actuation motor for seat design

[YateHaugan1]

hi folks,

I've been trying to spec a motor for a seat project I'm working on.

It needs to move a 100kg person up and down at 45 degrees by about 400mm. The actuation would be via a toothed rack cut into one of the motion guides, so I've estimated i'd need a pinion with about 300 Nm torque. The motor itself needs to be quite slim, and as light as possible.

So far, the best option I've found is a 480 watt 4-pole item from Maxon:

http://www.maxonmotor.com/maxon/view/product/motor/ecmotor/ec4pole/397800

This motor runs nominal load at 4350 rpm, so I'd need two stages of worm gear reduction (built into design of motor unit). However this wouldn't give me enough torque - not including losses, at a 263:1 reduction it would give 254 Nm, and with two worm gears, I'm looking at 30% or more in mechanical losses, is that correct?

So my question is, could I run more current through it to get more torque, and also, is there an alternative motor out there that anyone can think of? The Maxon motor is very expensive (around $500 I think) but it's very light (800g) and has a very small diameter, which is ideal. I get the impression that its more designed for automation though, and I don't really need that level of precision.

My alternatives at the moment seem to be adding some kind of spring balance to the mechanism, which would add a load of complexity, or more heavy-duty motors, which are more like 2500g, which is way too heavy.

thanks in advance - as you can probably tell, electronics are not my forte.

 

[imcjoek]

Raising 100kg up a 45deg incline = ~693N load. (Ignoring losses).

300Nm on a pinion driving a rack where the pinion is, lets say ~25cm diameter = 2400N force.

Either your pinion is rather bigger than I imagine or one of us did some math wrong someplace.

 

[imcjoek]

FWIW I was thinking 25mm diameter as a more realistic size... So 24,000N.

 

[itsmoked]

I see no mention of how fast. 'Fast' is about the power needed. If you can take all day you should be able to do it with fleas on a treadmill.

Also what about a cast metal counter-weight that conforms to the rack? Might be simpler than springs and generally works great if what-must-go-up always has to come down and vis-verse.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If this is a one time thing, don't re-engineer the wheel. Go to your local auto wrecker and buy a seat.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[YateHaugan1]

thanks imcjoek - if someone's got some maths wrong, it's almost certainly me.

The pinion will only be small - probably more like 3cm diameter, rather than 25, with a small gear pitch.

How does torque on a rack and pinion setup translate to vertical force? because 24,000Nm of torque sounds like rather a lot.

itsmoked - I need it to perform the movement in 30 seconds or less.

I can't use a counterweight, because it's an aviation seat, so weight is critical - hence the rumination around spring balances etc.

 

[imcjoek]

How does torque on a rack and pinion setup translate to vertical force? because 24,000Nm of torque sounds like rather a lot.

300Nm with 25mm pinion = ~24,000N Force. (Use pinion pitch-radius as the moment arm).

Where'd you come up with this 300Nm estimate, anyways?

 

[YateHaugan1]

Where'd you come up with this 300Nm estimate, anyways?

To be honest, I can't remember exactly.

Though using this (ignoring friction coefficients) I get just under 300Nm.

https://servotak.eu/tools/engineering_calculator

(radial force on pinion)

If I include a reasonable friction coefficient of 0.8, I get a radial force on pinion of 500N.

System inclination is 48 degrees and speed required is 0.013 m/s

 

[BrianE22]

The motor you referred to is a brushless DC. Brushless motors are small but the controls are more expensive than for a brushed, universal motor. You might want to price out the controls.

 

[YateHaugan1]

Thanks BrianE22 - could I get the same watts per kg with a brushed motor? I also like the low electrical noise and low maintenance aspect of brushless, since this is for an aviation seat.

A controller solution can be had for less than 10% of the cost of the motor, so it's not a massive diffidence, especially given the benefits.

 

[BrianE22]

Someone else will have to chime in the power density comparison. Years ago universal motors had the highest density but maybe now the brushless motors have caught up or surpassed them.

I'm curious, what controller did you find? The one thing that has kept me away from the brushless motors in our product is the size and cost of the controller.

 

[Compositepro]

A screw actuator would be soooo much more appropriate than a rack and pinion.

 

[YateHaugan1]

I'm curious, what controller did you find? The one thing that has kept me away from the brushless motors in our product is the size and cost of the controller.

I was just going to use an Arduino for the prototypes, and a basic rack mount supply

but one of these will run three at once

http://uk.rs-online.com/web/p/products/4184045/?grossPrice=Y

 

[YateHaugan1]

A screw actuator would be soooo much more appropriate than a rack and pinion.

I'd toyed with the idea early on. But there's not really enough space to get a motor and drive to a jackscrew, certainly not without adding a lot of weight and complexity. I'd put a motor by the passenger's feet, which connected to a jackscrew via a driveshaft, but it needed universal joints and spur gears to transmit the power to the right place.

The pinion and rack works because the toothed rack can be cut into one of the guiderails for the movement, plus having the motor as part of the moving structure means the motor housing can be used to add strength to the seat base for the 16g crash test.

 

[YateHaugan1]

BTW - thank you all for your replies so far.

 

[imcjoek]

Though using this (ignoring friction coefficients) I get just under 300Nm.

https://servotak.eu/tools/engineering_calculator

(radial force on pinion)

Look at their diagram again.

Radial force is great if you want to separate the pinion from the rack, but it's not what you want to push your seat...

 

[YateHaugan1]

Radial force is great if you want to separate the pinion from the rack, but it's not what you want to push your seat...

I'm confused now (easily done)

I thought radial force is applied in the direction of rotation, and axial force would separate the pinion from the rack? But there's little axial force if I use straight-cut teeth.

 

[imcjoek]

There's a picture with the forces labeled in the calculator. (You shouldn't need an online calculator for this Napkin Grade calculation, honestly).

Radial = force trying to separate the pinion & rack. (Primarily because of the pressure angle of the teeth)
Axial = force along the axis (shaft)
Tangential = the bit that moves the rack. The one you want.

 

[itsmoked]

Hi YateHaugan1;

You are indeed a little confused if you think that the controller you linked to will control three motors:

but one of these will run three at once

A "brushless DC motor" is essentially a three phase AC motor and that controller has an output for each one of those 3 phases to one motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[YateHaugan1]

(You shouldn't need an online calculator for this Napkin Grade calculation, honestly)

Probably back of a napkin stuff for you, but it's been a while since I've had to work out anything like this, and rather a lot of other crap has filled my brain since. Plus I have a screaming baby in my ear today.

However, just to check I'm not going crazy here: I need to move 1000N, 48 degrees upwards at a speed of 0.02 m/s with a 24mm spur gear.

Depending on the coefficient of friction (I'm not sure what lubrication I'll be using yet) I'm getting a tangental force on pinion of around 1400N.

I must have been using the wrong forumla before, because it seems I only need a torque at pinion of about 16Nm (rather than the 290-odd I previously had.