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nmoushon

Direct Drive vs GEM

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One bored evening while the wife was watching her show I decided to do the "If I won the lottery what would I buy" window shopping on the web and came across that a lot of the higher end mounts are a Direct Drive mount and not just a giant GEM. I know how a GEM works, well the basic premise anyways, but have no idea what a Direct Drive mount is or how it works. Apparently if the higher end mounts are using it it must be better than the GEM setup and of course more expensive. Anyone know any links or mind explainging what it is? My curiousity is driving me crazy.

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The "direct drive" bit is unrelated to whether the mount is GEM or alt-az or fork or whatever, I think. It refers to how the motors drive the axes of the mount. In a HEQ5 or NEQ6 for instance, the axes are driven by the motors through a set of gears. In other mounts they are driven by belts. In a direct drive system it's exactly what it says -- the motor output shaft drives the axes directly.

James

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Well that make since. I guess the style of tracking doesnt really change.

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Indeed as James said. Direct drive is a type of transmission system and it's unrelated to the mount style. Although there are a number of GEM like direct drive mounts that uses the drive torque to counter the telescope weight and did away with the counterweight found on GEM.

Most telescope mount uses a set of spur gears to reduce motor speed and then a worm wheel for the final reduction. Worm wheel has a self locking output, so telescope weights and off balance torques don't get transmitted directly back to the motor. Thus power is only needed to move the mount. Error in the worm wheel produces periodic error.

Some newer designs such as iEQ45 and AZ-EQ6 replaced the reduction spur gear with a belt drive. However, the final reduction is still achieved using a worm wheel. Belt drives are said to be quieter than spur gear and do not require lubrication, thus reduces the amount of noise and maintenance.

Moving further upmarket, Avalon introduced two all belt drive mounts that did away with the worm wheel which removed the periodic error. However, it is said to have problem with belt flex which will introduced non-periodic error.

Friction drive is another novel drive system. It is used by the Mesu mount and one or two others. It completely remove toothed transmission and instead relied on friction between steel rollers. Since there are no teeth, I think it is more important to have a balance system to ensure no slipping. I don't know exactly how it works, but I believe the diameter of the roller needs to be machined to a very high precision to ensure the drive produces the correct reduction ratio. (unless they have a absolute encoder on the mount axis)

When the price get to a silly level, you starts to see harmonic drive and direct drive.

Harmonic drive runs a deformable metal belt with external teeth inside a rigid shell with internal teeth. A set of roller runs in the inside the belt, as the rollers rotate and presses the some belt teeth against the shell teeth, it rotates the shell. Since the shell has more teeth than the belt, it will rotate less. Harmonic drive system have no backlash or periodic error, and it can achieve a very high reduction ratio in a very small volume.

Direct drives as its name suggest connect the output (mount axis) directly to the motor. Direct drive motor has a large number of winding and poles. Mount positioning is control by firing various combination of magnets. This type of mount have no transmission error at all, but requires expensive encoders and an advanced motor control system. Since everything is done by electromagnets, direct drive system is very power intensive. I think a few direct drive mounts requires industrial three phase electricity, normal 240V main isn't enough.

Finally, the last type of drive is not a transmission system, but a sensor. Absolute position encoder is attached directly to the mount axis so it can read orientation of the axis directly. A feedback mechanism will tell the motors how much to move to reach the desired position. Unlike conventional servo motors used by many mount, absolute encoder is attached to the the mount axis rather than the motor, so it completely ignores transmission error between the motor and the mount axis. The biggest drawback of absolute encoder in astronomy is the need for the encoder to have sub arcsecond accuracy, which make them extremely expensive. Also some absolute encoder system needs to build a model of the mount transmission, so it's not very straight forward to used.

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Wow thanks for such an explanatory post. Really clears up whats what and why prices are so high.

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A great explaination from Keith, just one thing to add on the direct drive mount.

As I understand it, the ASA DDM mounts (as an example of the direct drive type) use optical encoders on each axis and they will increment the correct amount per second, so following a perfect track. Should the wind catch the scope, it will vary the power to the drive to compensate and so maintain the track irrespective of the conditions. Equal and opposite action.

So, to add to Keith's comments, yes, they will consume more power, but partly to compensate for drift.

The use of 3-phase motors is not because they need more power than the mains can supply, it's only to allow smooth movement, as one phase smooths out the next.

The big 0.7m Planewave AltAz mount uses the 3-phase motors, I think and also dynamically corrects for wind gusts. But at $200k ++, it's way out of my league.

Gordon.

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A great explaination from Keith, just one thing to add on the direct drive mount.

As I understand it, the ASA DDM mounts (as an example of the direct drive type) use optical encoders on each axis and they will increment the correct amount per second, so following a perfect track. Should the wind catch the scope, it will vary the power to the drive to compensate and so maintain the track irrespective of the conditions. Equal and opposite action.

So, to add to Keith's comments, yes, they will consume more power, but partly to compensate for drift.

The use of 3-phase motors is not because they need more power than the mains can supply, it's only to allow smooth movement, as one phase smooths out the next.

The big 0.7m Planewave AltAz mount uses the 3-phase motors, I think and also dynamically corrects for wind gusts. But at $200k ++, it's way out of my league.

Gordon.

Good point. It makes sense to use 3-phase to smooth the current input. It still needs 3 phase mains, which can be very expensive to install.

However, I guess if you can afford a $0.1M set up then the cost of getting a 3 phase supply and the electricity bill will be small change.

I wonder if there are any commercial mounts with wacky transmission that I haven't mentioned. I think I've seen some ATM projects of all belt drive mounts that uses smooth V belt rather than toothed belt. I don't think I have seen any hydraulics or chain drive mount yet. :D

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Explore Scientific sell an add on absolute encoder type device which they call the "Telescope Drive Master" for the right ascension axis of some popular mounts such as the EQ6. The graph on their website shows tracking errors in the sub second range for an EQ6, a result normally only available in mounts costing several times the price of an EQ6.

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Hydraulic mount? I can see that working actually, plenty of force, good precision, no gears to backlash.

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Keith, the 3-phase supply is derived using loads of electronics from a DC source. It does not run at 50hz, or high voltages, or anything like that.

Think of the phases working like a 3-way tug-of-war match, with the encoders directing the power to each set of drive coils to move about the axis. It is all about fine movement control.

Planewave did publish a paper on the development some time ago, though I have not seen it in ages. It talked about the development of the drive system and the 3-phase drive circuits. I am vague on detail, as it was read soon after they first developed it..

My favourite, only because it is so off-the-wall is the harmonic drive design from Chronus.

Not needing to balance or counterbalance huge payloads is a recipe for power consumption, though.

Gordon.

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