The Servo Sizing Process

The servo sizing process described below can be very accurate. However, caution is warranted - the results are only as good as the inputs are accurate.

Step 1: Drive Mechanism

The sizing process begins by identifying the drive mechanism to be used
Obtaining the design specifics such as the inertias, efficiencies and dimensions of all components
Identify mechanical constraints such as torque and force limits that must not be exceeded

Related topics: Drive Mechanics

Step 2: Motion Profile Requirements

Depending on the type of application, certain aspects of the motion profile requirements may be more important than others
For pick and place applications such as gantry robots, move time, peak velocity, acceleration and deceleration are key
For continuous running applications such as feed rolls in paper mills, velocity may be the only key requirement
Specify all motion profiles in the sequence including:
1. Distance
2. Velocity
3. Acceleration
4. Deceleration
5. Jerk
6. Dwell
7. Payload
8. Thrust during Acceleration, Constant Velocity, Deceleration and Dwell

Related topics: Motion Profiles, Sequence

Step 3: Gearbox Selection

The gearbox selection is often an iterative process as a balance is struck between the gearbox output torque and speed limits and the motor torque and speed limits
Selection criteria
1. Maximum output torque
2. Maximum input speed
3. Continuous rating - if the duty cycle is greater than the duty cycle for continuous operation, then the nominal output torque and mean input speed ratings must not be exceeded
4. Ratio - select a gearbox ratio that allows the motor to operate within its intermittent and rms rated torque curves, and ensure the inertia mismatch is appropriate for the application
5. Motor-Gearbox interface - if the gearbox is directly coupled to the motor, ensure the motor shaft diameter and length are suitable for the gearbox input interface

Step 4: Motor Selection

Ensure the following ratings meet the application requirements
1. Maximum torque
2. Maximum speed
3. Rms torque at mean speed
4. Inertia mismatch (ensure the inertia mismatch is appropriate for the application)
5. De-rate accordingly for environmental factors (ambient temperature and elevation)
6. Motor shaft diameter and length are suitable for the gearbox input

Step 5: Drive Selection

Depending on the vendor and its specific product lines, drive selection can vary from simply sizing the inverter, to sizing for the internal bleeder, as well sizing for bus power requirements in multi-axis shared bus configurations
For multi-axis shared bus configurations, the bus power can only be calculated after all axes have been entered into the model
Ensure the following ratings meet the application requirements
1. Inverter
  1. Continuous and peak current ratings meet the application requirements demanded by the motor
2. Bleeder
  1. Continuous and peak power
  2. Energy dissipation
  3. Resistance to provide enough peak power for the application (based on nominal bus voltage)
3. Bus power
  1. Output continuous and peak power
  2. Regenerative (if equipped) continuous and peak power
4. De-rate accordingly for environmental factors (ambient temperature and elevation)

Step 6: Infeed Selection

Depending on the vendor and its specific product lines, infeed selection may be required
Infeed selection can vary from sizing bus power requirements to sizing for the module’s internal bleeder requirements as well
For multi-axis shared bus configurations, the bus power can only be calculated after all axes have been entered into the model
Ensure the following ratings meet the application requirements
1. Bus power
  1. Output continuous and peak power
  2. Regenerative (if equipped) continuous and peak power
2. Internal bleeder
3. 1. Continuous and peak power
  2. Energy dissipation
  3. Resistance to provide enough peak power for the application (based on nominal bus voltage)
De-rate accordingly for environmental factors (ambient temperature and elevation)

Step 7: External Bleeder Selection

Add an external bleeder in a shared bus configuration if the existing drives and infeed modules do not have an internal bleeder, or their internal bleeder power ratings are insufficient for the application
Ensure the following ratings meet the application requirements
1. Continuous power
2. Peak power
3. Energy dissipation
4. Resistance

Related topics: Bleeder Module, Bleeder Margins, System Power

Step 8: External Capacitor Module Selection

In non-regulated bus systems, increasing the system bus capacitance with an external capacitor module can reduce the bus and bleeder power requirements, improving the overall efficiency of the system
Many vendors place limits on the amount of bus capacitance that can be connected to the bus. Typically, the limitation is the result of the infeed module’s maximum allowed inrush current.

Step 9: Guidelines for Sizing Cables, Filters and Fuses

Typically, sizing components at the infeed supply begins with calculating the infeed supply RMS and peak currents.
The power factor at the infeed supply is not calculated by the program. Therefore, the current values are not provided. However, the necessary power values are provided, can can be used to calculate the infeed supply RMS and peak currents.