Converting Servo to Stepper

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Apr 2002
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Just a bit northeast of nowhere
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Greetings all,

I'm working on a project where, for cost reasons, we've decided to use a stepper instead of a servo on the second system. The positioning is not critical, I'm moving to limit switches.

I'm checking my math.

On the servo I have:
1.5 A phase current while moving under full load
0.9 amps phase current while holding
Torque constant is 0.19 Newton-Meters

0.9 A* 0.19 Kt = 0.171 nM holding torque, or 24.2 ounce-inches.

1.5 A * 0.19 Kt = 0.285 nM load torque, or 40.4 ounce-inches.

(Thanks to www.unitconversion.org for their useful site)

Seems like the plucky STP-MTR-17048 from Automation Direct (83 oz-in, 2.0 A phase current) should have plenty of chutzpah to do the job.

Am I missing anything here?

Thanks,

TM
 
Am I missing anything here?


Maybe the big picture. o_O That would be the original data and calculations of load (mass) & acceleration & speed. I would be hard pressed to use Phase current to determine how much torque you'll need. How was this current measured ?

Remember that a stepper may not have the accel and speed capabilities of a servo.
 
The key points of Stepper vs. Servo:-

1. In most cases steppers are an open loop devices vs. Servos with an encoder. You never know the position of the motor!
If you miss steps due to the load exceeding the torque output of the stepper you won't know it has happened. You must carefully size the motor for load and and desired motion profiles [Speed, accel. and decel.]
Furthermore I would highly recommend a method to establish a starting position [limit switch, prox. switch]. This should preferably be reconfirmed during each machine cycle as well as a recovery scheme [re-home to start switch] to correct for any lost steps.

There are some stepper drive/motor combinations that do have feedback. [Danaher P7000 series and Anaheim Automation are two that come to mind].

2. Torque/Speed curves. Pay very close attention to these for your application. In general steppers are fantastic for low speed since they have high starting torque. But the torque rapidly drops off depending on the motor winding and applied voltage. Motor impedance is the culprit, to get high torque at higher rpm the only way is to increase the voltage in order to maintain current in the windings.

3. Low vs. high voltage drive/motor combination. Depending on your application you can select low voltage [typically up to 72V] or high voltage [up to 300V]. Low voltage requires a power supply and a drive. With the high voltage drives [Input of 110VAC or 220VAC] the power supply is built into the drive. This reduces panel space and wiring.

Applied correctly, stepper motors are a lower cost, simpler method of achieving motion in NEMA 34 frame size and below.
Applied incorrectly, particularly when it comes to matching the torque/speed requirements of the application they can be a real PITA.

My 10 cents.
 
Other things to consider with a stepper:

-Steppers have the quality of decreasing torque with increasing speed. This can be partially mitigated with the use of a higher voltage supply. In general, go with the highest voltage your drive and motor can safely handle.

-I'd recommend talking with the folks at Lin Engineering:

www.linengineering.com

They can select a particular winding arrangement and motor size based on the voltage available, RPM, and torque you need to achieve. They also sell NEMA 34 gearboxes if you need them.

-Steppers are typically oversized so there's less chance that they will loose pulses due to running right at max torque.

-When programming a stepper system, make sure to program a accel ramp into any move. Trying to get a stepper to accelerate from zero to any appreciable speed instantly will probably cause the stepper to stall and not move (this is very pronounced with any load on the shaft).

-Steppers as a brake when their windings are energized but no pulses are being sent. This can be very handy; most drives have an enable/disable input to allow the motor to freewheel.

-If your motor is to be used as a brake for any appreciable amount of time (longer than a few min at a time) set the drive to reduce current on standby. If you don't, the motor can become a rather good heater.

-Many drives, including the one I list below, can do micro-stepping. Use it, as it makes the motor's motion much smoother. I usually use 10x micro-stepping. That is to say, if I want to move the motor 1,000 steps at 100 pulses per second, I need to send 10,000 pulses to the drive at 1,000 PPS.

-I have used the following setup for steppers:
AutomationDirect DL-05; Y0 and Y1 outputs configured for pulse and direction respectively.

R710 stepper drive from Lin Engineering

Traco 72v, 300w Power supply here. Depending on your motor's current requirements, you may need the 600w version.

I contacted Lin for my particular motor needs.

-The R710 drives need some interface electronics to talk to most PLCs. I've designed a small interface card that makes this really easy; PM me if you're interested.


I hope this gets you started, and let us know how it turns out.


-rpoet
 
Hi guys!

Thanks for all the useful information, it's really going to help alot.

We have an existing servo-based system in place. The phase currents are actual measurements taken from the drive.

The movement in this case is very slow, and positioning is not relevant. The motor will move to one of three limit switches (lower travel, upper travel, top of stack). Even the stepper would be overkill, except I need holding torque to keep it at the "top of stack" position once found.

Thanks for the advice on standby mode, I'll definitely put that into work.

Thanks again!

TM
 
I need holding torque to keep it at the "top of stack" position once found.
With a servo gets a disturbance when holding position, it will bounce a little and then return. With a stepper, if you get a sharp load increase, it may loose rotor sync and your load will fall.

Better get a brake on it.

ps, always go with a micro-stepping drive.
 

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