediately flip the TTL signal.

As I have already finished and tested the probe while writing this, I have tried to apply such a gentle pressure so as to NOT make the TTL signal flip. It is hard to do, which means that it is a very sensitive sensor!

My implementation will flip the TTL output from high to low when the input surpasses 50mV. It will flip the TTL output from low to high when the input goes below 0V.

Below schematics show:

Terminal T1: 5V supply voltage for the OpAmp
Terminal T2: The TTL output (0V or 5V) that can be directly connected with a µC
Terminal T3: The analog input from the piezo element
Terminal T4: GND

Touch Probe analog to digital conversion circuit

 

 

It consists of the following components:

Inverting Schmitt trigger (U1, R1, R2)

I chose an easily available OpAmp LM358N. I chose the inverting configuration to completely isolate input from output. I chose R1 and R2 such that the input threshold voltage would be approx. 50 mV (a very light touch of the probe generates this easily). The threshold voltage is simply the voltage between the voltage divider R1 and R2. I chose R1 = 5k4 and R2 = 55.

V_threshold = Vcc * R2 / (R1 + R2) = 5 * 55 / (5k4 + 55) = 50 mV

Pulldown resistor (R3)

The negative input of the OpAmp and the piezo element on T3 are very high impedance and would float if not pulled down. In addition, the real OpAmp leaks a bit of charge into this terminal, bringing it up to 60mV which is even above the switching threshold. To bring the voltage much closer to 0V, I added a rather strong pull-down resistor R3 = 87k. The piezo has no problems with this load.

Low-pass filter (R4, C4)

In my wiring, T3 will be connected to a long line subject to interference. R4 together with C4 form a simple low-pass filter which a time constant t = 0.24 µs , which is shorter than what will be expected from the signal of the piezo element.

LED (R5, D1)

The LED and its limiting resistor R5 = 480 Ohms (for approx 10mA) is just there for immediate optical feedback.

 

Implementation on a development board

Above schematic can be transferred into the following equivalent simple development wiring layout (C4 is labelled C1):

 

Touch Probe implementation

 

The resistors and capacitors can be easily exchanged for trial-and-error fine-tuning:

Touch Probe electronics on a development board

 

Verification with Oscilloscope

The following probes were connected:

Yellow: The input of the piezo element before low-pass filter
Blue: The input of the piezo element after low-pass filter
Pink: The output of the circuit

Touch Probe test setup

 

 

 

Input voltage without pull down resistor

Input voltage without pull down resistor (yellow graph). The OpAmp is leaking some charge into the input terminal, causing the input voltage to raise to about 60mV. Connecting a pull down resistor will rectify this.

 

 

<img class="wp-image-304" src="http://michaelfranzl team project management tools.com/wp-content/uploads/2016/05/DS1Z_QuickPrint7.png” alt=”Input voltage with pulldown resistor” width=”598″ height=”337″ />

Input with pull-down resistor applied. Input voltage is now down to less than 5mV.

 

 

 

Triggering1

Touching the piezo probe increases voltage on the input. At 50mV, the output switches to LOW.

 

 

 

Triggering2

A faster, oscillating signal. When going over 50mV, the output switches to LOW. When going below 0V, the output switches back to HIGH.

 

Triggering3

An even faster oscillating signal (I hit the probe holder with a screwdriver). Behavior is as expected.

OpenGL programming in Python: pyglpainter

This was a recent hobby programming project of mine for use in a CNC application, using Python and OpenGL. The source code is available at https://github.com/michaelfranzl/pyglpainter .Simple OpenGL output using pyglpainter library

This Python module provides the class PainterWidget, extending PyQt5’s QGLWidget class with boilerplate code neccessary for applications which want to build a classical orthagnoal 3D world in which the user can interactively navigate with the mouse via the classical (and expected) Pan-Zoom-Rotate paradigm implemented via a virtual trackball (using quaternions for rotations).

This class is especially useful for technical visualizations in 3D space. It provides a simple Python API to draw raw OpenGL primitives (LINES, LINE_STRIP, TRIANGLES, etc.) as well as a number of useful composite primitives rendered by this class itself ( Grid, Star, CoordSystem, Text, etc., see files in classes/items). As a bonus, all objects/items can either be drawn as real 3D world entities which optionally support “billboard” mode (fully camera-aligned or arbitrary- axis aligned), or as a 2D overlay.

It uses the “modern”, shader-based, OpenGL API rather than the deprecated “fixed pipeline” and was developed for Python version 3 and Qt version 5.

Model, View and Projection matrices are calculated on the CPU, and then utilized in the GPU.

Qt has been chosen not only because it provides the GL environment but also vector, matrix and quaternion math. A port of this Python code into native Qt C++ is therefore trivial.

Look at example.py, part of this project, to see how this class can be used. If you need more functionality, consider subclassing.

Most of the time, calls to item_create() are enough to build a 3D world with interesting objects in it (the name for these objects here is “items”). Items can be rendered with different shaders.

This project was originally created for a CNC application, but then extracted from this application and made multi-purpose. The author believes it contains the simplest and shortest code to quickly utilize the basic and raw powers of OpenGL. To keep code simple and short, the project was optimized for technical, line- and triangle based primitives, not the realism that game engines strive for. The simple shaders included in this project will draw aliased lines and the output therefore will look more like computer graphics of the 80’s. But “modern” OpenGL offloads many things into shaders anyway.

This class can either be used for teaching purposes, experimentation, or as a visualization backend for production-class applications.

Mouse Navigation

Left Button drag left/right/up/down: Rotate camera left/right/up/down

Middle Button drag left/right/up/down: Move camera left/right/up/down

Wheel rotate up/down: Move camera ahead/back

Right Button drag up/down: Move camera ahead/back (same as wheel)

The FOV (Field of View) is held constant. “Zooming” is rather moving the camera forward alongs its look axis, which is more natural than changing the FOV of the camera. Even cameras in movies and TV series nowadays very, very rarely zoom.