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Advice on Digital Servos for drive by wire throttle

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Advice on Digital Servos for drive by wire throttle

Old 01-30-2004, 11:32 AM
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niallcaldwell
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Default Advice on Digital Servos for drive by wire throttle

Hi there

I am trying to retrofit a full-size car with a drive-by-wire throttle system for a research project (Suzuki Grand Vitara 2.0 petrol)

I am considering the option of using a hobby servo and wondered if anyone out there can give me some advice.

I have looked at fitting a production electronic throttle as made by SiemensVDO and Bosch for VW cars. But these rely on the ECU to close the throttle position loop which complicates my system.

The alternative is to drive the existing throttle butterfly with a digital hobby servo which would be easier from a control point of view because the position loop is taken care of. I think I need about 1Nm = 130 oz-in (if I remove the return spring) and I want to achieve <100ms response.

some questions I would really appreciate some answers to:

1. what accuracy could I expect in terms of bits of resolution? I guess I need 9 or 10 bits.
2. do digital servos have an integral term in their feedback loop to take care of steady-state error? if not what is the typical proportional gain in terms of degrees of error or pulse width for full torque?
3. my local hobby shop has a Futaba 9451 in stock. Is this a good example of state-of-the-art in terms of deadband, accuracy and speed? (I realise some other makers offer PC tuning)
4. my plan would be to replace the servo internal feedback pot signal with the the throttle position sensor pot signal. Is this a common and sensible mod?

Thanks a lot in advance for any help!

Niall Caldwell
Old 01-30-2004, 11:51 AM
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niallcaldwell
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Default RE: Advice on Digital Servos for drive by wire throttle

One more question- how do you think a servo would stand up to the temperature under a car bonnet? I think I can expect 80-100 degrees centigrade. I would select a metal geared, coreless design. Are the case materials up to it ie. glass filled nylon rather than cheap ABS? Could I bond an aluminium heatsink to it?

thanks!
Old 01-30-2004, 02:47 PM
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3D Joy
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Default RE: Advice on Digital Servos for drive by wire throttle

I know some Futaba servos use the bottom part of the case as an aluminum heat sink. I am pretty sure though that these servos are of resin gears instead of metal gears.

Just wondering. You maybe could take advantage of Hitec digitals as they are programmable. You can set end points and ceter point directly in the servo, making mechanical hook up real easy. You could change dead band and servo speed, if you want it slower. You may want a 5945 for torque or a 5925 for blistering speed. They both are coreless servos and both use metal gears. You can swap gears to make the high torque or high speed as you want.

These servos need only a fraction of degree between desired position (signal) and actual position to reach full operating torque.

I don't know about what is a "bit" of resolution but these servos are capable of something like 2000 or 3000 steps throught their entire travel range. Sure there is some slop in the gear train but these servos are about what is best for overall value. Hitec also sell the programmer for these servos.

I hope I help some...
Old 01-30-2004, 02:55 PM
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Phil Cole
 
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Default RE: Advice on Digital Servos for drive by wire throttle

I'd be hesitant to use hobby parts in an automotive application.

As far I as can tell, RC parts are designed to consumer grade requirements with maybe some attention paid to vibration and water/dust proofing, but not much. 100 C is going to have a noticeable effect on failure rate.

In the hobby application, the servos are typically in use for only 10 to 20 minutes at a time. People who fly a lot (say four flights every day for competition practice) will replace servos each season as a PM item.

My personal experience with about 60 servos over the last five years has resulted in four servo failures, and a couple that occasionally misbehaved in a way that discouraged me from using them in mission critical applications. Most of the failures were premium servos in demanding applications (mostly helicopter tail rotors with fast gyros).

As for your specific questions, I'll answer what I know.

The absolute best resolution you could expect would be 10 bits. Servos typically have a deadband specified, which is the amount of input pulse width error required for the servo to take corrective action (i.e. move). A very low deadband might be 1 us. The input pulse width range is 1 ms, giving around 10 bit resolution. Deadbands higher than this are much more common. I would expect something less than 10 us in most cases.

There is no error integration. I've never measured digital servos, but the normal analogue servos will typically need about 5 - 10° deflection to develop rated torque. There is no standard for the amount of position error required for torque measurement. Attempts have been made to find this out from the manufacturers, but a definitive answer has not been forthcoming.

The 9451 is representative of the state of the art.

Some servos come with Al heatsink cases.
Old 01-30-2004, 06:26 PM
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niallcaldwell
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Default RE: Advice on Digital Servos for drive by wire throttle

Thanks 3D and Phil for the replies so far.

I can now see the advantages of the programmable servos like Hitec, not just for tuning the position loop but also for range and offset adjustment.

So it seems that they have just about enough resolution, but that the position loop may not be stiff enough to achieve the full accuracy under load. The servo will be slaved to a microcontroller which can sample the throttle position sensor, so it's possible that I could do the integration as a seperate outer loop and modify the servo command signal to achieve zero steady state error. This would not be difficult as sample speed for integration is much slower than for proportional.
But hobby servos might be lacking in terms of reliability, especially in the harsh environment of the underbonnet.

I suppose I could drive the throttle via a cable from the much better environment of the cabin. But a spring would be needed to keep the cable taught, which would increase current consumption.

hmmmm...
Old 02-18-2004, 01:03 AM
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PaShe
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Default RE: Advice on Digital Servos for drive by wire throttle

Look at Alu-Star digital/UV-MKI on www.volz-servos.com
IUV-MKI even salt water resistant, but operating temp -30..+70C
Old 02-18-2004, 05:03 AM
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ZAGNUT
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Default RE: Advice on Digital Servos for drive by wire throttle

volz also makes industrial grade servos that are designed for a very long service life. some of these even have hall effect pots.


dave

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