In today’s blog we’re going to examine a bipolar chopper drive, a common method for precision current control in stepper motors. The simplest way to take a look at its operation is by using a basic schematic of it.
Let’s use our one ohm, one millihenry, and one amp motor again and wire it into the circuit as shown. As with the unipolar drive we’ll assume the transistors are ideal and have no forward drop across them when they are turned on. We’ll also start with the assumption that the power supply (+V) is 1.0 volt, the “current level control/digital to Analog Converter” (DAC) is calling for one amp and the “control logic” has turned on Q1 & Q3.
Current flows from the +V power supply through Q1 and into the “un-dotted” end of motor winding “A.” (Do you remember the transformer “dot convention”? If not, try Googling it for a refresher.) The current exits the “dotted” end of the winding and flows through Q3 and the current sense resistor “Rsense” to power supply common.
The current sense resistor usually is small in comparison to the motor winding’s resistance and in our case we’ll make it 0.01 ohms, or 1% of the motor winding, which we’ll consider negligible. Thus, we have 1 volt and a 1 ohm motor that limits the winding current to 1 amp. There’s nothing new here, right?
Now let’s take a step by turning off Q1 & Q3 and turning on Q2 & Q4 and what happens? The current flows through Q2 and into the dotted end of the winding. Then it flows out the un-dotted end of the winding, through Q4 and Rsense. The bipolar drive just reversed the current through the motor winding without having a negative power supply. Is that cool or what? And the drive uses the full winding, not just half of it.
Using the full winding produces 40% more torque than the unipolar drive straight off.
The power section of a bipolar drive circuit is typically referred to as an “H” bridge
Put Q1 in the upper left leg of the “H,” Q2 in the upper right leg, Q3 in the lower left leg, Q4 in the lower right leg and the motor winding in the horizontal part of the “H” and you can see where the “H” designation comes from.
Now lets increase the +V from one volt to two volts. We don’t have a series resistor to limit the current in this circuit as we did with the unipolar drive, but we do have a “comparator” that checks the current flow signal from “Rsense” and compares it to the commanded level that’s defined by the “current level control,” which in our example is set to one amp.
If +V goes from zero to two volts at time t = 0, the current will rise from zero to one amp in a little under one time constant (L/R = 1 MH / 1 ohm = 1 msec) or less than a millisecond. Remember that the current will rise to 63% of its maximum value in one time constant. Thus, 63% of 2 amps is 1.26 amps.
However, when the current reaches one amp the comparator and the control logic turn off the transistors that are on and lets the motor current decay to, let’s say 0.99 amps. At that point the control logic/comparator circuitry turn the transistors back on only to turn them off again when the current reaches one amp. This on-off-on-off cycling is referred to as “chopping” the phase current. Thus, the name of drive is called a bipolar chopper drive. Sounds like an appropriate name to me.
If a 2 volt power supply can pump the winding current up to one amp in under a millisecond what do you think a 48 volt supply can do?
More on bipolar chopper drives next time.