Servo System Sizing: The Need, the Present & the Future

By Christian Vardin, P.Eng

What is the need for a properly sized servo system?

What every customer and business wants - better performance and reliability at a lower cost.  However, more often than not, a servo system is either undersized or oversized.  An undersized system will fail to meet performance requirements, and in extreme cases, can overheat and damage components. Therefore, most engineers err on the side of caution and oversize since the numerous calculations can be complex and time consuming. However, an oversized motor is not a cost-effective solution. Larger motors require larger drives, and most often lead to larger infeed modules, filters, bleeders, cables, etc., which in turn require a larger cabinet enclosure to house the components. Over sizing has a ripple effect throughout the entire design. Usually larger components, be they motors, drives, gearboxes, etc., are higher priced than smaller units. Finally, an over sized design increases the long term operating costs as the design is inherently less energy efficient.

In summary, most engineers will over size because it is the easier and safer approach. However, over sizing increases the initial system cost as well as the ongoing operating cost of a servo system.

What is the current method of selecting components?

Consider an engineer who is designing the servo system for a 3-axis gantry robot. The design must meet all the performance requirements including velocity profiles over a given cycle.   The first phase of the sizing process is a collaborative effort with the mechanical engineering team.  It begins with the mechanical design, which must be able to achieve the reach as well as sustain the forces created during the entire cycle. The gearbox and motor are then selected in an iterative process. Most sizing tools perform this task adequately. But this is where most sizing tools stop. The servo system engineer is then left to perform the remaining calculations manually to size the drive, filter, cables, bleeder module, etc.  Typically, this is a very time consuming process, which applies rules of thumb to approximate the calculations.  Rules of thumb are applied at various stages, from a constant gearbox efficiency over the entire speed range, to a linear torque constant (kt) for the motor, as well as the amount of bus stored energy and the overlapping of bus power requirements from each axis.  Hence, rules of thumb are applied on top of each other.  By the time the requirements of the infeed power module are defined, the design is based on layers of assumptions which can have an error of 20% to 50%.

Let’s look a little deeper at the power flow throughout the system.  Most modern day servo systems are designed to be electrically coupled to share energy, minimize unused or oversized components, and reduce both initial hardware cost as well as operating costs by improved energy efficiency.  Consider the image to the right which illustrates one possible configuration for a 3-axis gantry robot.  For this example, each drive is connected to form one larger bus, and there is only one power supply connection which feeds all three axes. The internal bleeder in the Y axis drive is available to dissipate excess braking energy.  Now consider what the engineer does today attempting to calculate the power flow for parallel moves - say the Y axis is decelerating while the Z axis begins to accelerate. This is virtually impossible to determine accurately by hand or by spreadsheet without extensive algorithms.  Hence, the engineer resorts to more rules of thumb to approximate the loading each axis will actually draw from the supply, and then adds safety factors on top of the approximations to ensure the design will perform.  The result is an oversized and more costly design.

What will the future bring for sizing servo systems?

Precise sizing calculations for even the most complex, multi-axis, shared bus configurations.  The gearbox efficiency will be based on the actual efficiency at any given speed, and the motor model will precisely calculate the non-linear torque-to-current conversion.  Complete power flow calculations will be handled with ease, including precise modeling of a floating bus.  The user will click on any component, say the Z axis motor cable, or the system infeed cable, and the precise results will be presented. No guessing, no rules of thumb, just accurate results, immediately.  In a few seconds, scenario analysis will allow engineers to quickly and confidently assess the value of additional capacitor modules as it relates to energy efficiency and a lower cost solution as less bleeder capacity is required.

And the good news - the future of servo sizing tools is here.  Today’s sizing tools are complete, accurate, and immediate. In large organizations such as OEMs who manufacture hundreds or even thousands of machines and robots per year, the cost savings and improved customer value are multiplied through better and efficient designs.  In these organizations, the value of today’s servo sizing tools is leveraged significantly.  By dramatically reducing the servo system design process by as much as 50% from weeks to days, design cycles decrease, products reach the market sooner, and resources are freed up to work on other projects. All the while, the customer gets a better performing, more reliable and efficient machine.  Win, win, and win.

Christian Vardin, P.Eng, is the Product Manager for ServoSoft – The complete sizing tool, by ControlEng Corporation (www.controleng.ca)