tmckeown
Electrical
- Jul 3, 2008
- 11
Hi,
First time posting here. This is a great forum. I've learned a lot from just reading it all day.
I've been assigned to make a cable hoist for our company. I finished the first batch of them and they turned out pretty well, but I'm having a few problems that I could use some expertise on. First, I'll give you the parts that I'm using, so you get a better picture of what's going on.
1) ABB ACS350 vector drive
2) Brother mid series 1HP gearmotor (1268 in-lbs @45RPM)
3) Dynapar encoder (on drum shaft for positioning)
4) 8" Diameter grooved drum
5) Custom controller (we made it)
The spec I needed to meet was:
Must be able to accurately position a 300# load
Must be able to move it from zero speed to 100 feet per minute
Must fit into a specific lighting truss
Description:
A user sends a command from a lighting console (DMX) to the controller. The controller keeps track of positioning as well as translating the desired speed and destination info from the lighting console. The controller sends the drive speed (0-5 volts) and direction info. The controller also creates the ramps up to speed, down to stop and it sets the brake when at the destination. The drive obviously controls the motor. The drive is set to have the fastest ramp .1 sec. This is so the controller can handle the ramping. We are running the drive in vector torque mode, though we have done tests in vector speed mode as well. The motor has a brake. Both the controller and the drive have to tell the brake to be released before it gets released. When the drum is moving, the controller counts the encoder pulses to determine its position. When its within X steps from the destination, it start a ramp down of the control voltage feeding the drive. When the destination is reached, we set the brake and remove voltage to the drive as well as telling it to not move either up or down.
For the most part, it works well, though we're not completely satisfied with the performance. I'm hoping someone might be able to fill in the blanks of our knowledge and help us get the project completed.
We chose the Brother gearmotor due to it's size 15"H and it's weight 44#. We chose the ABB drive simply because a sales rep said it would do what we needed. On an 8" drum, the motor is able to give us 317# of torque. We pushed the max frequency of the drive to 69HZ, which lets us hit the desired 100' per minute. We can get very good repeatability on positioning.
So, what's the problem?
We are having trouble controlling the motor at very slow speeds. We find that we are not able to release the brake immediately upon start of a move, or it will drop the load. So, to get around that, we have tried two things. (not simultaneously)
1) We set the drive to release the brake once it's reached a certain speed.
2) We send the drive a minimal control voltage and keep it there for a few seconds before releasing the brake.
In both cases, it does prevent the load from falling, but the initial move is a bit of a jump. If we lower either of those two values, the load drops. We could almost live with the jump, but we see another related problem when we try to do very short 1"-2" moves. When we do a very short move, occasionally after the move is completed and the controller has turn on the brake, the drive won't shut off. We can hear it pulsing; trying to move. We checked the current to the motor, and the drive is sending the max current we allowed (6.2A). If we then try to do another move in the opposite direction, the drive still retains control and tries to move the motor in the previous direction. When that happens, the drive keeps moving the motor until it reaches the torque that is set by our control voltage and then quickly starts moving in the correct direction. Pretty odd eh? For the longest time, we though the controller had a software bug. After days of testing, we found that we can reproduce this glitch at will.
We have spoken to the drive engineers at ABB. They tell us it is because we are not allowing the drive to ramp down before we set the brake. We don't want the drive to do a ramp. We are creating the ramp in the controller and sending the control voltage to the drive. If we allow the drive to handle the ramp, there is no way for us to stop at the correct destination. The encoder for positioning connects to our controller.
This brings up a number of questions:
1) Is a servo motor a better option for a cable hoist? They seem very expensive and much larger than the Brother motor.
2) Should we change the Brother motor to one with an encoder? The brother has no place to mount an encoder. The brake is on the end of the motor and there is only about 1/4" of shaft sticking out from there, but it's all under the fan housing. Maybe someone knows of an encoder we could add?
3) Should we change the drive? Maybe there is one that could give us full torque holding without a motor encoder.
4) Have we reached the limit of what we can do? Do we need to de-rate the hoist to a lower lifting weight, so we can move slower and avoid these problems?
I know I've probably created even more questions with my questions. Thanks for the help.
Tom
First time posting here. This is a great forum. I've learned a lot from just reading it all day.
I've been assigned to make a cable hoist for our company. I finished the first batch of them and they turned out pretty well, but I'm having a few problems that I could use some expertise on. First, I'll give you the parts that I'm using, so you get a better picture of what's going on.
1) ABB ACS350 vector drive
2) Brother mid series 1HP gearmotor (1268 in-lbs @45RPM)
3) Dynapar encoder (on drum shaft for positioning)
4) 8" Diameter grooved drum
5) Custom controller (we made it)
The spec I needed to meet was:
Must be able to accurately position a 300# load
Must be able to move it from zero speed to 100 feet per minute
Must fit into a specific lighting truss
Description:
A user sends a command from a lighting console (DMX) to the controller. The controller keeps track of positioning as well as translating the desired speed and destination info from the lighting console. The controller sends the drive speed (0-5 volts) and direction info. The controller also creates the ramps up to speed, down to stop and it sets the brake when at the destination. The drive obviously controls the motor. The drive is set to have the fastest ramp .1 sec. This is so the controller can handle the ramping. We are running the drive in vector torque mode, though we have done tests in vector speed mode as well. The motor has a brake. Both the controller and the drive have to tell the brake to be released before it gets released. When the drum is moving, the controller counts the encoder pulses to determine its position. When its within X steps from the destination, it start a ramp down of the control voltage feeding the drive. When the destination is reached, we set the brake and remove voltage to the drive as well as telling it to not move either up or down.
For the most part, it works well, though we're not completely satisfied with the performance. I'm hoping someone might be able to fill in the blanks of our knowledge and help us get the project completed.
We chose the Brother gearmotor due to it's size 15"H and it's weight 44#. We chose the ABB drive simply because a sales rep said it would do what we needed. On an 8" drum, the motor is able to give us 317# of torque. We pushed the max frequency of the drive to 69HZ, which lets us hit the desired 100' per minute. We can get very good repeatability on positioning.
So, what's the problem?
We are having trouble controlling the motor at very slow speeds. We find that we are not able to release the brake immediately upon start of a move, or it will drop the load. So, to get around that, we have tried two things. (not simultaneously)
1) We set the drive to release the brake once it's reached a certain speed.
2) We send the drive a minimal control voltage and keep it there for a few seconds before releasing the brake.
In both cases, it does prevent the load from falling, but the initial move is a bit of a jump. If we lower either of those two values, the load drops. We could almost live with the jump, but we see another related problem when we try to do very short 1"-2" moves. When we do a very short move, occasionally after the move is completed and the controller has turn on the brake, the drive won't shut off. We can hear it pulsing; trying to move. We checked the current to the motor, and the drive is sending the max current we allowed (6.2A). If we then try to do another move in the opposite direction, the drive still retains control and tries to move the motor in the previous direction. When that happens, the drive keeps moving the motor until it reaches the torque that is set by our control voltage and then quickly starts moving in the correct direction. Pretty odd eh? For the longest time, we though the controller had a software bug. After days of testing, we found that we can reproduce this glitch at will.
We have spoken to the drive engineers at ABB. They tell us it is because we are not allowing the drive to ramp down before we set the brake. We don't want the drive to do a ramp. We are creating the ramp in the controller and sending the control voltage to the drive. If we allow the drive to handle the ramp, there is no way for us to stop at the correct destination. The encoder for positioning connects to our controller.
This brings up a number of questions:
1) Is a servo motor a better option for a cable hoist? They seem very expensive and much larger than the Brother motor.
2) Should we change the Brother motor to one with an encoder? The brother has no place to mount an encoder. The brake is on the end of the motor and there is only about 1/4" of shaft sticking out from there, but it's all under the fan housing. Maybe someone knows of an encoder we could add?
3) Should we change the drive? Maybe there is one that could give us full torque holding without a motor encoder.
4) Have we reached the limit of what we can do? Do we need to de-rate the hoist to a lower lifting weight, so we can move slower and avoid these problems?
I know I've probably created even more questions with my questions. Thanks for the help.
Tom