A printed circuit board (pcb) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. pcb's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.
Design
A PCB motor is comprised of a rotor (custom made to the specific application) and a stator with a number of piezo-elements mounted on a Printed circuit board (PCB) in segments of 8 elements as shown in the drawing below. The PCB might also hold other circuits such as position sensor, driver, uController etc as required by the specific application. The stator ring is milled out during the PCB manufacturing before the pz-elements are soldered onto the raw PCB.
The purpose of the pz-elements is to generate at travelling wave in the stator ring. The applied voltage (100-200Vrms) will cause the elements to either expand or contract depending on voltage polarity. In this way the PCB surface will be forced to follow the movements of the elements and thus generate a small bending (<10μm) of the laminate. If you apply 2 phases at 90° (a sine and a cosine) and use the same frequency as the mechanical resonance of the stator this movement will be amplified and be of sufficient magnitude to push the rotor around. By switching SIN and COS signals you can change the direction of the travelling wave and thereby the direction of the rotation.
Each mechanical wave in the stator ring is supported by 8 polarized pz-elements and the number of waves ranges from 4-16 for different sizes of rings. In order for the SIN and COS signals to support the resonance of the ring is it important that the above sequence of orientation and connections is maintained.
Pz-elements
Pz elements are small box-formed components which will change size when a voltage is applied to the terminals. The element is metallised on both end surfaces with approx. 10 um of silver (Ag) which is suitable for soldering.
Information for the mounting process is included with the piezo components.
Tape & Reel
Pz elements can be supplied from PCBmotor.com on standard 8 mm tape. Reels are 7” and each reel contains 5000 pcs.
Handling
Pz elements are shipped in sealed vacuum bags. After breaking the seal we recommend to store the components in a non-oxidizing atmosphere to avoid corrosion of the terminals.
Resonance design
The resonance frequency is determined by the material properties of the PCB laminate. We have found that a resonance of approximately 45 KHz works well. This can be obtained by using a 1.6 mm FR4 laminate (incl. copper layers), an approx. center-to-center distance of 2 mm for the elements and a width of the ring of from 3.25 mm. Mechanical dimensions are critical to yield a clean resonance curve.
See example below for ø30mm diameter recorded using our Labkit:
Physical layout
Below is a cross section of a double sided stator.
Content in attached zip files
In the layout zip files you will find an Eagle formatted board file which can be used to generate output for your choice of SMD machine. We have good results using the Fabmaster format. Alternatively you can export the layout to your preferred PCB layout system.
PCB stator bridge design
In order to allow the stator ring to oscillate freely it is suspended in 3 or more “bridges”. These structures also provide a routing path for Sin, Cos and Gnd connections of the pz elements. Dimensions depend on axial force applied in the application. The via holes helps to establish a good mechanical interface between piezo and PCB. Via diameter is 0.2 mm in order to prevent melted solder to go through the PCB.
Using our jigsaw concept you can focus on designing your application while taking advantage of our preassembled stators. As shown above you can use a simple cut-out in the motherboard and fit a jigsaw stator either in-plane, on-the–board or at any height above the board. Later – when volume picks up – you can decide to fully integrate the stator on your motherboard and do the piezo assembly along whit your other components.
Rotor design
We recommend always using two interlocked rotor discs even if you are designing a single sided stator. In this way you can avoid axial forces on the stator bridges which potentially can damage the PCB. The rotor must be spring loaded against the stator. The force needed is dependent on application but a typical value of 0.6 N per wave (8 piezos) will work for most applications.
The rotor can be any material; however it must be observed not to short circuit the piezo elements. The material and surface properties has a strong impact on friction between stator and rotor and therefore affects the mechanical output and may affect the noise level. We recommend using a tape on the rotor. ITW Stokvis Tape Group, RHO GL96 has proven to have good friction and wear properties.
This is the rotor example with inline spring used in some of our Evaluation kit. The two discs are clamped and locked together to the stator PCB by the force from the central spring.
In order to give a stable rotation the planarity of the rotor discs should be better than 0.02 mm.
Temperature dependence and driver design
The motor resonance frequency is mostly determined by mechanical constants of the PCB-material and it is necessary to compensate for the change in resonance frequency due to ambient temperature changes and self-heating in the motor.
During testing and continuous operation the power dissipated in the stator will raise the temperature in the PCB slightly. This will soften the laminate and result in a lower resonance frequency. For 1.6 mm FR4 this resonance-TC is approx -40 Hz/°K.
The 2-phase driver for the stator must supply a sine and a cosine voltage of 100-200Vrms to the pz-elements. As the pz-elements are a capacitive load it is desirable to design the output stage as a resonant circuit with an electrical resonance close to the mechanical resonance of the stator ring. This is typically done by using a tuned transformer in the output stage, at the same time stepping up the voltage from the supply voltage (3-5V) to the high 100-200Vrms required to drive the stator. Some small losses will occur in the transformers (one for each phase) and in the Mosfet drivers for the primary side of the transformers, but 80-90% efficiency is possible. The basic TestKit driver is shown below with the output transformers and the Mosfet drivers to the right:
Capacitors C3 and C5 are used to tune the electrical resonance to match the mechanical resonance and C3 & C5 will have different values for different stator designs. Since the mechanical resonance changes with temperature while the electrical resonance is stable, some mismatch will occur at the ends of the temperature range and the power efficiency will be highest when the two frequencies match.
For temperature control of the frequency it is suggested to use a tracking controller that monitors either the speed or power consumption and sets the frequency for optimum performance.
The DC current supplied to the motor driver is closely correlated to the motor performance (resonance, speed & stall momentum) and is convenient to measure during operation as well as for production testing. The controller in Development Box v.5 continuously measures the DC-current supplied to the output stage in the driver and steps the frequency up and down until a maximum is achieved. For continuous operation this tracking feature is an effective way of setting the optimum frequency to compensate for the self-heating in the motor. Please contact PCBmotor.com for details of the implementation.
Test
To do a quick electrical test on the stator assembly you can run a frequency sweep while monitoring the current consumption of the driver. This will look like the curve below (mA vs. kHz) on a free stator without rotor. For more information on our Labkit please see:
You can also measure the capacitance which should be approx 6 pF per pz element. From SIN to GND you will measure half of the piezos in parallel and from COS to GND the other half. As an example, an Ø20 mm stator, single sided, 32 pz-elements should be approx 100 pF in each phase.
PCBWay, PCB prototype and Fabrication the Easy Way!
