Voltage mode and current mode

The difference between voltage control and current control

There are two types of stepping motor control methods: constant voltage control (voltage mode) and constant current control (current mode). STEP800 can be used in voltage mode, while STEP400 can switch between both voltage and current modes. The difference between these modes is well-explained in this presentation PDF by STMicroelectronics.

The differences for the users can be described as follows;

  • Voltage mode is quiet and smooth, but can only drive at low speed.
  • Current mode is noisy, but can drive to higher speed.

Test with the STEP400

Here is a video about:

  • The difference between constant voltage control and constant current control
  • The difference between full-step and micro-stepping drive

First, the motor runs under constant voltage control (hereinafter referred to as “voltage mode”). After around 800 step/sec, the motor cannot run properly and starts to vibrate, then it stalls at about 1,400 step/sec and stops completely. The motor runs quietly until the vibration starts, but in this video, a microphone is attached to the motor so that we can capture the smallest noise.

Next, the motor is switched to constant current control (hereinafter referred to as "current mode"). It is noisy, but it can drive to higher speed. In this setting, we were able to achieve more than 11,000 steps/sec.

KVAL and TVAL

KVAL register values are applied to control the drive voltage in the voltage mode, and TVAL registers are used to control the drive current in the current mode. Although they are actually same registers in the driver, our firmware keeps them separated, rewrites them when mode changes to avoid unintended values to be set.

Voltage mode setting

In the voltage mode of the STEP400 or STEP800, registers called KVAL is used to set what percentage of the power supply voltage should be applied to the motor. Also if a high voltage power source is used, excessive current may flow when the motor is spinning at lower speed. To compensate this current imbalance, there is a group of registers to lower the drive voltage at low speed and supply higher voltage at higher speed. The calculation of these register values is described in the STMicroelectronics application note. In STEP400, these registers can be set with /setBemfParam command, and from the Config Tool as well.

We have calculated the register values for some motors based on our actual measurements and have made them available as configuration files. We have only a small numbers of configuration files at the moment, but we are planning to add more in the future.

Current mode setting

In the current mode which available in STEP400, the TVAL registers are used to set the target current value. The current can be set up to 5A in increments of 78mA in the STEP400. Yo need a high voltage power supply to deliver the target current when the motor is running at high speed. Although the PowerSTEP01's actual current drive capability is 10A, the STEP400 has the upper rating limit of 5A due to the power rating limitation of the current sensing resistor. At 5A phase current, the torque is considerably strong, and the tiniest mistake may lead to great physical danger. In such situations we recommend to use use an industrial grade motor drivers.

Switching modes

These commands are available for switching between modes;

The motor must be in the high impedance (High Z) state before switching the mode. For example, if you are going to switch the Motor 1 to current mode, the command sequence is in the following order;

  1. /hardHiZ 1
  2. /setCurrentMode 1

The microstepping is limited to minimum 1/16 in current mode. Any lower value than 1/16 will be regarded as 1/16. When you change the microstepping mode, the coordinate system will also change. For example one rotation in 1/128 microstepping mode is 200x128=25600 steps for a 200steps/round motor, but one rotation in 1/16 microstepping mode is 200x16=3200 steps.

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