Inexpensive Piezoelectric Elements for Nozzle Contact Detection and Bed Levelling in FFF 3D Printers

Inexpensive piezoelectric diaphragms can be used as sensors to facilitate both nozzle 1 height setting and bed levelling in FFF (Fused Filament Fabrication) 3D printers. A variety of probes 2 have been developed by the authors and others to utilize piezoelectric diaphragms both under the 3 build stage and in the printer head. The reliability, repeatability and sensitivity of these probes has 4 been investigated along with such practical considerations as usability in different environments, the 5 functional life of piezoelectric diaphragms in this use and what improvement to print quality may 6 be obtained. A probe using a piezoelectric diaphragm has been developed and released as an open 7 source product, this probe as well as kits for making probes are available and are proving reliable. 8 The conclusion is that piezoelectric diaphragms are equal to or better than other technologies used 9 for nozzle probing. 10


12
At the core of the RepRap project is the objective that RepRap printers should be able to print 13 many or most of the parts that are used in their own construction while those parts that cannot 14 be printed should be readily available and inexpensive [1]. Piezoelectric diaphragms are readily 15 available and inexpensive as they are used as sounders in many manufactured goods. As piezoelectric 16 discs will also function as sensors they are useful components for making RepRap printers: This paper 17 addresses the suitability of piezoelectric diaphragms as sensors for bed levelling in FFF printers..  The early RepRap printers levelled the build stage manually by adjusting three or four sprung 29 adjusting screws. [4] As manual adjustment was laborious and may be required frequently, methods 30 were sought to automatically check the height of the printer nozzle without resorting to tools such as 31 feeler gauges. Once the earliest automatic methods of measuring the relative distance from the nozzle 32 to the build stage it became possible to use software to compensate for distortion of the build stage 33 and ultimately to compensate for geometric errors in the printer itself. 34 Submitted to Entropy, pages 1 -11 www.mdpi.com/journal/entropy The first methods measured the distance between the print nozzle and the build stage using a 35 switch which could be manually, mechanically or electrically deployed. [5]  and any functionally similar replacements from unidentified manufacturers. These diaphragms are 52 used in musical novelties, as the voice in toys, to produce the warning sound in alarms, to replace the 53 mechanical click sound in tactile keyboards and in a great many other ways.

54
Conversion of electrical energy to mechanical energy in piezoelectric diaphragms is by what 55 is correctly termed the "Inverse Piezoelectric Effect" however piezoelectric materials also exhibit the 56 "Direct Piezoelectric Effect" [9] where mechanical energy is converted to electrical energy: It is this 57 effect which is used by the sensors described in this paper. The diaphragm consists of a piezo-active 58 ceramic disc bonded to a metal disk and a conductive layer on the opposite surface which form the 59 electrical connections.

60
The design intent of these piezoelectric diaphragms is the conversion of electrical energy to 61 mechanical movement when an electrical potential applied to the piezo-active ceramic causes the 62 centre of the diaphragm to bow relative to the periphery. The ceramic used will also operate in the 63 reverse sense, a pressure that causes the diaphragm to bow or to bend will generate an electrical 64 charge between the electrodes. In addition, a pressure applied directly between the face and the 65 substrate without causing it to bend will also generate an electrical charge.

66
In order to assess the usefulness of inexpensive piezoelectric diaphragms as sensors in FFF The limitation of use at higher temperatures is investigated as well as the effect of large numbers 75 of simulated nozzle contact events at room temperature and at temperatures near the limit of 76 sensitivity. Data is compared for diaphragms before and after thermal cycling to assess the ageing 77 of the diaphragms in service.

78
The development of a Z probe integrated into the printer hotend is described by Simon Khoury 79 in the discussions section of this article. action. The jig as depicted in Figure 1 has a small table mounted on an actuator rod which is 84 connected to a 3D printed parallel mechanism, the parallel mechanism transferring pressure to the 85 piezoelectric diaphragm through a 3D printed pressure pad. A load spring maintains an upward 86 pressure on the actuator rod and on the diaphragm through the parallel mechanism. A preload 87 adjuster centres the parallel mechanism at its resting position and provides a small force on the 88 piezoelectric disc after the spring load has been removed. The CNC machine is programmed to start 89 a probe moving towards the actuator from 1mm above it and to continue for 0.5mm after striking the  In Figure 3 the probe strikes the actuator at 1mm per second and the peak voltage obtained from 121 the piezoelectric diaphragm was 8.1 Volts which occurred 90ms after the first contact. Oscilloscope 122 settings were 5V per cm vertical with trigger set to 2.4V and horizontal was set to 50ms per cm.  The probe strikes the actuator and over-travels by 20µm each cycle from 20 to 220µm. The 124 voltage response is shown in Figure 4. Note that the travel at greater than 120µm is more than the 125 90µm implied by the first test. This is thought to be due to the deceleration phase from the CNC 126 software. To obtain data on the force response the solid probe was replaced with a light spring and travel 128 was set so that with each cycle the force applied by the spring was increased by 20 grams force to 129 a maximum of 100 grams force. To obtain the required spring rate an Entex stock no. 3352 spring 130 was shortened to give a rate of 125 grams per mm. The resulting voltage is shown in Figure 5, the 131 available voltage being significantly reduced by resistive leakage through the oscilloscope probe 132

Results
The remaining nine piezoelectric diaphragms were all checked for basic voltage output and did 133 not differ visually from the first one shown in Figure 3. To investigate any change that may occur at higher temperatures the piezoelectric diaphragm  its print head assembly, was already known. However the system of placing the discs below the 172 build stage, required at least three piezo discs, sometimes four, so was considered more complex than 173 necessary.

174
The build stage assembly is frequently mounted on a moving axis, the Y-axis in some cases The key innovation, was to drill a hole through the centre of the piezoelectric disc, in such a way is driven into the melting chamber above the printers nozzle (hotend). In the case of a 3mm filament 201 no guide tube was used (as this filament is stiffer due to its larger diameter). It is noteworthy that 202 piezo-ring devices already exist with holes centrally located but the cost of these devices is several 203 orders of magnitude higher than for piezoelectric discs such as the Murata 7BB series, and they are 204 available only from specialist suppliers.

205
Having determined by test probing, and testing of various drilled piezo electric discs on an 206 oscilloscope, that the disc still functioned as it did when un-drilled, albeit with a reduction in voltage 207 generated equal to the proportion of ceramic material removed, but well within the range at which 208 detection with high sensitivity is possible, the next step was to mount the disc above the extruded 209 polymer heater assembly. nozzle is also undesirable but so long as it is less then 100µm, its effect on the accuracy of the print 244 is acceptable. The sensor unit's design therefore is a compromise between having high sensitivity 245 for nozzle contact which would be achieved by having a relatively loose assembly which allows for 246 greater compression/flex in the piezoelectric disc, yet an unstable nozzle, and having an extremely 247 tight assembly which would have much less sensitivity due to pre-loading of the piezoelectric disc, 248 but exhibit greater nozzle stability.

249
Another aspect considered was that in the first prototype shown here, which used a 27mm 250 piezoelectric disc, the mechanism by which force was imparted to the piezoelectric disc was by 251 uniform compression. Whilst this achieves reasonable sensitivity, greater sensitivity can be achieved 252 by flexing the disc. In this version four screws were used to hold the assembly together. This allowed a 253 reasonably firm assembly to be constructed. Another version with three screws holding the assembly 254 together was deemed to be too flexible and polymer pins were introduced alongside the screws, the 255 idea being that the lower part could slide on the pins, the pins acting to limit lateral movement in the 256 assembly and attached hotend/nozzle. This was later designed-out as the unit became smaller and 257 this lateral movement was reduced. one another the diaphragm is bent centrally against its upper support and placed in light pre-load.

263
This enhances sensitivity whilst achieving much less movement laterally at the nozzle. Another 264 change was to make the unit smaller, in order to do the size of piezoelectric disc reduced from 27mm 265 to 20mm. the foregoing, the output from these components is so large that even a poor quality piezoelectric 273 diaphragm is able to give an output much greater than is needed for accurate detection of the 3D 274 printer build surface.

275
In order to promote the widespread adoption of this technology and method of probing the build 276 stage of a 3D printer, the company Precision Piezo [14] has been formed which has during its first 6