9 replies to this topic

[Kicking Horse]

Alright guys,

Can someone explain what the Cable Tach does?

[Emax]

A "cable tach" creates a pulse train that varies its output frequency in direct proportion to the rope speed. It will will produce these pulses all the way down to zero speed. The output signal from this "tach" (it's actually known as an encoder) is used for many purposes - from displaying rope speed to actually measuring the passage of rope. Lift safety subsystems have need of such information to determine overspeed, accel and decel rates, brake set speed and off-ramp performance by the drive. Normally, safety functions require their own rope tach. On detachable machines, rope tach measurements are needed for carrier spacing and terminal anti-collision schemes.

This post has been edited by Emax: 03 December 2008 - 01:47 PM

[Kicking Horse]

Thanks Emax.

[2milehi]

Pomas use a tachogenerator driven by a terminal sheave to determine rope speed. It is a generator that produces a voltage proportional to the speed of the haul rope. Older Doppelmayrs use a tachogenerator that is attached to the drive motor for line speed. And as Emax stated, certain lift safeties examine this value.

[Emax]

2milehi, on Dec 5 2008, 01:57 PM, said:

Pomas use a tachogenerator driven by a terminal sheave to determine rope speed. It is a generator that produces a voltage proportional to the speed of the haul rope. Older Doppelmayrs use a tachogenerator that is attached to the drive motor for line speed. And as Emax stated, certain lift safeties examine this value.

Are they also attempting to determine rope passage (in feet, meters, cubits) using this analog signal?

I don't understand why anyone uses a DC tach generator anymore. Perhaps it's an AC unit?

[Aussierob]

Virtually all our lifts have DC tachs for drive speed feedback and/or line speed. Its just the way they come from the manufacturer. The newer lifts are going to encoders and we have retro fitted some. The Peak to Peak has DC tachs on both drive and return bullwheels, so they are not obsolete yet.

[mthornton]

I have to admit that after installing a few encoders lately, I now love them. Tacho-generators, with their maintenance & wear issues, will become obsolete none too soon. Mounting an encoder is a snap, and they are easy to wire into the modern ABB drives. Picking off a speed signal for other purposes can be a challenge, (like for connecting to a data-logger for a load-test), but there are other ways to get a speed signal.
Does anyone know any practical disadvantages of encoders (for new installations)?
MitchT, Pano

[2milehi]

Emax, on Dec 5 2008, 02:07 PM, said:

Are they also attempting to determine rope passage (in feet, meters, cubits) using this analog signal?

I don't understand why anyone uses a DC tach generator anymore. Perhaps it's an AC unit?

It is a DC tach - it goes into an analog input port on the PLC and yes the signal is used for rope speed, overspeed, and zero speed. The AB prox pulses that are used for anti-collision do not come in at a fast enough rate. There are 16.67 pulses per second from each AB prox (at 5 m/s) so there can be 16 to 18 pulses per second, depending on the phasing of the target. So right off the bat there is 6% error.

Here is the problem with trying to determine zero speed with AB proxs. Say we want to set the service brake at 0.3 m/s when the lift commanded to stop. At 0.3 m/s, there is 1 pulse per second, so it takes about a second to determine that speed. By then the lift has already come to a stop and may roll back before the service brake even sets.

One could use quadrature with the AB prox signal to increase the "events" to ~67 per second, but now the PLC's scan time is not fast enough to capture quadrature. So the next fix is to add a high-speed counter module and a capable PLC, but even 67 events per second yield 1.5% error, so another prox and "higher-count" target is added to give a better value.

Now I am not knocking encoders, I have done some upgrades with replacing a tachogenerator with an encoder on a DC motor. No maintenance, ability to run the lift at 0.1 m/s, and no concern of noise on the speed signal going to the drive is a great benefit. But the ABB drive was constructed with a high speed counter on board.

This post has been edited by 2milehi: 06 December 2008 - 06:33 AM

[2milehi]

mthornton, on Dec 6 2008, 12:05 AM, said:

I have to admit that after installing a few encoders lately, I now love them. Tacho-generators, with their maintenance & wear issues, will become obsolete none too soon. Mounting an encoder is a snap, and they are easy to wire into the modern ABB drives. Picking off a speed signal for other purposes can be a challenge, (like for connecting to a data-logger for a load-test), but there are other ways to get a speed signal.
Does anyone know any practical disadvantages of encoders (for new installations)?
MitchT, Pano

The ABB drive has an analog output port that can generate a voltage proportional to speed and be scaled to 0 - 10 volts. Fine when running on electric, not so good when running on diesel.

As for disadvantages, use a bearingless encoder and make sure the encoder won't be tweaked by the magnetic fields of the DC motor.

This post has been edited by 2milehi: 06 December 2008 - 06:41 AM

[Kicking Horse]

Guys thanks for the info on this. :)