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Understanding Pulse Width Modulation (PWM) February 21, 2013

Posted by Servo2Go.com in Technical Support Information.
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Servo amplifiers are used extensively in motion control systems where precise control of position and/or velocity is required.  The amplifier basically translates the low-energy reference signals from the controller into high-energy signals (motor voltage and current).  These reference signals can be either of an analog or digital nature.  An analog +/-10 VDC signal is still the most common.  This signal can represent either a motor torque or velocity demand.

Although there exist many ways to “amplify” electrical signals, pulse width modulation (or PWM) is by far the most efficient and cost-effective approach.  At the basis of a PWM amplifier is a current control circuit that controls the output current by varying the duty cycle of the output power stage (fixed frequency, variable duty cycle).   A typical setup is shown below in Figure 2.3  (here for a single phase load):

PWM Current Control Circuit

PWM Current Control Circuit

S1, S2, S3 and S4 are power devices (MOSFET or IGBT) that can be switched on or off. D1, D2, D3, and D4 are diodes, which guarantee current continuity.  The bus voltage is depicted by +HV.  The resistor Rc is used to measure the actual output current.  For electric motors, the load is typically inductive (due to the windings used to generate electromagnetic fields).  The current can be regulated in both directions (+ and -) by activating the appropriate switches.  When switch S1 and S4 (or S2 and S3) are activated, current will flow in the positive (or negative) direction and increase.  When switch S1 is off and switch S4 is on, (or S2 off and S3 on) current will flow in the positive (or negative) direction and decrease (via one of the diodes).  The switch “ON”-time is determined by the difference between the current demand and the actual current.  The current control circuit will compare both signals every time interval (typically 50m sec or less) and activate the switches accordingly (this is done by the switching logic circuit, which also performs basic protection functions).  The picture below shows the relationship between the pulse width (ON-time) and the current pattern.  Note that the current rise time depends on the bus voltage (+HV) and the load inductance.  Therefore, certain minimum load inductance requirements are necessary depending on the bus voltage.

Output current and duty cycle relationship

Output current and duty cycle relationship

Make sure minimum inductance requirements are met!  Pulse Width modulation (PWM) servo drives deliver a pulsed output that requires a minimum amount of load inductance to ensure that the DC motor current is properly filtered.  The minimum inductance values for different drive types are shown in the individual data sheet specifications.  If the drive is operated below its maximum rated voltage, the minimum load inductance requirement may be reduced. Most servo-motors have enough winding inductance.  Some types of motors (e.g. “basket-wound”, “pancake”, etc.) do not have a conventional iron core rotor, so the winding inductance is usually less than 50 μH.  If the motor inductance value is less than the minimum required for the selected drive, use an external filter card.

More information on PWM Servo Amplifiers from Servo2Go can be found at the link below-

http://www.servo2go.com/category.php?cat=10020&sub=10002

For more information, please contact:

Editorial Contact:

Warren Osak
sales@servo2go.com
Toll Free Phone:  877-378-0240
Toll Free Fax:   877-378-0249
www.servo2go.com

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