GM3 motor suppressor

I have also been asked for more details on my motor suppression circuit that I first blogged in dc-to-daylight, so here goes :-



The Solarbotics GM3 generates large amounts of RF noise from 20MHz up to at least the TV band, which is 470- 850MHz in the UK. I know this because I can see the 20 MHz on my scope and it was also affecting our TV reception.

This is the circuit I used :-



The 1nF capacitors were axial ceramics and the 10nF was a radial ceramic, mainly because that is what I had to hand. I don't know the spec of the ferrite beads because I salvaged them from an old disc drive. Here is what they look like though :-



They should be a low Q type rated for at least 1A. The current rating is not so much about how much current they can carry but about the point where the magnetic field saturates the ferrite and the inductance disappears.

We want them to have a high impedance from 20 MHz to 800 MHz. I don't have much knowledge in this area but think this is quite a big ask for a ferrite and that I fell lucky with these. To get more impedance at the low frequency end it is normal to increase the number of turns to increase the inductance which is proportional to their square. The problem with that is that it increases the capacitance, reducing the attenuation at the high frequency end.

These beads are a good compromise: they have nearly a whole turn compared to a straight through bead which is half a turn, hence four times the inductance, but the wires maintain 0.1" separation so minimizing the capacitance.

The first two 1nF capacitors are soldered to the motor case. This is easier than you might imagine because steel is such a poor conductor of heat compared to copper, although it has to be said I am using a 50W temperature controller soldering iron. I cleaned the area first with a PCB cleaning block.



This is the rest of the circuit before it was soldered on top of the two capacitor leads. Spot my mistake!



Ignore the back emf diode, it is specific to my controller and should really be part of it. I used twin screened cable with the braid grounded at the controller end and left unconnected at the filter end.

Measuring temperature the easy way

I have been asked for more details on my temperature measurement scheme so I have consolidated some of my previous articles :-

The objective is to measure temperature from room temperature to about 250°C using a thermistor. The thermistor resistance is a extremely non linear. It is approximated by a negative exponential of the reciprocal of absolute temperature.

Ro is resistance at known temperature To, in this case 25°C, expressed in Kelvin. Beta is a second parameter of the thermistor which can be calculated if you know the resistance at two different temperatures or can be found on the data sheet.

The RepRap thermistor is an Epcos B57540G0103+, data sheet here. R₂₅ is 10KΩ and beta is around 3500. Several values are given on the datasheet for different temperature ranges illustrating that the above equation is only an approximation. Here is a graph of its resistance against temperature :-


This can be made more linear by putting a fixed resistors in parallel. The magic value to use appears to be the value of the thermistor at the middle of the temperature range. In this case it is about 470Ω. Here is the resulting combined resistance, the formula for two resistors in parallel is :-

1/R = 1/R₁ + 1/R₂

I.e. the total conductance is the sum of the two conductances.


The resulting resistance is a lot more linear, however to measure temperature with an ADC we need a voltage rather than a resistance. This is easy, instead of wiring the resistor in parallel connect it in series to a voltage source equal to the full scale voltage of the ADC.



The voltage across the thermistor is then :-

V = V_ref . R_th / (R + R_th)

Here is a graph of the the output voltage when Vref is 5V.



Note that the voltage decreases as the temperature rises. This could be inverted by swapping the resistor and thermistor but I prefer to keep one end of the thermistor at 0V so I can use single screened cable. It is also a good idea to put a capacitor across the ADC input to filter out any noise when using long leads like RepRap does. I used a 10uF tantalum bead.

Another consideration is how much power is dissipated in the thermistor as it will cause heating and alter the reading. The maximum dissipation will occur when its value equals the value of the resistor. At this point half the voltage is across the thermistor so the power dissipated in it is :-

P = (V_ref / 2)² / R

In the example above this works out at 13.3mW. The thermistor datasheet specifies a maximum of 18mW and a dissipation factor (in air) of 0.4 mW /K. I think this means that the temperature will rise by 33°C by self heating. The error would be less when not in air, but it is still perhaps a bit high. My system uses a V_ref of 1.5 volts which, because it is a square law, only dissipates 1.2mW giving a 3°C rise at the mid range temperature in air.

For a 5V system is is probably worth sacrificing some of the ADC resolution to reduce the self heating error. This can be done by using two resistors :-



The full scale voltage is now :-

V_fsd = V_ref * R1 / (R1 + R2)

We also want the source impedance of this voltage, which is R1 in parallel with R2, to be 470Ω.

1/R = 1/R1 + 1/R2

Solving these simultaneous equations gives :-

R1 = R / (1 - V_fsd / V_ref)

R2 = R . R1 / (R1 - R)

So for V_fsd = 1.5V, V_ref = 5V and R = 470:

R1 = 671Ω and R2 = 1569Ω, preferred values are 680 and 1K6.

And finally here is the Python code to work out the temperature :-

from math import *

class Thermistor:
   "Class to do the thermistor maths"
   def __init__(self, r0, t0, beta, r1, r2):
       self.r0 = r0                        # stated resistance, e.g. 10K
       self.t0 = t0 + 273.15               # temperature at stated resistance, e.g. 25C
       self.beta = beta                    # stated beta, e.g. 3500
       self.vadc = 5.0                     # ADC reference
       self.vcc = 5.0                      # supply voltage to potential divider
       self.vs = r1 * self.vcc / (r1 + r2) # effective bias voltage
       self.rs = r1 * r2 / (r1 + r2)       # effective bias impedance
       self.k = r0 * exp(-beta / self.t0)  # constant part of calculation

   def temp(self,adc):
       "Convert ADC reading into a temperature in Celcius"
       v = adc * self.vadc / 1024          # convert the 10 bit ADC value to a voltage
       r = self.rs * v / (self.vs - v)     # resistance of thermistor
       return (self.beta / log(r / self.k)) - 273.15        # temperature

   def setting(self, t):
       "Convert a temperature into a ADC value"
       r = self.r0 * exp(self.beta * (1 / (t + 273.15) - 1 / self.t0)) # resistance of the thermistor
       v = self.vs * r / (self.rs + r)     # the voltage at the potential divider
       return round(v / self.vadc * 1024)  # the ADC reading

Laying it on the line

I decided to investigate the conditions necessary for multiple layers of HDPE filament to stick together so I wrote a little Python test script to extrude 20mm squares stacked on top of each other. From my graphs in equations-of-extrusion I chose an output rate of 3mm/s which gives a filament diameter of about 1.2mm. That only requires about 60% PWM which I thought was not too stressful for a 5V motor running from 12V. I set the heater temperature to 200°C. Here is the first run :-



The first two layers look reasonable and then we are into basket work! The z-axis was raising 1.2mm between each layer but, although the nominal filament diameter should be about 1.2mm, the sides were not growing at the same rate. That meant the filament was dangling allowing it to wiggle around. Next I reduced the z-increment to 1.1mm :-



Better, the first four layers are OK this time, so obviously I tried z-increment 1.0mm next :-



Much better! What is happening is that the filament is no longer cylindrical. Each layer is about 1.0mm high and 1.4mm wide. It could be due to gravity but I think it is more to do with being bent through 90° as it comes out the end of the nozzle.

The fact that the filament weaved about when the nozzle was too high made me think that the feed rate might be too fast so I did a taller test with the XY travel 20% faster :-



Another basket case! What is happening here is that there is not enough material so the filament slumps down and holes start appearing.

I went back to the original feed rate and did a couple of 20mm high tests to check consistency :-



These are actually incredibly strong in the vertical direction. I can stand on one and it takes my full weight. Here is a video of the one on the right being made, the middle section is sped up 8 times :-



I also ran a test at 160°C to see if the filament would still weld to the layer below. It did but it did not stick to the foam board.

As you can see the main defect is that the bottom corners curl up. This was completely expected from the work Forrest published here: Ten-layers-with-no-curling, so next I will try his solution of laying down a raft first.

Another defect is that the filament width varies in waves. These seem to be related to the rotation of the extruder drive screw. You can hear the motor labouring more on part of the revolution. I think it is because something in the drive is a bit eccentric but more investigation is required.