5 Rules of Thumb for Sizing Servo Drives in Warehouse Automation
Guest blog by MHI member ADVANCED Motion Controls
In modern material handling, a motion control system is only as good as its weakest link. If you choose the wrong servo drive for your robot arm, automated guided vehicle (AGV), or automated storage and retrieval system (AS/RS), the entire system can suffer from stalls, nuisance faults, or blown budgets.
While exact model selection always requires detailed analysis of feature requirements and performance demands, engineers need a quick way to baseline their designs during the early phases of a project. Although some applications might warrant exceptions, here are five reliable rules of thumb for selecting a servo drive that will keep your automated warehouse systems running smoothly and safely.
1. Add 25% Current Headroom
The Rule: Include an additional 25% headroom to the current limit above what you calculate is actually needed for the application.
The Application: It is easy to calculate the continuous current needed to move a standard payload across a perfectly flat, clean warehouse floor. But real-world conditions inevitably change.
An AGV might encounter a steeper ramp than it was originally tested for, or wheel friction might increase as the machine ages and mechanical components wear down. Furthermore, end-users are notorious for pushing payload limits. On a flat surface, hitting the drive’s current limit might just result in sluggish, delayed acceleration. On an incline, hitting that limit means your mobile robot stalls entirely. Adding a 25% current buffer prevents slow acceleration, following errors, and unexpected stalls in the aisles.
2. Allow a 25% Margin for Voltage Fluctuations
The Rule: Select a power supply and servo drive combination that provides at least a 25% margin between the nominal supply voltage and both the undervoltage and overvoltage limits of the drive.
The Application: Voltage is rarely perfectly stable in warehouse automation, and this buffer protects you from two major material handling realities:
• Battery Drain on Mobile Robots: For battery-powered AMRs and AGVs, the available supply voltage drops as the battery runs low. If a low battery suddenly needs to draw full power to accelerate a heavy rack, the voltage can dip significantly. The 25% cushion ensures the drive doesn’t trip an undervoltage fault and strand the vehicle.
• Regeneration on Lifts: Regeneration is that critical moment when a motor stops consuming power and acts as a generator, by design or not. This is a massive factor for vertical applications like AS/RS cranes. When a heavy load decelerates during its descent, it pumps voltage back into the system. If the drive’s overvoltage limit is too tight, this regenerative spike will cause an immediate fault.
3. Ensure You Have Proper Isolation
The Rule: You must have an isolation transformer between the AC line and the power supply’s power ground, OR optical isolation between the servo drive’s power ground and signal ground.
The Application: When power comes from an AC source, isolation protects your sensitive control equipment from damage and prevents electric shock hazards. Power supplies rated below 100VDC usually use step-down transformers that naturally provide isolation. However, if you are designing high-voltage systems (like heavy-duty conveyors or large robotic arms operating above 200VDC), you must ensure your servo drives feature built-in isolation to handle safety and severe electrical noise concerns.
This voltage difference can create a current path strong enough to destroy the servo drive and other connected network components if not properly isolated.
4. Don’t Oversize Your Power (Too Much)
The Rule: It is good to have extra power, but don’t go overboard.
The Application: If your lift system only requires 2A continuous and 6A peak, it does not make sense to install a servo drive that outputs 30A continuous. Oversizing comes with direct engineering and commercial penalties:
• Lost Space: A more powerful drive is physically larger. In the cramped, highly optimized chassis of an AMR, internal real estate is at an absolute premium.
• Poor Current Control: A drive scaled for massive current often lacks the fine current control and resolution needed for smooth, precise, low-speed motion.
• Wasted Budget: You are paying for current capacity you will never use. Size closely to your application (while keeping the aforementioned 25% headroom) to optimize both performance and your bill of materials.
5. Talk to Technical Support
The Rule: Again, none of these rules are absolute, but talking to the servo drive provider before finalizing your decision can practically be considered a must.
The Application: Every automated machine and warehouse environment is unique. Time is money, and it is usually much faster to ask an application engineer to review your torque, speed, voltage, and feature requirements than to guess and test. Leverage the expertise of your motion control partner to ensure your drive is perfectly specified before you ever finalize the electrical cabinet or cut metal.
