Gearing Axes

Gearing is used when one axis (the geared axis) must move incrementally and proportionately to another axis (the gear master). This topic describes a method of gearing one axis to another by using a gear ratio. There are three other types of gearing available, described in the following topics:

• Sine Move Command

• Follow Spline Segment Command

• Follow Axis with Offset - see the SoftPID Position Move Command

The gear master axis can either be controlled by the RMC (internal) or just a position input from an encoder or transducer that is not under the control of the RMC (external, also called an half axis). A numerator and denominator specified in the geared axis's command area determines the ratio of the movement between the gear master and the geared axis.

The only limitation for gearing axes together is that axes must not be geared in a loop (that is, if axis 0 is geared to axis 1, axis 1 cannot also be geared to axis 0).

The Gear Master Axis:

An axis does not need to do anything to become the master of a gearing relationship. That is, the axis may be controlled in open or closed loop, or it may even be only an input (in the case of a velocity or position reference such as a joystick). The geared axis will select its gear master axis.

Setting up a Geared (Slave) Axis:

The slave axis starts gearing when issued a Go command with the command parameters set up as follows. It continues gearing until another motion command is issued to the same axis.

1. Specify the Gear Master and Type.

In the Mode command parameter, do the following:

• Set the Gear Bit (bit 12, 0x2000 hex). If this bit is not set, then the Go command will not initiate a geared move. It will instead do a point-to-point or speed control move.

• Use the Gear Master Select bits (bits 4-6) to select the desired master axis. Multiple slaves can be geared to the same master.

• Set the Gear Type Bit (bit 14, 0x4000 hex) to be geared to the master's Actual Position. Clear this bit to gear to the master's Target Position.

2. Specify the Gear Ratio.

The slave-to-master gear ratio is specified by the Command Value command parameter divided by the Speed command parameter, as shown in the below three equivalent equations:

* The master speed and position used may be based on either the Actual or Target Position, as selected by the Gear Type bit in the Mode word.

Note: When the gear ratio is clutched between two ratios--such as from non-geared to 2:5 gearing--the gear ratio will be ramped at a user-specified rate (see below). This clutching will take place by ramping the gear ratio numerator from its original value to the final ratio numerator. However, the interim numerator values must always be integers. Therefore, it is recommended that relatively large values be used for the gear ratio components. For example, instead of specifying values of 2 and 5 for a 2:5 ratio, you might specify 2000 and 5000. This allows clutching through up to 2000 gear ratios instead of only two, for a much smoother clutch.

Example:

To obtain a gear ratio of 1:2, such that the slave axis moves at half the speed of the master axis, you will enter a Command Value that is one half the value of the Speed. A Command Value of 1000, and a Speed of 2000 would work. As described in the above note, values of 1 and 2 would also work, but there would be no clutching: the axis would go directly from a 0:2 to a 1:2 gear ratio. By using the equivalent 1000:2000 gear ratio, clutching from 0:2000 to 1000:2000 can be made in up to 1000 small gear steps.

3. Specify the Gear Ratio Clutching.

Whenever a Gear command is given to an axis that is already geared, the gear ratio is ramped from the current ratio to the requested ratio. Unless the gear ratio is ramping to or from zero, only the numerator (given in the Command Value command parameter) can change; the denominator (given in the Speed command parameter) must remain constant.

When a Gear command is given to an axis that is not currently geared, then its initial gear ratio is computed such that the computed geared speed of the axis will match its current speed.

Note: RMC CPU firmware prior to 20020222 always starts a previously non-geared axis with a zero gear ratio.

The ramping up or down of the gear ratio may be specified in several ways. The Acceleration/Deceleration Mode Select bits in the Mode word determine the method:

Mode 0: Reserved; do not use.

Mode 1 (Rate): Acceleration indicates the number of counts that the numerator is increased or decreased each millisecond until it reaches the requested ratio. The Deceleration field is not used in this mode.

Mode 2 (Distance): This mode uses the following two parameters to specify the range of master positions over which the gear ratio will be ramped:

Acceleration: Indicates the total distance spanned by the master positions. The span must be less than 32767 position units for proper operation.

Deceleration:  Indicates the starting master position for the slave's ramp.

Speed: The sign of this field, which is the denominator of the new gear ratio, indicates the direction that the range of master positions span from the starting master position.

Note: A negative number x may need to be entered as 65536 - x. For example, -1000 is entered as 64536.

Once the final gear ratio is reached, the slave will be locked into that gear ratio. Therefore, if the master backs up after the final gear ratio is reached, the slave will move at that ratio.

If the master backs up before the final gear ratio is reached, the slave will follow the clutching ramp profile backwards. If the master moves before the clutching start point, and then back into the clutching area, the slave will resume the clutching at the correct position as specified by the command.

Example 1 of Mode 2:

Acceleration: 600

Deceleration: 1000

Speed: Positive

The slave will change the gear ratio beginning when the master is at 1000 position units, and reach the requested gear ratio when the master is at 1600 position units.

Note that if the axis is given this gearing command when the master axis is at a position greater than 1600, the gearing will start instantaneously.

Example 2 of Mode 2:

Acceleration: -200

Deceleration: 500

Speed: Negative

The slave will change the gear ratio beginning when the master is at 500 position units, and reach the requested gear ratio when the master is at 300 position units.

Note that if the axis is given this gearing command when the master axis is at a position less than 300, the gearing will start instantaneously.

Mode 3 (Time): The Acceleration field indicates the time in milliseconds that the ramp will take. The Deceleration field is not used in this mode.

4. Issue the Command.

After the command parameters (Mode, Acceleration, Deceleration, Speed, and Command Value) have been set up as described in the preceding steps, the Go (G) command can be issued.

Gear Ratio Status

State Bits A and B in the Status word are used to indicate the state of the gear ratio on the slave axis in the following manner:

Limitations

Because both the Command Value and Speed fields, which make up the gear ratio, are limited to –32,768 and +32,768, the number of teeth on the master and slave gear are likewise limited. However the Speed field (denominator) of the slave axis cannot be zero. Also, be careful when setting the number of master teeth larger than the number of slave teeth. It is possible to cause the slave speeds to exceed 65,535 position units per second, which would cause the speed to wrap and incorrect feed forward values to be calculated.

Example 1:

In the gear master axis's Mode word, the Rotational and S-Curve bits are set to indicate that the axis will control speed using S-Curved ramps. See Speed Control for details on this feature. Also in its Mode word, the Acceleration and Deceleration Mode is selected as type 1. As a result the axis will move continuously in the positive direction at 10,000 position units per second.

In the Mode word of the geared axis, Gearing mode is selected with axis 0 as the master and will use Acceleration/Deceleration Mode 1. The Gear Type bit is cleared indicating that the axis will gear to the master's Target Position. It will move at half the speed of the master because the gear ratio is 5000 divided by 10,000. Assuming that the gear ratio is 0 when the slave is issued this command, it will take 100 milliseconds to ramp the gear ratio from 0/10,000 to 5000/10,000 at 50/10,000 per millisecond.

Example 2:

In this example, the master is given the exact command as the previous example.

The slave axis's Mode word is the same, except that Acceleration/Deceleration Mode 2 is used, which ramps the axis based on the master's distance. The final ratio is 1:1 (10,000/10,000). The ramp up to this ratio will occur while the master moves from 1000 to 2000 position units (as given by the starting position in the Deceleration field, the master move length given in the Acceleration field, and the master move direction given by the sign on the Speed field).

As the master moves the 1000 position units, the slave will move 500 position units since its average speed is one half of the master speed while ramping. This means that if the master is at a position of 1000 and moving toward 2000 and the slave is at a position of 1500 when the slave receives the command, both axes will reach 2000 at the same time. This is useful for flying cutoff saw applications.

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