Gear Ratio

The gear ratio of an axis is set using the Gear Ratio Numerator and Gear Ratio Denominator parameters. The gear ratio applies a multiplier between the unit of position inside WMX3 and the pulse unit of the commands that are sent to the servo and the feedbacks that are received from the servo.

The unit of position inside WMX3 is called the user unit, and is discussed in Axis Position and User Units. The relation between the user unit and the pulse is expressed with the following equation.

Pulse = User Unit * Gear Ratio

For example, if the Gear Ratio is 2, each user unit equals 2 pulses. In this case, a motion from position 0 to position 10000 in WMX3 causes the command position that is sent to the servo to change by +20000. Similarly, a velocity command of 10000 in WMX3 causes the command velocity that is sent to the servo to be 20000 pulses per second.

The gear ratio converts between user units and pulse units for all commands and feedbacks, including the following:

• Command Position

• Feedback Position

• Command Velocity

• Feedback Velocity

• Velocity Offset

Changing the gear ratio changes the home position (zero position). To obtain the correct home position, homing must be executed again after changing the gear ratio.

Parameters and Statuses with Pulse Units

Some parameters and statuses of WMX3 have units of pulses instead of user units. These include the following:

Parameters:

• Single Turn Encoder Count

• Absolute Encoder Home Offset

• Servo On Following Error

Statuses:

• Encoder Command

• Encoder Feedback

• Accumulated Encoder Feedback

• Actual Following Error

Setting the User Unit to a Physical Unit

The gear ratio can be configured so that the user unit is equal to a physical unit. For example, if the servo encoder resolution is 2^20 pulses per revolution, the distance per encoder revolution is 10mm, and the desired user unit is 1 um (micrometer), the gear ratio should be set to:

2^20 [pulse/revolution] / (10 [mm/revolution] * 1000 [um/mm]) = 1048576 / 10000 [pulse/um]

Gear Ratio Numerator = 1048576 [pulse/revolution]

Gear Ratio Denominator = 10000 [um/revolution]

After setting the gear ratio as above, the user unit will be equivalent to um (micrometers). A StartMov function with the target set to 1000 will move the axis 1000um.

The minimum and maximum allowable values for the velocity, acceleration, and jerk of motion profiles are defined in user units. Changing the gear ratio can effectively change the minimum and maximum velocity, acceleration, and jerk. See Profile Parameters for more information.

Command Position and Feedback Position According to Gear Ratio Change

Even if the gear ratio is set, the current command position and feedback position will not change. This is a product specification designed to maintain the command position and feedback position during the process of changing the gear ratio.

While the command position and feedback position set by the gear ratio do not change, the modified gear ratio will be reflected in the relationship between the subsequent output ‘command position and feedback position’ and ‘Encoder Command and Encoder Feedback’.