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Changing Torque Limits on Kinetix Servo Drives Using an SSV Instruction

Kinetix servo drives allow users to adjust torque limits through the Logix Designer programming environment. Different torque limits may be required during production. Rather than manually changing the drive configuration, the controller can write a new torque limit value to the axis when needed.

Kinetix servo drives allow users to adjust torque limits through the Logix Designer programming environment. One way to accomplish this is by using the SSV (Set System Value) instruction in Studio 5000.

The SSV instruction allows a controller to modify selected axis attributes at runtime, making it possible to adjust torque limits based on machine operating conditions, recipes, or other application requirements.

For example, an application may require different torque limits during normal production, setup, commissioning, or troubleshooting. Rather than manually changing the drive configuration, the controller can write a new torque limit value to the axis when needed.

This article explains the general SSV workflow, the torque-limit attributes to look for, and important considerations when implementing runtime torque-limit changes.

1. Understanding the torque-limit attributes

Studio 5000 provides three relevant torque-limit attributes for servo axes:

Attribute

Description

TorqueLimitPositive

Limits commanded positive torque.

TorqueLimitNegative

Limits commanded negative torque.

TorqueLimitBipolar

Applies a common limit in both directions.

The positive and negative attributes are runtime-readable and writable using GSV and SSV, respectively. Their values are expressed as a percentage of the motor's continuous rated torque.

For example, setting the positive limit to 50% and the negative limit to -50% establishes symmetric runtime limits of 50% in either direction, subject to the other configured drive limits.

Important distinction:

Runtime torque limits are different from the Peak Torque Limit configured in the axis properties. The configured peak limit still constrains the available torque; an SSV cannot increase the drive's configured maximum capability.

2. Configuring the SSV instruction

The SSV instruction writes a new value to an attribute of a specified object. For a Kinetix motion axis, the general configuration is:

Example: Set positive torque limit

Illustrative SSV configuration in Studio 5000

SSV field

Value

Class Name

AXIS

Instance Name

YourAxis

Attribute Name

TorqueLimitPositive

Source

PositiveTorqueLimit

To configure the instruction:

  1. Create a REAL tag to hold the desired torque limit.
  2. Add an SSV instruction to your ladder logic.
  3. Select the axis object and the appropriate torque-limit attribute.
  4. Assign the source tag containing the desired percentage.
  5. Trigger the SSV when the machine needs to change its torque limit.

To change the negative-direction limit, use another SSV with TorqueLimitNegative and a negative source value, such as -50.0.

 3. Verifying the torque limits

Before implementing runtime changes, use a GSV instruction to read the existing torque-limit attributes.

You can monitor the values online in controller tags and compare them with the requested values after each SSV executes.

Also monitor the axis torque-reference and torque-limit status information, where available, to help determine whether the drive is actually limiting the commanded torque.

4. Important considerations

  • Negative values matter. TorqueLimitNegative must be assigned a negative number. For example, use -50.0, not 50.0, for a 50% reverse-direction limit.
  • Stay within configured limits. Runtime limits cannot override the drive's hardware, motor, or configured peak torque restrictions.
  • Allow sufficient torque for stopping. An excessively low limit can prevent the axis from following its commanded motion or producing sufficient deceleration torque.
  • Test both directions. Verify positive and negative torque limiting under controlled conditions.
  • Do not use torque limiting as a safety function. These programmable limits are not a substitute for a validated safety-rated motion function or appropriate mechanical protection.

Rockwell describes torque limits as a means of restricting the maximum commanded current/torque relative to the motor's continuous rating. The appropriate limits depend on the machine's mechanical constraints and motion requirements.

The SSV instruction provides a way to dynamically change the positive and negative torque limits of a Kinetix servo axis from the Logix controller.

By writing to TorqueLimitPositive and TorqueLimitNegative, machine builders can implement different torque limits for different operating modes, such as setup, homing, and production.

The key is to use the correct axis attributes, apply the correct sign to each value, and verify the resulting limits under controlled operating conditions.

Helpful Links:

 AXIS Attributes (S-Z) 

 Logix5000 Controllers Motion Instructions