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AWR Intelligent Handset - Microstep Drive Box


DESCRIPTION

The drive box operates a standard hybrid stepper motor (200 steps per rev) in microstep mode generating 12800 microsteps per revolution. The step increment in angular resolution has been measured and found to be very consistent. This allows connection of the motor directly to a slow motion shaft and will drive worm wheel ratios as small as 144:1 with a microstep rate of 21.438 microsteps per second. With worm wheel ratios bigger than 144:1 there are more microsteps per second to give the same Sidereal rate. The benefit of a direct motor mechanical connection is a removal of a source of backlash, namely the gearbox. All drive rates are programmable with onboard non-volatile memory and settable to 0.001 arcsec per sec. The serial connection can be connected to a PC for programming (usually only done in the factory) or to the intelligent handset for normal operation mode.

The microprocessor circuit generates two independant frequencies for RA and DEC and both can be operating in any mode. Acceleration is achieved to a maximum speed of 3 revolutions per second (38400 microsteps per second). The motor is also decelerated to maintain a uniform torque loading across the full speed range. The high speed implies a telescope movement of several degrees per second across the sky allowing computer control with a GOTO facility a real feature.

Position information is available by a virtual encoder (as driven but not measured) providing a clock pulse every 64 microsteps with a direction line. This information appears as TTL signals (4 wires) on the IH connector port. The actual position is then counted in the IH from a reference (calibrated) position.

A power supply of 12V DC 2 Amp to 8 Amp rating is required (depending on motors). In normal use only the RA motor is operating and DEC is static (but a bias current is flowing to maintain the microstep positional accuracy).

The guiding accuracy is controlled by quartz crystal and is better than 0.1%. At high speed the maximum phase jitter between pulses is designed to be less than 0.5%. All steps will be counted and transformed into coordinate positions when used with the Intelligent Handset.

The box containing the electronics is 6cm x 10cm x 20cm and is designed to sit on the floor close to the telescope. A 3m lead connects the dumb handset. The motor leads are 0.5m for RA and 1m for DEC. There are inline connectors in each motor lead. Power inlet connector (screw down terminals) for connection of 12V DC is present on this box. It is reasonably weather proof.

The angular resolutions achieved are as follows:

Telescope Reduction Arcsec per ustep Encoder Resolution KING rate
144:1 0.70 45 arcsec 21.438 ustep/s
360:1 0.28 18 arcsec 53.596 ustep/s
720:1 0.14 9 arcsec 107.8 ustep/s

The torque available depends on the motor used. The loading indicated above is from a 50Ncm motor and this will maintain positional accuracy when loaded up to 10Ncm. This is adequate for small balanced telescopes (up to 20cm aperture but depends on telescope type). DRAWING of MOTOR. We can supply with motors 32Ncm to 210Ncm with 1 or 2 amp per phase required.

The actual motor has been characterised with a torque loading of 9.5Ncm. A mirror was attached to the motor shaft in addition to the torque load. A laser beam directed at the mirror produced a spot measured at a distance of 8.8 metres. The loading effect is at follows: When at the pole (full step position) there is a static shift of +10 microsteps. During a 64 microstep sequence (one pole to the next one) the torque available varies and superimposes a wobble of +/-7 microsteps. There is an additional variation from pole to pole of +/-3 microsteps. This all adds up to a backlash due to the torque loading and a ripple (periodic error). With our system driving a 360 tooth worm these errors correspond to a positional variation no more than 6 arcsec including a ripple of +/-1 arcsec.

The protocol between the DriveBox and the Intelligent Handset (or PC) easy to use (see PROTOCOL document). The drive rate parameters that can be programmed are SIDEREAL RATE, GUIDE (30%), CENTRE (2x), MOVE (32x), SLEW (1000x) MAX ACCELERATION RATE in Hz per second per second and BACKLASH. This programming can be done by a serial lead (RS232) from the Microstep drive box to a PC. You do not need the Intelligent Handset as the communication interface, but if you require the Planetarium link and the GOTO facility you will need the Intelligent Handset.

The time to slew speed is the SLEW value (Hz) divided by the MAX ACCEL value and should be normally within the range 1 to 10 seconds. The acceleration algorithm achieves a linear increase in frequency with time (accelerating and decelerating) and so the distance moved and time for the move is calculable.

ustep system drawing
STANDARD and ADVANCED MICROSTEP BOX drawing


MICROSTEP WITH MUCH HIGHER TORQUES

AWR have solutions for telescopes requiring higher torques than that available from the 50Ncm motors.

The 120Ncm motors (MOTOR/120) can supply about 80Ncm and stay within the torque loading and ripple limits as detailed above. These motors require a power supply of 6.8 Amps at 12V DC. The drive box generates extra heat and is 6cm x 10cm x 28cm.

There is also a new motor MOTOR/220 which has 220Ncm of torque, usable on loads up to about 140Ncm of torque. It is half the size you would expect due to the use of rare earth magnetic materials. See details of some large systems supplied.

We have also fitted a gearbox (MOTOR/G) to boost the torque available by 5 times (or more) to drive in excess of 200Ncm using microsteps. This introduces extra backlash but will be removed by the Intelligent Handset. DRAWING of MOTOR ASSEMBLY


ADVANCED MICROSTEP DRIVE BOX

For the advanced telescope facility, AWR offer expanded input output facilities in addition to the standard MICROSTEP drive box. These include:

The diagram above shows the functionallity of the drive box in diagrammatical form.


SIMPLE HANDSET

A simple handset provides four buttons for the four directions and a three position switch to select the adjustment speeds. These are GUIDE (30% sidereal) 2x (switch in centre) and SLEW by accelerating to between 2 and 5 degrees per second as detailed above. It is supplied with a 3 metre lead and connector.


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