Index

 

synchr. motor

gearbox select

motor control

timing diagram

circuit description

motor voltage

loop tuning

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.

 

 

 

Beschrijving: Beschrijving: Beschrijving: Motor met vertraging

 

 

 

 

Close-up of the motor plus gear box. The double cylinder construction is clearly visible. The phasing capacitor is outside this picture and inside the control box.

 

The photograph has been taken at an actual magnetic loop capacitor controller. Note the isolating coupling (white) between the gearbox and the tuning capacitor at the right hand side.

 

 

 

 

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 Europe. When multiplying this value by 360 degree for one complete rotation, we find the number of degrees the motor will handle per minute. Since we are interested in the angle per step, (angle / step) we have to divide by the number of seconds per minute and the number of cycles per second (50 Hz.):

 

                        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.

 

 

 

Beschrijving: Beschrijving: Beschrijving: TUNINGBE

 

 

Tuning construction of the magnetic loop antenna.

The motor plus gear-box  is  visible at the left-hand site, the differential capacitor at the right.

Note the capacitor to 'tune' the range of the tuning capacitor to the frequency range (white stretch of coaxial line on top, and the double strands of branding to connect the tuning capacitor to the loop.

 

 

 

 

Beschrijving: Beschrijving: Beschrijving: Magloop 4 klein

 

 

 

 

The completed magnetic loop antenna for 14 - 30 MHz. with the tuning unit on top.

 

The loop is matched to 50 Ohm over the entire tuning range by means of a gamma match, the short parallel stretch of tubing at the bottom of the loop.

 

In front at the bottom the small control cabinet may be seen with the controls for forward-off-reverse in the middle and push buttons for 'step' and 'continuous' at each side.

 

 

 

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   

 

 

 

 

                                       Figure 1: Timing diagram

 

 

 

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.

 

 

Beschrijving: Beschrijving: Beschrijving: MOTORS

 

                                          Figure 3: Motor connections

 

 

Controlling the loop antenna

 

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