Installation of Telescope Drive Systems
Alan Buckman, AWR Technology

Open the box
Upon receipt of your drive system unpack it and check the contents. Before fitting to the telescope, connect the system together and power it up to familiarize yourself with its operation. A 12V DC regulated supply will be required unless otherwise stated. The features of the system will be detailed on the User Instruction Sheets. The Factory Test Sheet specifies all drive rates in terms of steps per second and can be related to the sky by noting the sidereal step rate which is equivalent to 15 arcsec per second. A sheet describing options may be included and the function of the fitted option will be detailed on this.

In use you will find the motors get warm, even if not rotating. For the motor to deliver its rated mechanical torque the temperature of the motor can rise 40 degrees centigrade above the ambient temperature. This is normal. It is permitted for the motor parts to reach 70 deg C in operation without causing a safety hazard. Motors driven with SYSTEM 5 drive electronics operate at reduced power when not rotating, so these motors will not get quite so warm.

The RA and DEC motor assemblies need to be fitted securely so when they rotate they are able to transfer the torque to the load rather than twisting the mounting bracket. The mechanical power must also be transferred to the slow motion shaft, usually by a coupling. AWR can supply an OLDHAM coupling which allows for slight shaft mis-alignment. Some telescopes suppliers fit the gearbox shaft directly into the slow motion shaft by means of a grub screw - this is fine if the shafts are not out of alignment, otherwise they will bind and could stall the motor. This can be checked by loosening off the motor mounting screws, running the drive and watching to see if the gearbox assembly wobbles over a complete rotation of its output shaft.

The RA and DEC rotation sense can be changed electrically by jumper links contained on the circuit boards. To test the sense as supplied, set the adjust speed to slew or coarse, then press the four direction buttons. Each fitted motor should move in both directions depending on the button pressed. If the directions are not to your liking then the jumpers need to be adjusted to reverse the sense. This will make sure that when a button is pressed on the handset the motor will turn in that direction. The location of the jumpers is indicated on the documentation supplied with the drive system.

A Synchronous motor has a small speed range, usually +/- 20% and cannot go backwards unless it is a reversible type. Reversible motors are identified with wires labelled PHASE1, PHASE2 and COMMON. (There may be two common wires). All three wires need to go to the drive box connector by suitably rated cable (3 amp mains cable). The reversible type require a phasing capacitor and this will be present within the drive box if the correct unit has been ordered. If it is possible to fit an earth wire to the body of the synchronous motor (when operating in an observatory with mains earth available) then do so.

Getting the best out of your system
When using an OLDHAM COUPLING to transfer the mechanical power from the gearbox shaft to the slow motion shaft, small periodic errors can result if the shafts are not in exact alignment. There can be a small wobble introduced by this component which is 1/4 of the period of rotation of the output shaft. All causes of periodic error can be measured if a star is allowed to trail in declination whilst taking a prime focus photo of approx 10 minutes in duration.

The system as designed should supply sufficient torque to drive the telescope at all drive rates. It is possible for motors (of whatever type) to stall if the mechanical load is too great. This can happen if additional accessories are added to the telescope at a later date, or the telescope becomes out of balance. Other little problems can take away useful torque, such as if the worm is meshed too tightly in the main gear reduction, or the battery supply voltage droops. Balancing is crucial and needs to be carried out with the telescope in many orientations to make sure this is corrent. It should be possible for the telescope to remain in any orientation with the clamps removed from both axes.

The torque requirement is a very easy thing to measure. The principle is as follows: wrap a string around the shaft you need to find the torque on. Attach the end of the string to a spring balance (the type Fishermen use). Then pull on the spring balance at right angles to the shaft until the shaft begins to turn. The torque can then be calculated by noting the reading and measuring the diameter of the shaft where the string is. If it were 2.3kg and the shaft is 8mm diameter the torque is
weight x gravity x radius
= 2.3 x 9.82 x 0.4 [in units of written Ncm]
torque = 9.0 Ncm
You would do this on the slow motion axis (without the motor fitted) to find the torque required from the motor. If you reverse the direction you wrap the string round you can check the torque requirement when the shaft rotation direction is reversed. If it is identical then the load on that shaft is balanced. It is also possible to do this measurement with weights hanging off the string but only if the shaft is horizontal.

Tidying Up
It makes sense to tidy away cables by tying to the mount (if possible) so that they do not cause a hazard in the dark. It is not unknown to trip over wires causing damage to yourself or the telescope. For a portable telescope it is feasible to mount the battery at the base of the tripod, but fixed to it with a specially built tray. It is then very easy to secure all the cables leaving only the handset on a long lead, to allow proper use at the telescope. A hook can be added to the handset to hook over something attached to the mount, so keeping the handset out of the way.

Power Requirements
AWR drive systems require 12 volts DC (regulated) at the required current rating for the system to work properly. The maximum current requirement is measured and noted on the Factory Test Sheet. It would be wise to provision a larger rated 12V DC power supply in case any other equipments are to be powered from the same source. It is not adequate to have raw DC unsmoothed and unregulated.

A rechargeable 12V battery such as a car or motor cycle battery can be used for operation at remote sites. These batteries can supply a very large current, in excess of 100 amps which is sufficient to fuse wiring in the event of a short circuit. Therefore it is wise to fit an in-line fuse to such a supply, with a fuse rating equal to the current rating of the wire. It is suggested that this rating be also double the actual requirement. Automotive fuses with blade fittings can be plugged into wiring fitted with the appropriate fittings. Sundry fittings and wire are available from AWR Technology.

Batteries for use at remote sites should be able to supply the maximum current requirement for the duration of the observing session. If this is to be three hours and the current requirement is 2 amps then a battery with at least 6 ampere-hours capacity is required, and it must start out fully charged.

Primary cells (non-rechargeable Alkaline cells or equivalent) do have odd characteristics - they can have no effective capacity when working below zero. Beware of this if you are using battery packs. One way of overcoming this is to bury the battery pack within your clothing on cold nights.

Mains adapters to provide 12V DC need to be regulated and used with caution. Damp conditions out of doors and mains 240 volts AC are a safety hazard and the correct safety precautions need to be taken. Read the paragraph on safety of mains operated equipment.

Safety of Mains Operated Equipment
When using mains operated equipment (240V ac) such as equipment used in an observatory it is advisable to make sure the observatory outlets are covered by a circuit breaker such as an RCD or RCCB. These act fast in disconnecting the mains supply in the event of flashover to earth, which can happen if plugs are damp. All metal on a mains operated appliance should be bonded to earth with a substantial wire (capable of carrying the fuse rating current). If in doubt consult a qualified electrician, or run a low voltage cable from the transformer in a safe environment.

AWR inverter drive units supplying 110V or 240V ac from 12V DC supply cannot deliver more than 5 or 10 watts depending on the type supplied and so a 3 amp rated mains wire is sufficient to act as an earth conductor. This can be fitted if there is an earth point to fix it to, otherwise this equipment is generally safe.

Trouble Shooting
If there is no light on the handset, check the power supply is good and the fuse is still good. Most systems supplied have an internal fuse fitted on the circuit board within the drive box. This should be checked. Fuses can sometimes blow with no apparent reason, if the current requirement is near to the rating of the fuse. Internal fuses are 20mm x 5 antisurge type T fuses. These can be obtained from AWR Technology.

On dual axis systems it is very often the case that the two motor assemblies are nearly identical. This allows a certain amount of trouble shooting before reaching for the phone. If one motor refuses to work the following procedure should be adopted. Remove the motor assembly from the telescope and retry the drive. If the motor now turns then the scope may require too much torque from the motor. If the motor does not turn then try swapping over the two connectors to the motors. Does the fault transfer to the other motor? If not then there is a problem with the motor. If it does transfer then there may be a problem in the electronics. Mechanical components such as connectors are the usual things to fail, especially if they are unplugged by pulling on the cable. A missing phase will cause a buzzing sound but no rotation of the motor.

The drive for these develop either 110V or 240V ac so beware. These should not be tampered with. Trouble shooting should be limited to checking the fuse. It is possible to measure the generated output voltage under no load conditions using a multi-meter (switch to 1000V AC VOLTAGE range before prodding the outlet terminals).

If the image wobbles in RA with a period near 1 second then there is a lack of torque to maintain the microstepping accuracy. This can be caused by an insufficiently rated power supply, or too much voltage drop along the cable to the drive box. You can check with a multi-meter the DC voltage present on the 12V inlet terminals to the drive box when the drive is operating. This should be close to 12 Volts. You can also check the AC voltage content by switching to AC Voltage range, this should be less than 100mV. If at the other end of the cable the DC voltage is a lot higher then a solution would be to fit a larger rated cable for the DC supply, perhaps using 10 Amp rated mains cable.

Guarantee and Service
All AWR products are guaranteed for 1 year from date of purchase against faulty workmanship and materials. We do take every care in the building of all units, determining exactly what is required at the ordering stage, with a thorough test and quality inspection before goods leave us. If there is a problem with the installation, operation or features then please give us a call. We would like to think that our products have been designed and built to give you years of trouble free service and that you will recommend us to others.

Our help line is 01304-365918. Please ring before returning goods.

	AWR Technology
	The Old Bakehouse
	Albert Road
	DEAL, Kent, UK
	CT14 9RD
Date of purchase . . . . . . . . . . . . . . .

Serial number . . . . . . . . . . . . . . . .

[Alan Buckman, 17th August 1998]