Gearing

Gearing is used when one axis (the slave axis) must move incrementally and proportionately to a register (the gear master), which is typically the position or velocity of another axis. The RMC has several commands to cover a wide range of simple and advanced gearing applications. This topic describes the basics of gearing and gives an overview of the gearing commands.

If you need to gear using a non-linear profile, see the Curves Overview topic.

In general, to gear an axis, issue a gearing command to the axis to be geared (the slave axis). The axis will remain geared until another command is issued to the axis. A register does not need to do anything to become the master of a gearing relationship. The slave axis will select its gear master.

The RMC provides both relative gearing, which defines only the rate at which the slave moves based on the rate of the master, and absolute gearing, which defines exactly the position of the slave based on the value of the master.

Gearing Commands

The RMC offers the following gearing commands:

Absolute Gearing Commands

Absolute gearing defines exactly the position of the slave based on the value of the master. The absolute gearing commands work very well for making an axis follow a reference input (half axis).

Relative Gearing Commands

Relative gearing defines the rate at which the slave moves based on the rate of the master.

Gearing with Motion Limits Commands

The Track commands provide gearing, with limits on the position, velocity, acceleration and jerk. These commands are useful for smoothly tracking a signal containing noise or step-jumps, or for gearing to another position while not exceeding specified motion limits.

Gear Modifying Commands

These commands modify an existing gearing relationship.

Gear Ratio

The gear ratio specifies the gearing of the axis to its master. To make gearing infinitely accurate in more applications, most of the RMC gear commands use a Numerator and Denominator to specify the ratio. For example, if a user has a rotary application and needs a gear ratio of 1:3, it cannot be represented with a single decimal number. However, the Numerator and Denominator will represent this accurately and the system can gear for any number of revolutions without losing the position.

The Numerator is the distance the slave axis travels as the master travels the distance specified by the Denominator. This relationship is shown below in equivalent equations:

Gear Ratio of Zero (0)

A gear ratio of zero will cause the slave axis to stop. This can be very useful if the axis is already geared. By specifying a gear ratio of zero and clutching by distance, the slave can be instructed to stop when the master reaches a certain position.

High Gear Ratios

High gear ratios can cause unstable control. A high gear ratio will cause the slave axis to move a large distance while the master moves a small distance. Any noise in the master will be amplified in the motion of the slave axis. In addition, the feedback increments will be amplified. Try to avoid high gear ratios.

If a high gear ratio cannot be avoided, try to use the lowest noise and highest resolution feedback possible on the master axis. This will minimize the amplification of the feedback increments and noise. If the master axis is a control axis, make sure the motion is smooth and minimize quick changes in velocity. If the master axis is a reference axis, consider adding filtering to it to reduce noise.

Gear Absolute

The Gear Absolute commands do not use a numerator and denominator. Instead, the gearing relationship is defined with Master Point A, Master Point B, Slave Point A, Slave Point B. The ratio can be calculated as follows. See the Gear Absolute (25) command for more details.

Gear Ratio = (Slave Point B - Slave Point A) / (Master Point B - Master Point A)

Clutching

Clutching is used for relative gearing commands. Clutching is the transition of the motion of the slave axis at the time the gearing command is issued to the final specified ratio. Most often, this cannot be done instantaneously.

Example 1

Consider a basic gearing application where Axis 1 (the slave axis) is to gear to Axis 0 (the gear master) with a 1:1 ratio. Before gearing, Axis 0 is moving at 10 in/sec and Axis 1 is stopped. If the gearing were to start instantaneously, then Axis 1 must go from 0 in/sec to 10 in/sec instantaneously, which is impossible.

The RMC supports several methods of clutching, making it very flexible for use in many gearing applications.

Clutching Methods

• By Rate

Clutching by rate specifies the rate at which the slave axis' velocity changes in order to reach the requested gear ratio. The rate is specified with an Acceleration and a Jerk parameter. In Example 1 above, if the clutching Acceleration is 20 and the Jerk is 1000, then Axis 1 will change from 0 in/sec to 10 in/sec at that rate. Once it reaches 10 in/sec, it will lock in at the 1:1 ratio.

For more details, see the Gear Pos (Clutch by Rate) (39) command, which starts gearing with clutching by rate.

• By Time

Clutching by time specifies the amount of time it should take to reach the requested gear ratio. In Example 1 above, if the clutching time is 0.5 seconds, then Axis 1 will go from 0 in/sec to 10 in/sec in 0.5 seconds. It will then be geared at 1:1.

For more details, see the Gear Pos (Clutch by Time) (30) command, which starts gearing with clutching by time.

• By Distance

Clutching by distance specifies the distance in which the requested gear ratio should be reached. This allows the user to specify the exact positions at which the axes should be locked in at the specified gear ratio. This is especially useful for flying-cutoff type applications.

Consider a gearing application where Axis 1 (the slave axis) is to gear to Axis 0 (the gear master) with a 1:1 ratio. Before gearing, Axis 0 is moving at 10 in/sec and Axis 1 is stopped. With clutching by distance, the user can specify that the axes should start clutching when the master reaches 5 in. and reach the final gear ratio when the master reaches 10 inches. The user also wants the slave to be at 2 inches at this point.

Therefore, when the master reaches 5 inches, the slave will start moving. When the master reaches 10 inches, the slave will be at 2 inches, and will be geared 1:1 with the master.

For more details, see the Gear Pos (Clutch by Distance) (32) command, which starts gearing with clutching by distance. This command is used in flying-cutoff type applications.

Transitions

Transitions are used for absolute gearing commands. When an absolute gearing command is issued to an axis, the axis must already be on the gearing relationship, or a Transition command must previously have been issued to the axis to define how the axis should move from it's current position onto the gearing relationship.

See the Transition Rate (56) command or more details.

Possible Gear Masters

The gear master can be any register in the RMC. A register does not need to do anything to be a gear master. Most registers in the RMC are not useful as a gear master. Some practical gear masters are described below:

• Target Position or Actual Position of a control axis:
  In this case, the axis is geared to the Target or Actual Position (or pressure or force) of another control axis. For example, one motor may need to gear to another motor, or one cylinder may have to move at the same rate as another cylinder. Typically, if you are gearing to a control axis, gearing to the Target Position provides smoother motion than gearing to an Actual Position because Actual Positions, as an actual feedback signal, tend to be less smooth due to transducer noise and quantization.

• Actual Position of a reference input (half axis):
  In this case, the axis is geared to the position (or pressure or force) of a reference input, such as from a joystick, an analog output from a PLC, or a belt position sensor. The Gear Absolute (25) and Gear Absolute (Prs/Frc) (59) commands work very well for this type of application.

• A Variable:
  Using the Expressions command in User Programs, you can create a Variable that varies in time. By gearing to the variable, you can move the axis in that profile.  The Gear Absolute (25) and Gear Absolute (Prs/Frc) (59) commands work very well for this type of application.

The RMC will process the slave after the master, which reduces latency in the slave axis—as long as there are no g"gear chains". For example, if axis A is geared to axis B, which is, in turn, geared to axis C, there is no guarantee that C will be processed before B which will be processed before A. However, if A and B are both geared to directly C, then C will be processed before both A and B.

Noisy Masters

Gear masters that are not very smooth, such as Actual Positions or reference inputs, may cause chatter in the slave axis. The velocity, acceleration and jerk of the slave are calculated from the master. Any noise on the master will be exaggerated in these calculations, especially the acceleration and jerk. To reduce chatter, you may need to set the Acceleration Feed Forward and Jerk Feed Forward to 0 on the slave axis.

If the gear master is a reference input, it can be filtered to make it smoother.

Using a Virtual Axis as a Gearing Master

A virtual axis can be used as a gearing master axis. It is sometimes desirable to gear to a virtual axis rather than executing the motion as a function of time. All the axes geared to the virtual axis can be sped up or slowed down by speeding up or slowing down the virtual axis. The virtual axis can even be moved backwards causing the geared axes to back up too. This cannot be done using time-based commands.

When using a virtual axis as a master as described above, it is often useful to set it up as a rotary axis because it will never need to be reset. When used as a master, the virtual axis is typically commanded to move with a Move Velocity (37) command. Moving it at 1 unit/sec as the standard velocity makes gearing ratio calculations very easy. The acceleration and deceleration provide a smooth start and stop for the geared axis.

Halt Groups

For safety, both the master and slave axis should be included in the same Halt Group. If one axis in a Halt Group halts due to an error, all the other axes in that halt group will also halt.

Target Generator Components

The behavior of the components of the gearing target, including the Target Velocity, Target Acceleration, and Target Jerk, depend on the type of register used as the master:

• Target Position
  When gearing to a Target Position, the slave Target Velocity will be the ratioed master Target Velocity, the slave Target Acceleration will be the ratioed master Target Acceleration, and the slave Target Jerk will be the ratioed master Target Jerk.

• Actual Position
  When gearing to an Actual Position, the slave Target Velocity will be the ratioed master Actual Velocity (filtering applies), the slave Target acceleration will be the ratioed master Actual Acceleration (filtering applies), and the slave Target Jerk will be zero (0).

• Actual Pressure/Force
  When gearing to the Actual Pressure/Force, the slave Target Velocity will be the ratioed master Actual Pressure/Force Rate, the slave Target Acceleration will be the ratioed simple derivative of the velocity, and the slave Target Jerk will be zero (0).

• Other Registers
  When gearing to any other register, the slave Target Velocity will be the ratioed simple derivative, the slave Target Acceleration will be the ratioed simple derivative of the velocity, and the slave Target Jerk will be zero (0).

See Also

Synchronizing Axes

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