Index
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Step /
continuous control for small synchronous motors
(published in Electron #4, 2001) Introduction
Many radio systems are equipped with small
electric motors to control dials, inductors, switched, capacitors etc. Also
moving parts in automatic registering equipment are usually controlled by
these small motors e.g. drums in Morse and Hell writers. In most of these
applications some type of stepping motor is the active element, usually
controlled by a microprocessor. Regularly small synchronous motors are being
offered at ham fest markets, that in effect are stepping motors as well but
optimized for direct drive by mains frequency at a direct or lowered voltage.
Next to these motors also small gearboxes that connect to these synchronous
motors are being offered at various speed ratios, so speed of the final axes
may be selected over a wide range. It appeared to me these small motor were an
excellent means to control the capacitor in a magnetic loop antenna because
of the simple construction. These motors are easily controlled by components
from the junk box without the need for a micro-processor since only simple
actions are required like step or continuous movement and forward and
backward control of the tuning capacitor. The
synchronous motor
Small synchronous motors may be recognised by
their construction. At the outside these often look like two cylinders welded
together. From each of these cylinders two wires will emerge, of which two
are directly connected and the other two through a capacitor. At low voltage
motors the capacitor usually is around 2,2 μF. These synchronous motors are in fact optimized
stepper motors for a single control frequency which is clear to see if you
were to open up one, unfortunately to end its life there. Each of the two
cylinders is containing a coil (hence the two wires) and a number of metal
teeth. The position in the upper compartment of these teeth is shifted in
respect to the bottom half. A sinusoidal voltage is connected to one of the
coils and phase shifted (by the capacitor) to the other. The rotor is consisting of a (series of)
permanent magnet(s) that will orient to the magnetic field, directed by the
teeth. A little moment later the maximum field strength is directed by the
phase shifted teeth of the other coil and the rotor will follow. Next the
upper coil will have the upper hand and the rotor will follow again. In this
way the rotor is turning around, always following the strongest magnetic
field. The number of 'steps' per minute depends of cause on the number of
teeth and the number of permanent magnets in the rotor. It is good to notice these motors are
perfectly symmetrical. When connecting the voltage to the other coil with the
first one now being fed through the capacitor, the motor will run backward at
the same speed. Also, at presenting one sinusoidal period at the time, the
motor is following with just one step. This is exactly the kind of action we
require to control a capacitor in a magnetic loop antenna that should be
capable of making a small step (forward and backward) for accurate control
and continuous motion to quickly find the tuning point.
Selecting
the motor and gearbox
First question to answer is the optimal motor
- gearbox combination, since both are offered in many variations. The optimal
combination is depending on the magnetic loop parameters, we will investigate
at first. With the final axes from the gearbox stepping
too coarsely, accurate loop tuning will be very difficult since loop antennas
usually are exhibiting a very high Q (small band-width). At too small a step
size, we will have to wait very long before the tuning position will arrive. A simple calculation is giving a clue about
the required positioning sensitivity:
f0 C0 C0 Q
= ----- ~
-------- and also: δC ~
------
2.δf δC Q with f0 as the loop resonance
frequency, C0 the capacitor at this frequency. The parameter δC represents a small change in resonating
capacitance until the voltage at the circuit is down to 0,7 x maximum; δf is the corresponding frequency change. It may be
appreciated that a high Q is translatating to a
small step size of the capacitor. A useful (differential) tuning capacitor at
HF frequencies has a capacity range of say 12 - 57 pF over an angle of 180
degrees of rotation, so ca 0,25 pF / degree. At the
most sensitive side of the tuning range (minimum capacity) we may calculate
the maximum allowable step-size at a realistic system Q, say 400.
12 δC
~ ------- pF
~ 0.03 pF
400 Maximum change of angle per step at the
tuning capacitor (characterized at 0,25 pF / degree) therefore is: angle / step = 0.03 / 0.25
= 0.12 degree per step. Small synchronous motors often are specified
by rotational speed in rounds per minute (rpm), always at a specific
frequency, usually 50 Hz. in rpm * 360
angle/step
= --------------- =
0.12 * rpm. 60 * 50
According to our calculations we are looking for
a maximum rotation of 0,12 degree / step, so we have all figures ready to
calculate the gearbox ratio (gbr) 0.12 0.12
gbr =
------- = ---------------- = 1
/ rpm
angle / step 0.12 * rpm
This is a surprisingly simple formula that is
telling us, when we find a synchronous motor specified at 60 rpm, we need a
gear box with a ratio of 1 : 60 for our HF magnetic loop antenna. As it
happens, gear box ranges of 1 : 10 to 1 : 100 are not difficult to find. If
we have to select one to the nearest calculated gearbox ratio, we better
select a somewhat higher, than a lower ratio. Note in the above selection we are obtaining
a rotational speed of about one turn per minute at 50 Hz. at the outgoing
axis, so we have to wait for a maximum
of 30 seconds (half turn) for the tuning punt to pass. When we would like to
tune more accurately, we need a higher gearbox ratio and so have to wait on
average somewhat longer for this tuning position. Although in all above examples we have been
playing with real-life numbers, the actual loop-Q has to be measured before
an accurate calculation should be made for the motor / gearbox combination.
Synchronous
motor control
To control the synchronous motor I designed a
simple circuit from materials laying around in the junk-box. The circuit is
based on two HEF-type of integrated circuits that will operate in the range of
5 - 15 V. This comes in handy when applying the circuit 'in the field' at
battery power. Mind to apply the exact IC-types, including the NAND-gate as
this is a hysteresis type which is controlling the oscillator and counter
acts switching 'bounce'. Diodes are 'general' types provided these
allow burst currents up to 200 mA and a reverse voltage up to 40 V. (most
general purpose types will), with no special frequency requirements. Timing
diagram
Timing diagram may be found in figure 1. The
synchronous motor is connected to V+, shortened, to V- and shortened again.
This is a first approximation of a sinusoidal supply with switching moments
when a real sinus is at a value of 0,6 respectively - 0,6 as in Q0
to Q1, Q3 to Q4, Q4 to Q5
and Q7 to Q0
Diagram
The switch timing is generated by a Johnson
counter (HEF 4017), that will shift a digital '1' through the outputs at each
clock period. A diode matrix is translating this stepping order into the
right switching moments for the driver transistors, to generate the timing of
figure 1. This control circuit may be found in figure 2. When output Q8 is set high, the
Johnson counter is reset through an inverter and a flip-flop (the
cross-coupled gates); in this position output Q0 will be high,
shortening the motor by means of the driver transistors. At a pulse by the 'step' switch, the
flip-flop is reset and the counter will make one complete tour until reset
again. Holding the step switch has no effect; for the next step, it should be
released at first. At holding the 'continuous' switch, the
counter will keep on making its tours until the switch is released. The
counter will then stop at the 'reset' position. Other components around the flip-flop ensure
the system to always start at the 'reset' position. Figure 2:
Control circuit Different
motor voltages
At ham fests synchronous motor will be
available for different voltages, with 12V. and 24 V. to be most abundant.
The circuit is easily adaptable for different motor voltages by means of a
small mains voltage transformer. The control circuit has been designed to
generator a frequency just above 50 Hz., so these mains transformers will
operate quite nicely, without being troubled by the higher harmonics. In figure 3 we may find the 'standard' 12 V.
connection at the left-hand site and a 24 V. connection at the right hand.
Note the 12 V. winding to end up at the a - b terminals. The high-voltage
side of the transformer is being left open. As already mentioned, these motors will run
forward and in reverse by connecting to different terminals. When applying a
three position tumbler with double throw, a no-current mid-position may be
created to have the circuit only consume power when in use; this will extend
battery life in the field. When applying the indicated transistors,
motors up to several Watt may be controlled.
Figure 3: Motor connections The magnetic loop antenna plus controller has
been around in the shack and outside for quite some time now. The procedure
to control the loop appears to be easiest when following steps will be taken: Switch the transceiver to a low-power CW
position, set the motor control switch to 'forward' and watch the needle of
the SWR meter. Suddenly a strong dip will appear, but before you can release
the switch, the dip has already been gone. You now set the motor control
switch to 'reverse' en 'step' backward until you reach the low SWR position
again. When overshooting, you simply reverse again and step forward. Tuning
in this way also take the back-lash out of the gearbox since we are only
looking at the tuning effect; this will enable you to accurately position the
loop to frequency. Bob J. van Donselaar, on9cvd@veron.nl |
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