If you design motion control systems, chances are you’ve worked with pulse width modulated (PWM) drives. Thanks to their stability, availability and familiarity, PWM drives are the default choice in most day-to-day motion control jobs. But PWM drives tend to be noisy—making them less than ideal in some high-fidelity motion applications. And that’s where linear drives come into play.
Let’s explore the difference between PWM and linear drives, as well as some of the steps you can take to ensure you size your linear drive correctly for positioning applications.
PWM Versus Linear Drives
Rather than the continuous voltage switching that defines PWM, linear drives scale an input voltage to arrive at a desired current or voltage output. This constant gain approach results in fast, accurate current loops and also eliminates any deadband at the drive’s zero crossing—giving linear drives an edge in high-precision motion control applications.
The price you’ll pay for the improved precision will mostly come in the form of heat. Linear drives typically maintain small amounts of power inside the drive circuits—increasing heat. Excess voltage not needed by the motor is also dissipated as heat. To manage these thermal conditions while meeting application requirements, it’s important that you size linear drives correctly.
Define System Requirements. The most important system variables from a sizing standpoint are:
- Peak motor velocity in RPM
- Peak motor torque or force
- Average velocity and torque values
Load characteristics and friction effects are implicit in the motor, torque and force requirements. You’ll also need to take the motion profile into consideration when calculating both peak and average torque requirements.
Select a Motor and Drive. After defining your motion parameters, it’s time to make your initial motor and drive selection. First, choose a motor family capable of satisfying peak torque and speed requirements. Next, calculate the required voltage and current for each motor in the system and look for a matching drive that can handle these voltage and current values.
Determine Safe Operating Area (SOA). Next, calculate your application’s SOA to ensure the power doesn’t surpass the drive’s capacity. Although certain linear drives feature a reactive safety capability, if the SOA is exceeded significantly and quickly, the drive may not be able to shut down in time to prevent damage.
Calculate Continuous Dissipation. It’s important to calculate the continuous operating limits. Exceeding the average wattage could damage the drive and degrade overall system performance. Keep in mind that average velocity and torque are calculated in relation to time. To keep things simple, assume a trapezoidal motion profile and use half the torque and velocity during acceleration and deceleration.
To learn more about properly sizing linear drives for positioning applications, download our latest white paper.