Stepper motor moves To and Fro

[RP.3]

Hi,

We have used L6208 for driving bipolar stepper motor in our application. We would like to run the motor in full step mode. We implemented the circuit given in the datasheet on our PCB. When we program and try to drive the stepper motor, the shaft rotates one step forward and one step backward.

1. The clock pulse which we are supplying to CLK pin is 50ms.
2. The following are the connections of the bipolar stepper motor with L6208 Driver

Coil 1

1. +ve Terminal to Bridge A Output 1
2. -ve Terminal to Bridge A Output 2

Coil 2

1. +ve Terminal to Bridge B Output 1
2. -ve Terminal to Bridge B Output 2

I am posting a snapshot of the waveform observed after probing the outputs of Bridge A and Bridge B of the Driver.

Bridge A - Green

Bridge B - Yellow

Looking at the waveform, I feel that one of the bridge is not functioning properly and hence this to and fro motion of the motor shaft.

Request you to kindly let me know if you know how to fix this issue.

[Cristiana SCARAMEL]

Hello @RP.3​ welcome to the ST Community.

Please could you share more info about your application?

• Are you implemented the typical application of L6208 datasheet Figure 25 with components value of table 7?
• Could you confirm supply voltage of 30 V (from waveform)?
• What is the value of reference voltage (VREF pins)?

If it is possible in the same snapshot monitors (of the same full bridge):

• OUT voltage
• VREF voltage
• SENSE voltage

With these additional info, we try to help you.

[RP.3]

Hi Christiana,

Thank you for your response.

Yes, I have use the same circuit with same values mentioned in table 7 of the datasheet .I have attached a snapshot of the circuit implemented on our PCB.

Image is not available

Supply voltage is 24V.Reference voltage is 0.58V. I am using a potential divided arrangement for setting the Vref.

I have attached a snapshot of the waveform

CLK Frequency 10ms

Supply Voltage 24V

VREF 0.594V

OUT vs SENSE for Bridge B ( Yellow - Output Voltage | Green - Sense )

OUT vs VREF ( Yellow - Output Voltage | Green - VREF )

 

Please let me know if you need any more inputs to understand this issue better.

I have also taken snap shots of of both the sense signals. Adding them here just in case they might be useful.

SENSE A Vs SENSE B

Close up view of the pulses in SENSE A and SENSE B

[Cristiana SCARAMEL]

Hello @RP.3​ ,

please check the motor connection, from your schematic looks like you swapped two OUT nets:

• OUT2A becames OUT1B
• OUT1B becames OUT2A

With reference to device pins, the right motor connection is: one coil connected between OUT1A and OUT2A and the other one to OUT1B and OUT2B.

Some additional notes on your schematics:

With Vref = 0.58 V and sense resistors of 0.3 ohm, the internal PWM current controller is working with OFF time of about 20us (39 kohm / 1 nF) and it set a motor current of: I_target = 0.58 / 0.3 = 1,93 A

This current value is your target?

Moreover, I would suggest you add a capacitor on EN pin.

The external RC network connected to EN pin have to be sized to ensure a safe over-current protection.

In details, you can set the disable time (time before recovering normal operation after an OC event) and the delay time (time before turning off the bridge after an OC event).

With 100 kohm and adding 4,7 nF that allow obtaining about 1us of delay time and 280us of disable time (with high voltage level of 3,3V).

Let me know if you solve the issue checking the motor connection.

[RP.3]

Hi @Cristiana SCARAMEL​ 

Thank you very much for sharing your observations. You were right. The net names got changed and I was making the motor connections as per the names. I have corrected the connections now and the stepper motor is rotating properly.

I was not sure about how much current our stepper motors would draw, so i've set it to 1.9A as you've shown in your calculations.

So going by your suggestion above, Should the disable time be very much larger than the delay time ? Can you please elaborate on how you've arrived at those values for Delay time and disable time ?

Also can you please let me know what is the max and min frequency of pulses which can be given to the CLK pin.

I am currently driving the stepper motor with a 10ms pulse and the shaft seems to rotate little slow. If i try to increase the frequency( by decreasing the pulse duration) the motion of the shaft is becoming abnormal( it stops abruptly for an instance and rotates normally again) . and if i try to increase the frequency futher shaft stops rotating and motor makes a high pitch noise and the current consumption is too high.

[Cristiana SCARAMEL · Accepted solution]

Hi @RP.3​,

well, we solved your motor issue.

The delay time is the intervention time for overcurrent protection and must be shorter so that it intervenes as soon as possible.

The disable time must be long enough to ensure a safe r.m.s. current value in case of short circuit.

For more details refer to the dedicated section 7.5 in the L6208 datasheet and to the Application AN1451 (paragraph 2.12) where you can find the formula for the timing calculations.

The CLOCK pin is the Step clock input: the internal state machine makes one step on each rising edge, a minimum frequency is cannot be defined.

Concerning the maximum frequency you can find the value in the Table 5. Electrical characteristics at page 9: fCLK max is 100 kHz.

The abnormal motor behavior that you observed might be related to the motor resonances.

In the stepper motor each time a step is performed, the final position is not immediately asserted, but the rotor vibrates around the target position before stop. When the step rate reaches the frequency of this vibration, the mechanic of the motor resonates. This effect is named mid-point resonance. Possible effects can be: strong vibrations, reduced torque or discontinuous motion.

Are you running the motor without load? In many cases adding a small load at the motor shaft dramatically reduce the resonances allowing the motor to run correctly.

[RP.3]

Hi @Cristiana SCARAMEL​ ,

Thank you for the clear explanation.

Yes, I was running the motor without load. I am currently away from my lab for few days. Will try running it with load once i get back.

Thanks once again. Have a great day ahead.